A/D Conversion Clock Select bits confussion ..

[gunayburak]

Hello everyone ..

For a long time I've been struggling with these settings ... When one would like to perform an ADC reading what kind of settings he must add into his code ?

As far as I read the datasheet of 16F1827 , I see the following details about this setting



ADCS<2:0>:A/D Conversion Clock Select bits
000=FOSC/2
001=FOSC/8
010=FOSC/32
011=FRC(clock supplied from a dedicated RC oscillator)
100=FOSC/4
101=FOSC/16
110=FOSC/64
111=FRC(clock supplied from a dedicated RC oscillator)


So , If I'm running my PIC at 32 MHZ internal oscillator , the setting should be 011 or 010 ?

and the SECOND QUESTION :

What is the maximum ADC conversion time at 32MHZ for the PIC16F1827 ...
When I look at the conversion tables that Microchip has published for this PIC , I can't understand which is the exact time at the selected OSC frequency that takes for a complete ADC conversion time ..

Thanks in advance ...

[richard]

010=FOSC/32 tad=1.0uS
011=FRC tad=1.0uS to 6uS (vcc and temperature dependant)
110=FOSC/64 tad=2.0uS

a 10 bit adc takes 11.5 tads

any divider less than fosc/32 @32mhz is out of reliable conversion range

[elcrcp]

FOSC is Frequency of OSCillator, so if you are using 32MHz internal osc, your fosc is 32MHz if you are using 8MHz external osc then your fosc is 8 MHz and if you are using 4 MHz external with PLL then your FOSC is 16MHz.
011 and 111 settings are for an internal dedicated oscillator with 1.6us period if I remember correct...
You can always use fosc/2 timing since faster is generally better. But again, it depends on your timing need. If you don't need a special timing, just choose whatever you like.

Edit: I'm not sure minimum conversion time but I believe it has to be larger then 10us

[gunayburak]

Thanks for your answers but I still do not understand something .. Which one of these oscillator settings should be chosen for a robust operation .. Or in other words , for a "classic proper" operation ... What I understand from your explanations is that I can choose any of these settings non regarding if the OSC is internal or external ...

As far as I know the Frc stands for internal oscillator ... Please correct me If I'm wrong at this point .. So If I choose 4MHZ with PLL operation , I'm running it with 32 MHZ internal oscillator which is also called the internal RC osc block , so in this case I should go with Frc for a more accurate conversion ... or with Fosc/32 or Fosc/2 for a faster conversion ... Right ?

[aratti]

32 MHz is a too high frequency to obtain a good ADC reading. The minimum TAD is 1.6 us and If you set fosc/64 you will get a TAD of 2 us and you are barely inside the correct timing. Since you will be in the low end side as far as the TAD value is concerned. Dropping your fosc to 16 MHz with the same setting you will have a TAD of 4 us which is much better than 2 us.

Cheers
Al.

[gunayburak]

Thanks Al .. You may be right ...
Well the question is how to set the define adc sampleus parameter in pbp ? I should adjust it according to the decided Fosc ?

[aratti]

The sampleus parameter is the time given for the acquisition ( changing the sample & hold capacitor). This timing is critical since you must charge the capacitor no more than 1T..(up to 1T the charging curve is quasi linear) Now If you have decided to use fosc = 16 MHz than the minimum pauseus you can get is 5 us so a Define adc_sampleus 15 should give you good results.

Cheers

Al.

[gunayburak]

Should we not calculate it 1.6x11 Tad = 17.6 uS ? Thus is it not better to set it to 20 uS ?

[aratti]

The 11 TADs is the time taken by the MCU for the 10 bits convertion! Please do not confuse acquisition with convertion. The setting suggested is the time you will give to the sample & hold capacitor to charge with your input dc value. This time should be enough to charge the cap not beyond the 1T, because beyond this point the charging function is no longer quasi-linear, but totaly logaritmic.

You can set it to 20 us and see If your ADC reading are consistent and adjust this value to suite your need. (Remember that the best acquisition time will depend by the hardware setting, so every circuit has his own best acquisition time)

Cheers
Al.

[gunayburak]

Grazie tante per la risposta cara aratti .. :)

[Heckler]

I remember saving a link to this... http://www.edaboard.com/nextoldesttoentry1570.html
page. Be sure to hit the "next" button at the bottom of the page where there is discussion of acquisition time.

hope this helps you.

[Heckler]

I remember saving a link to this... http://www.edaboard.com/nextoldesttoentry1570.html
page. Be sure to hit the "next" button at the bottom of the page to go to Part 2 where there is discussion of acquisition time.

Possibly something of value there.

[gunayburak]

The only part I cant understand is the line he somehow calculated 40 + 38.4 us = 78.4 uS .. where did that 40 come from ?

[aratti]

40 us is the value the author arbitrarely gives as acquisition time "sampleus parameter".

Remember:

A too high acquisition time will give issues when the analog input approch the V(ref+)

A too low acquisition time will give issues when the analog input approch the zero

So, if you do not see any issues in your reading at the extreme of the reading field then the acquisition time used is correct!

Ciao, complimenti per il tuo italiano!

Al.

[tumbleweed]

A too high acquisition time will give issues when the analog input approch the V(ref+)
A too low acquisition time will give issues when the analog input approch the zero


Where do you get this from?

The aquisition time is the MINIMUM amount of time you have to wait for the holding capacitor (and it's RC network) to fully charge/discharge to the voltage level you're sampling.

If you set the acquisition time too low then you won't have allowed enough time for the charge on the capacitor to reach the desired level, and you'll get an incorrect conversion. It doesn't matter so much if the input is near Vref or zero, it'll be wrong.

If you're measuring a DC voltage, there is no "too high" an aquisition time.

[aratti]

Where do you get this from?


From the charging function of a capacitor!

Al.

[tumbleweed]

So you're saying that if you wait too long the voltage on your sampling capacitor will be wrong?

If you put a small RC network across a voltage source, what will the voltage across the capacitor be if you wait a second? A minute? An hour?

[aratti]

You cannot use the full function because it is foundamentally a logaritmic function. For the ADC convertion you need a linear function. Once time, resistence and capacity are fixed than the charging function depend only from the voltage applied. If during the charging time you do not pass the 1T point of the function, than the charging function can be considered "quasi liner" and the system work. In other words you can deduce the value of the voltage applied. If you pass the 1T point then thing become more complicated since from that point on the function is purely logaritmic.

Cheers
Al.

[tumbleweed]

You're somehow mixing Acquisition time with the linearity of the ADC conversion function.

For the Acquisition time, you need the sample cap to charge to whatever value you want the ADC to convert. Period.

The time constant of an RC network is T=RC, and after one T the voltage across the cap is roughly 66% of the input voltage.
If you look at T vs percent, you get:


1 63.2%
2 86.5%
3 95.0%
4 98.2%
5 99.3%
10 99.995%
20 99.9999998%

If you set the Acquisition time to 1T and then convert the value, you could be VERY wrong... 37% wrong. Of course, that depends on the new input voltage you want to convert and any existing charge on the sample cap, but T gives you a worst-case number.

For example, an 8-bit ADC (1 part in 256, or 0.39%) would need an acquisition time of better than 5T, but there's nothing wrong with giving it 10T or 20T or 100T, as long as nothing else changes in that time.

You do not have to hit some magic 1T window trying to get it into a "quasi-linear" range, or guess at things to "see if it's good enough".

[aratti]

No much to say, just keep your convintions if you are happy with them.

Al.

[HenrikOlsson]

Hi,
I must say I'm quite confused and intruiged by this statement as well.

As I'm currently messing around with the ADC in the 18F2431 I've been reading up on the acquisition time and conversion time requirement(s) in order to better understand it and I can't for the life of me find any references to issues with too long acqusition time.

The datasheet is pretty clear on the point that the conversion clock period (TAD) needs to be as short as possible, yet still longer than the minimum (418ns for 18F2431). This means that when operating at 40MHz the selection FOsc/32 must be used since that gives a TAD of 800ns. Going a step lower (FOsc/16) would violate the 416ns requirement. A conversion takes 12TAD so 9.6us in this case.

The minimum acqusition time, ie for how long the S/H capacitor is connected to the analog input before the conversion starts dependes on the source impedence driving the inputs, temperature range etc. For a 1k source impedence I calculated the minimum acquistion time to 2.38us which is ~3TAD in my case. I'm well below 1k in source impedence but I'm going to stick with a acqusition time of 4TAD.

This gives me a total A to D time of 3.2+9.6=12.8us.

Nowhere can I see any reference to issues with too long acqusition time.

Confused.....

/Henrik.

[richard]

if it were possible that the acquisition time could be too long then surely the optimum acquisition time would have to varied logarithmically according to the instantaneous sample voltage . the suggestion is not realistic ,unsubstantiated and unworkable

[tumbleweed]

Agreed.


Confused...
Don't be. You have it correct.

What you're computing is the minimum time it takes for the RC sample/hold network to charge to the input voltage assuming you're making a full-scale voltage change. Depending on the accuracy you're trying to reach that normally takes about 7T-8T. Once it's charged up (or down), you're good to go, but it does no harm in waiting longer (as long as nothing else changes).

The only thing to watch out for is if you change PIC's be sure to check the specs of the new device. The source resistance of the internal switch and the cap can be different for different families, so that can change the calculation.

[aratti]

Henrik, I am using a rather old pic (18F2620), I still have a couple of hundreds of them. But timing apart your and mine are very similar in the setting of the TAD value and the acquisition time.
As far as TAD value is concerned I can Select value up to Fosc/64 (minimum delay required for 1 TAD = 1.6 us)
As far as ACQUISTION TIME is concerned, I have a range selectable from 2 TAD to 20 TAD. (So you have a lower and upper limits)

I made some experimenting some years ago, using the manual acquisition time, so I could increase/decrease the acquisition time beyond the 2/20 TAD limits. Unfortunatly I was not able to find these data (I have been digging for the whole day without success) but I assure you they were very interesting. The experiment was done taking 100 reading at three specific points of the 10 bits range. First point was fixed to 10 ADC count (low end), second point in the middle at 512 ADC count and the third point at 1012 ADC count (high end). These reading were taken at different acquisition time going from 1 us up to 200 us in steps of 10 us each.
Once, all the data were collected ( they have been collected via RS232 connection and loaded directly into an Excel sheet), the statistic applied were : Average/Standard deviation/standard error.
Looking at the standard error, I noticed an increase in the two extreme regions when the acquisition time was too low or higher than the 2/20 TADs region , no significant variation of standard error were observed in the middle range.
I will continue to search these files (I am sure I still have them somewhere) and If I found them I will post them here in this thread.
But everybody, with some time to spare, can repeat the experiment on his own.
Cheers.
Al

[HenrikOlsson]

Hi,
OK, I did the test.

For this I was using a 18F25K20 running at 64MHz and 3.3V. Vref set to VDD/VSS and measured at 2.999V.
Actual voltages measured with a FLUKE189

The test program cycles thru all the available clock selections and all the available acquisition times (49 combinations). For each selection it takes 8 readings on AN0. Between each reading it makes a dummy conversion of another (floating) channel. After 8 readings it calculates the averages and presents the result.

The test was conducted at 3 different input voltages (25mV, 1.65V, 3.285V) with 3 different source impedences (1k, 10k, 39k). For the 18F25K20 10k is the max recommended source impedence.

Here's the code if anyone wants to repeat the test.
Please note the ADC clock and acquisition time may be different on other devices, this matches the 25K20 that I used:

'************************************************* ***************
'* Name : ADC Test.PBP *
'* Author : Henrik Olsson *
'* Notice : Copyright (c) 2015 Henrik Olsson *
'* : All Rights Reserved *
'* Date : 2015-07-23 *
'* Version : 1.0 *
'* Notes : Test program for verifying ADC clock selection *
'* : and acquisition times. *
'* : For 18F25K20. *
'************************************************* ***************
DEFINE OSC 64
DEFINE LOADER_USED 1 ' We're using a bootloader.
DEFINE HSER_RCSTA 90h ' Enable serial port & continuous receive
DEFINE HSER_TXSTA 24h ' Enable transmit, BRGH = 1
DEFINE HSER_CLROERR 1 ' Clear overflow automatically
DEFINE HSER_SPBRG 138 ' 115200 Baud @ 64MHz, -0,08%

SPBRGH = 0
BAUDCON.3 = 1 ' Enable 16 bit baudrate generator

ADCON0 = %00000001 ' Enable the ADC, select AN0
ADCON1 = %00000000 ' Vref is Vdd/Vss respectively
ADCON2 = %10001000 ' Right justified result, 2TAD, FOosc/2

TRISA.0 = 1 ' RA0/AN0 is input
ANSEL.0 = 1 ' Digital buffer disabled, analog enabled

Samples VAR BYTE
ACQT VAR BYTE
ADCS VAR BYTE
RESULT VAR WORD
Accumulator VAR WORD

GODONE VAR ADCON0.1

PAUSE 1000
HSEROUT["Start",10,13]

For ACQT = 1 to 7 ' We have 7 different TAD settings ranging from 2TAD to 20TAD
For ADCS = 0 to 6 ' We hace 7 different conversion clock selections
ADCON2 = 128 + (ACQT * 8) + ADCS ' "Build the ADCON2 word by combining the two settings.

PAUSE 25

Accumulator = 0

For samples = 0 to 7 ' Take 8 samples

' First make a dummy conversion of a channel we're not really interested in
ADCON0.2 = 1
GODONE = 1
WHILE GODONE : WEND

' Then switch to AN0 and make a conversion.
ADCON0.2 = 0
GODONE = 1
WHILE GODONE : WEND

' Get the result and add it to the accumulator
Result.HighByte=ADRESH
Result.LOWBYTE=ADRESL
Accumulator = Accumulator + Result

NEXT

' Divide accumulated result by 8 to get the average of our 8 readings.
Result = Accumulator >> 3

' These selections matches the 18F25K20
Select CASE ADCS
CASE 0
HSEROUT["FOSC/2 "]
CASE 1
HSEROUT["FOSC/8 "]
CASE 2
HSEROUT["FOSC/32 "]
CASE 3
HSEROUT["FRC "]
CASE 4
HSEROUT["FOSC/4 "]
CASE 5
HSEROUT["FOSC/16 "]
CASE 6
HSEROUT["FOSC/64 "]
END Select


SELECT CASE ACQT
CASE 1
HSEROUT["2 TAD "]
CASE 2
HSEROUT["4 TAD "]
CASE 3
HSEROUT["6 TAD "]
CASE 4
HSEROUT["8 TAD "]
CASE 5
HSEROUT["12 TAD "]
CASE 6
HSEROUT["16 TAD "]
CASE 7
HSEROUT["20 TAD "]
END SELECT

' Report the result.
HSEROUT[" Result: ", #RESULT, 13]

NEXT
NEXT

PAUSE 100

END

And here are the results:

************************************************** ***
************************************************** ***
VRef is VDD/VSS measured to 3.299V
1k resistor in series with input
************************************************** ***
************************************************** ***

Input voltage 25.7mV
FOSC/2 2 TAD Result: 8
FOSC/8 2 TAD Result: 8
FOSC/32 2 TAD Result: 7
FRC 2 TAD Result: 7
FOSC/4 2 TAD Result: 8
FOSC/16 2 TAD Result: 7
FOSC/64 2 TAD Result: 7
FOSC/2 4 TAD Result: 9
FOSC/8 4 TAD Result: 7
FOSC/32 4 TAD Result: 7
FRC 4 TAD Result: 7
FOSC/4 4 TAD Result: 8
FOSC/16 4 TAD Result: 7
FOSC/64 4 TAD Result: 7
FOSC/2 6 TAD Result: 8
FOSC/8 6 TAD Result: 7
FOSC/32 6 TAD Result: 7
FRC 6 TAD Result: 7
FOSC/4 6 TAD Result: 7
FOSC/16 6 TAD Result: 7
FOSC/64 6 TAD Result: 7
FOSC/2 8 TAD Result: 8
FOSC/8 8 TAD Result: 7
FOSC/32 8 TAD Result: 6
FRC 8 TAD Result: 7
FOSC/4 8 TAD Result: 7
FOSC/16 8 TAD Result: 7
FOSC/64 8 TAD Result: 7
FOSC/2 12 TAD Result: 8
FOSC/8 12 TAD Result: 8
FOSC/32 12 TAD Result: 7
FRC 12 TAD Result: 7
FOSC/4 12 TAD Result: 7
FOSC/16 12 TAD Result: 7
FOSC/64 12 TAD Result: 7
FOSC/2 16 TAD Result: 8
FOSC/8 16 TAD Result: 7
FOSC/32 16 TAD Result: 7
FRC 16 TAD Result: 7
FOSC/4 16 TAD Result: 7
FOSC/16 16 TAD Result: 7
FOSC/64 16 TAD Result: 7
FOSC/2 20 TAD Result: 8
FOSC/8 20 TAD Result: 7
FOSC/32 20 TAD Result: 7
FRC 20 TAD Result: 7
FOSC/4 20 TAD Result: 7
FOSC/16 20 TAD Result: 7
FOSC/64 20 TAD Result: 7

Input Voltage: 1.650V
FOSC/2 2 TAD Result: 497
FOSC/8 2 TAD Result: 509
FOSC/32 2 TAD Result: 511
FRC 2 TAD Result: 511
FOSC/4 2 TAD Result: 508
FOSC/16 2 TAD Result: 511
FOSC/64 2 TAD Result: 511
FOSC/2 4 TAD Result: 504
FOSC/8 4 TAD Result: 511
FOSC/32 4 TAD Result: 511
FRC 4 TAD Result: 511
FOSC/4 4 TAD Result: 512
FOSC/16 4 TAD Result: 511
FOSC/64 4 TAD Result: 511
FOSC/2 6 TAD Result: 507
FOSC/8 6 TAD Result: 511
FOSC/32 6 TAD Result: 511
FRC 6 TAD Result: 511
FOSC/4 6 TAD Result: 512
FOSC/16 6 TAD Result: 511
FOSC/64 6 TAD Result: 511
FOSC/2 8 TAD Result: 509
FOSC/8 8 TAD Result: 511
FOSC/32 8 TAD Result: 511
FRC 8 TAD Result: 511
FOSC/4 8 TAD Result: 512
FOSC/16 8 TAD Result: 511
FOSC/64 8 TAD Result: 511
FOSC/2 12 TAD Result: 511
FOSC/8 12 TAD Result: 511
FOSC/32 12 TAD Result: 511
FRC 12 TAD Result: 512
FOSC/4 12 TAD Result: 512
FOSC/16 12 TAD Result: 511
FOSC/64 12 TAD Result: 511
FOSC/2 16 TAD Result: 511
FOSC/8 16 TAD Result: 511
FOSC/32 16 TAD Result: 511
FRC 16 TAD Result: 511
FOSC/4 16 TAD Result: 512
FOSC/16 16 TAD Result: 511
FOSC/64 16 TAD Result: 511
FOSC/2 20 TAD Result: 511
FOSC/8 20 TAD Result: 511
FOSC/32 20 TAD Result: 511
FRC 20 TAD Result: 511
FOSC/4 20 TAD Result: 512
FOSC/16 20 TAD Result: 512
FOSC/64 20 TAD Result: 511

Input Voltage: 3.286V
FOSC/2 2 TAD Result: 511
FOSC/8 2 TAD Result: 1013
FOSC/32 2 TAD Result: 1019
FRC 2 TAD Result: 1019
FOSC/4 2 TAD Result: 995
FOSC/16 2 TAD Result: 1018
FOSC/64 2 TAD Result: 1019
FOSC/2 4 TAD Result: 511
FOSC/8 4 TAD Result: 1019
FOSC/32 4 TAD Result: 1019
FRC 4 TAD Result: 1020
FOSC/4 4 TAD Result: 1011
FOSC/16 4 TAD Result: 1020
FOSC/64 4 TAD Result: 1019
FOSC/2 6 TAD Result: 1006
FOSC/8 6 TAD Result: 1019
FOSC/32 6 TAD Result: 1019
FRC 6 TAD Result: 1020
FOSC/4 6 TAD Result: 1016
FOSC/16 6 TAD Result: 1019
FOSC/64 6 TAD Result: 1019
FOSC/2 8 TAD Result: 1014
FOSC/8 8 TAD Result: 1020
FOSC/32 8 TAD Result: 1019
FRC 8 TAD Result: 1019
FOSC/4 8 TAD Result: 1018
FOSC/16 8 TAD Result: 1019
FOSC/64 8 TAD Result: 1019
FOSC/2 12 TAD Result: 1017
FOSC/8 12 TAD Result: 1019
FOSC/32 12 TAD Result: 1019
FRC 12 TAD Result: 1019
FOSC/4 12 TAD Result: 1020
FOSC/16 12 TAD Result: 1019
FOSC/64 12 TAD Result: 1019
FOSC/2 16 TAD Result: 1019
FOSC/8 16 TAD Result: 1019
FOSC/32 16 TAD Result: 1019
FRC 16 TAD Result: 1019
FOSC/4 16 TAD Result: 1020
FOSC/16 16 TAD Result: 1019
FOSC/64 16 TAD Result: 1019
FOSC/2 20 TAD Result: 1020
FOSC/8 20 TAD Result: 1020
FOSC/32 20 TAD Result: 1019
FRC 20 TAD Result: 1019
FOSC/4 20 TAD Result: 1020
FOSC/16 20 TAD Result: 1020
FOSC/64 20 TAD Result: 1019

************************************************** ***
************************************************** ***
VRef is VDD/VSS measured to 3.299V
10k resistor in series with input
************************************************** ***
************************************************** ***

Input voltage 25.7mV
FOSC/2 2 TAD Result: 9
FOSC/8 2 TAD Result: 8
FOSC/32 2 TAD Result: 7
FRC 2 TAD Result: 7
FOSC/4 2 TAD Result: 8
FOSC/16 2 TAD Result: 7
FOSC/64 2 TAD Result: 7
FOSC/2 4 TAD Result: 8
FOSC/8 4 TAD Result: 7
FOSC/32 4 TAD Result: 7
FRC 4 TAD Result: 7
FOSC/4 4 TAD Result: 8
FOSC/16 4 TAD Result: 7
FOSC/64 4 TAD Result: 7
FOSC/2 6 TAD Result: 8
FOSC/8 6 TAD Result: 7
FOSC/32 6 TAD Result: 7
FRC 6 TAD Result: 7
FOSC/4 6 TAD Result: 7
FOSC/16 6 TAD Result: 7
FOSC/64 6 TAD Result: 7
FOSC/2 8 TAD Result: 8
FOSC/8 8 TAD Result: 7
FOSC/32 8 TAD Result: 7
FRC 8 TAD Result: 7
FOSC/4 8 TAD Result: 7
FOSC/16 8 TAD Result: 7
FOSC/64 8 TAD Result: 7
FOSC/2 12 TAD Result: 8
FOSC/8 12 TAD Result: 7
FOSC/32 12 TAD Result: 7
FRC 12 TAD Result: 7
FOSC/4 12 TAD Result: 7
FOSC/16 12 TAD Result: 7
FOSC/64 12 TAD Result: 7
FOSC/2 16 TAD Result: 8
FOSC/8 16 TAD Result: 8
FOSC/32 16 TAD Result: 7
FRC 16 TAD Result: 7
FOSC/4 16 TAD Result: 8
FOSC/16 16 TAD Result: 7
FOSC/64 16 TAD Result: 7
FOSC/2 20 TAD Result: 8
FOSC/8 20 TAD Result: 7
FOSC/32 20 TAD Result: 7
FRC 20 TAD Result: 7
FOSC/4 20 TAD Result: 7
FOSC/16 20 TAD Result: 7
FOSC/64 20 TAD Result: 7


Input voltage 1.650V
FOSC/2 2 TAD Result: 445
FOSC/8 2 TAD Result: 471
FOSC/32 2 TAD Result: 508
FRC 2 TAD Result: 511
FOSC/4 2 TAD Result: 448
FOSC/16 2 TAD Result: 494
FOSC/64 2 TAD Result: 511
FOSC/2 4 TAD Result: 457
FOSC/8 4 TAD Result: 493
FOSC/32 4 TAD Result: 511
FRC 4 TAD Result: 511
FOSC/4 4 TAD Result: 470
FOSC/16 4 TAD Result: 507
FOSC/64 4 TAD Result: 511
FOSC/2 6 TAD Result: 461
FOSC/8 6 TAD Result: 503
FOSC/32 6 TAD Result: 511
FRC 6 TAD Result: 511
FOSC/4 6 TAD Result: 485
FOSC/16 6 TAD Result: 510
FOSC/64 6 TAD Result: 511
FOSC/2 8 TAD Result: 472
FOSC/8 8 TAD Result: 507
FOSC/32 8 TAD Result: 511
FRC 8 TAD Result: 511
FOSC/4 8 TAD Result: 494
FOSC/16 8 TAD Result: 511
FOSC/64 8 TAD Result: 511
FOSC/2 12 TAD Result: 478
FOSC/8 12 TAD Result: 510
FOSC/32 12 TAD Result: 511
FRC 12 TAD Result: 511
FOSC/4 12 TAD Result: 510
FOSC/16 12 TAD Result: 511
FOSC/64 12 TAD Result: 511
FOSC/2 16 TAD Result: 493
FOSC/8 16 TAD Result: 511
FOSC/32 16 TAD Result: 511
FRC 16 TAD Result: 511
FOSC/4 16 TAD Result: 512
FOSC/16 16 TAD Result: 511
FOSC/64 16 TAD Result: 511
FOSC/2 20 TAD Result: 503
FOSC/8 20 TAD Result: 511
FOSC/32 20 TAD Result: 511
FRC 20 TAD Result: 511
FOSC/4 20 TAD Result: 512
FOSC/16 20 TAD Result: 511
FOSC/64 20 TAD Result: 511


Input voltage 3.286V
FOSC/2 2 TAD Result: 511
FOSC/8 2 TAD Result: 929
FOSC/32 2 TAD Result: 1012
FRC 2 TAD Result: 1019
FOSC/4 2 TAD Result: 880
FOSC/16 2 TAD Result: 981
FOSC/64 2 TAD Result: 1019
FOSC/2 4 TAD Result: 511
FOSC/8 4 TAD Result: 980
FOSC/32 4 TAD Result: 1018
FRC 4 TAD Result: 1019
FOSC/4 4 TAD Result: 926
FOSC/16 4 TAD Result: 1011
FOSC/64 4 TAD Result: 1019
FOSC/2 6 TAD Result: 511
FOSC/8 6 TAD Result: 1001
FOSC/32 6 TAD Result: 1019
FRC 6 TAD Result: 1020
FOSC/4 6 TAD Result: 959
FOSC/16 6 TAD Result: 1017
FOSC/64 6 TAD Result: 1019
FOSC/2 8 TAD Result: 511
FOSC/8 8 TAD Result: 1010
FOSC/32 8 TAD Result: 1019
FRC 8 TAD Result: 1019
FOSC/4 8 TAD Result: 979
FOSC/16 8 TAD Result: 1019
FOSC/64 8 TAD Result: 1019
FOSC/2 12 TAD Result: 511
FOSC/8 12 TAD Result: 1017
FOSC/32 12 TAD Result: 1020
FRC 12 TAD Result: 1020
FOSC/4 12 TAD Result: 1000
FOSC/16 12 TAD Result: 1019
FOSC/64 12 TAD Result: 1019
FOSC/2 16 TAD Result: 923
FOSC/8 16 TAD Result: 1019
FOSC/32 16 TAD Result: 1019
FRC 16 TAD Result: 1019
FOSC/4 16 TAD Result: 1010
FOSC/16 16 TAD Result: 1019
FOSC/64 16 TAD Result: 1019
FOSC/2 20 TAD Result: 990
FOSC/8 20 TAD Result: 1019
FOSC/32 20 TAD Result: 1019
FRC 20 TAD Result: 1020
FOSC/4 20 TAD Result: 1015
FOSC/16 20 TAD Result: 1019
FOSC/64 20 TAD Result: 1019


************************************************** ***
************************************************** ***
VRef is VDD/VSS measured to 3.299V
39k resistor in series with input
************************************************** ***
************************************************** ***

Input voltage: 25.7mV
FOSC/2 2 TAD Result: 11
FOSC/8 2 TAD Result: 7
FOSC/32 2 TAD Result: 7
FRC 2 TAD Result: 7
FOSC/4 2 TAD Result: 8
FOSC/16 2 TAD Result: 7
FOSC/64 2 TAD Result: 7
FOSC/2 4 TAD Result: 9
FOSC/8 4 TAD Result: 7
FOSC/32 4 TAD Result: 7
FRC 4 TAD Result: 7
FOSC/4 4 TAD Result: 7
FOSC/16 4 TAD Result: 7
FOSC/64 4 TAD Result: 7
FOSC/2 6 TAD Result: 8
FOSC/8 6 TAD Result: 8
FOSC/32 6 TAD Result: 7
FRC 6 TAD Result: 7
FOSC/4 6 TAD Result: 8
FOSC/16 6 TAD Result: 7
FOSC/64 6 TAD Result: 7
FOSC/2 8 TAD Result: 8
FOSC/8 8 TAD Result: 7
FOSC/32 8 TAD Result: 7
FRC 8 TAD Result: 7
FOSC/4 8 TAD Result: 8
FOSC/16 8 TAD Result: 7
FOSC/64 8 TAD Result: 7
FOSC/2 12 TAD Result: 9
FOSC/8 12 TAD Result: 7
FOSC/32 12 TAD Result: 7
FRC 12 TAD Result: 7
FOSC/4 12 TAD Result: 7
FOSC/16 12 TAD Result: 8
FOSC/64 12 TAD Result: 7
FOSC/2 16 TAD Result: 8
FOSC/8 16 TAD Result: 7
FOSC/32 16 TAD Result: 7
FRC 16 TAD Result: 7
FOSC/4 16 TAD Result: 7
FOSC/16 16 TAD Result: 7
FOSC/64 16 TAD Result: 7
FOSC/2 20 TAD Result: 8
FOSC/8 20 TAD Result: 7
FOSC/32 20 TAD Result: 7
FRC 20 TAD Result: 7
FOSC/4 20 TAD Result: 7
FOSC/16 20 TAD Result: 7
FOSC/64 20 TAD Result: 7

Input voltage: 1.650V
FOSC/2 2 TAD Result: 406
FOSC/8 2 TAD Result: 420
FOSC/32 2 TAD Result: 470
FRC 2 TAD Result: 507
FOSC/4 2 TAD Result: 407
FOSC/16 2 TAD Result: 441
FOSC/64 2 TAD Result: 496
FOSC/2 4 TAD Result: 408
FOSC/8 4 TAD Result: 439
FOSC/32 4 TAD Result: 495
FRC 4 TAD Result: 511
FOSC/4 4 TAD Result: 418
FOSC/16 4 TAD Result: 469
FOSC/64 4 TAD Result: 509
FOSC/2 6 TAD Result: 415
FOSC/8 6 TAD Result: 457
FOSC/32 6 TAD Result: 505
FRC 6 TAD Result: 511
FOSC/4 6 TAD Result: 428
FOSC/16 6 TAD Result: 485
FOSC/64 6 TAD Result: 511
FOSC/2 8 TAD Result: 418
FOSC/8 8 TAD Result: 469
FOSC/32 8 TAD Result: 509
FRC 8 TAD Result: 512
FOSC/4 8 TAD Result: 442
FOSC/16 8 TAD Result: 495
FOSC/64 8 TAD Result: 511
FOSC/2 12 TAD Result: 427
FOSC/8 12 TAD Result: 486
FOSC/32 12 TAD Result: 511
FRC 12 TAD Result: 511
FOSC/4 12 TAD Result: 456
FOSC/16 12 TAD Result: 506
FOSC/64 12 TAD Result: 511
FOSC/2 16 TAD Result: 450
FOSC/8 16 TAD Result: 496
FOSC/32 16 TAD Result: 511
FRC 16 TAD Result: 512
FOSC/4 16 TAD Result: 469
FOSC/16 16 TAD Result: 509
FOSC/64 16 TAD Result: 511
FOSC/2 20 TAD Result: 453
FOSC/8 20 TAD Result: 502
FOSC/32 20 TAD Result: 511
FRC 20 TAD Result: 511
FOSC/4 20 TAD Result: 478
FOSC/16 20 TAD Result: 510
FOSC/64 20 TAD Result: 511

Input voltage: 3.286V
FOSC/2 2 TAD Result: 768
FOSC/8 2 TAD Result: 826
FOSC/32 2 TAD Result: 931
FRC 2 TAD Result: 1010
FOSC/4 2 TAD Result: 801
FOSC/16 2 TAD Result: 873
FOSC/64 2 TAD Result: 986
FOSC/2 4 TAD Result: 511
FOSC/8 4 TAD Result: 867
FOSC/32 4 TAD Result: 984
FRC 4 TAD Result: 1019
FOSC/4 4 TAD Result: 824
FOSC/16 4 TAD Result: 929
FOSC/64 4 TAD Result: 1014
FOSC/2 6 TAD Result: 511
FOSC/8 6 TAD Result: 904
FOSC/32 6 TAD Result: 1006
FRC 6 TAD Result: 1019
FOSC/4 6 TAD Result: 847
FOSC/16 6 TAD Result: 963
FOSC/64 6 TAD Result: 1019
FOSC/2 8 TAD Result: 511
FOSC/8 8 TAD Result: 930
FOSC/32 8 TAD Result: 1014
FRC 8 TAD Result: 1019
FOSC/4 8 TAD Result: 874
FOSC/16 8 TAD Result: 985
FOSC/64 8 TAD Result: 1019
FOSC/2 12 TAD Result: 511
FOSC/8 12 TAD Result: 963
FOSC/32 12 TAD Result: 1019
FRC 12 TAD Result: 1019
FOSC/4 12 TAD Result: 901
FOSC/16 12 TAD Result: 1006
FOSC/64 12 TAD Result: 1019
FOSC/2 16 TAD Result: 847
FOSC/8 16 TAD Result: 985
FOSC/32 16 TAD Result: 1019
FRC 16 TAD Result: 1019
FOSC/4 16 TAD Result: 927
FOSC/16 16 TAD Result: 1015
FOSC/64 16 TAD Result: 1019
FOSC/2 20 TAD Result: 847
FOSC/8 20 TAD Result: 999
FOSC/32 20 TAD Result: 1019
FRC 20 TAD Result: 1019
FOSC/4 20 TAD Result: 950
FOSC/16 20 TAD Result: 1017
FOSC/64 20 TAD Result: 1019

I haven't analyzed any of it yet and I don't have any more time just now but I wanted to post the results. If anyone wants to graph the results in Excel or whatever to better see what's going on, please feel free to do so.

/Henrik.

[tumbleweed]

You certainly coverted all the bases! Good job.

Instead of bothering to graph all the data and try and make sense of it, Let's start with this:

The 25K20 has a TAD spec of 0.7us min.

At 64MHz, that means all ADCS settings except for FOSC/64 and FRC violate minimum times (Table 19-1).
Using the FOSC/64 setting gives a TAD of 1us

Computing Tc from equation 19-1 for each of the three source resistances gives:
1K = 0.27us
10K = 1.2us
39K = 4.2us

Computing TACQ = Tamp + Tc + Tcoff, given Tamp = 5us and Tcoff = 0 (25degC) produces:
1K = 5.27us
10K = 6.2us
39K = 9.2us

So, since TAD = 1us, TACQ must be a min of ~6-9 TAD for the three different resistances.

Removing all of the measurements that don't fit the above you get:


************************************************** ***
1k resistor in series with input (TACQ=5.27us)
************************************************** ***
Input voltage 25.7mV
FRC 6 TAD Result: 7
FOSC/64 6 TAD Result: 7
FRC 8 TAD Result: 7
FOSC/64 8 TAD Result: 7
FRC 12 TAD Result: 7
FOSC/64 12 TAD Result: 7
FRC 16 TAD Result: 7
FOSC/64 16 TAD Result: 7
FRC 20 TAD Result: 7
FOSC/64 20 TAD Result: 7

Input Voltage: 1.650V
FRC 6 TAD Result: 511
FOSC/64 6 TAD Result: 511
FRC 8 TAD Result: 511
FOSC/64 8 TAD Result: 511
FRC 12 TAD Result: 512
FOSC/64 12 TAD Result: 511
FRC 16 TAD Result: 511
FOSC/64 16 TAD Result: 511
FRC 20 TAD Result: 511
FOSC/64 20 TAD Result: 511

Input Voltage: 3.286V
FRC 6 TAD Result: 1020
FOSC/64 6 TAD Result: 1019
FRC 8 TAD Result: 1019
FOSC/64 8 TAD Result: 1019
FRC 12 TAD Result: 1019
FOSC/64 12 TAD Result: 1019
FRC 16 TAD Result: 1019
FOSC/64 16 TAD Result: 1019
FRC 20 TAD Result: 1019
FOSC/64 20 TAD Result: 1019


************************************************** ***
10k resistor in series with input (TACQ=6.2us)
************************************************** ***

Input voltage 25.7mV
FRC 6 TAD Result: 7
FOSC/64 6 TAD Result: 7
FRC 8 TAD Result: 7
FOSC/64 8 TAD Result: 7
FRC 12 TAD Result: 7
FOSC/64 12 TAD Result: 7
FRC 16 TAD Result: 7
FOSC/64 16 TAD Result: 7
FRC 20 TAD Result: 7
FOSC/64 20 TAD Result: 7


Input voltage 1.650V
FRC 6 TAD Result: 511
FOSC/64 6 TAD Result: 511
FRC 8 TAD Result: 511
FOSC/64 8 TAD Result: 511
FRC 12 TAD Result: 511
FOSC/64 12 TAD Result: 511
FRC 16 TAD Result: 511
FOSC/64 16 TAD Result: 511
FRC 20 TAD Result: 511
FOSC/64 20 TAD Result: 511


Input voltage 3.286V
FRC 6 TAD Result: 1020
FOSC/64 6 TAD Result: 1019
FRC 8 TAD Result: 1019
FOSC/64 8 TAD Result: 1019
FRC 12 TAD Result: 1020
FOSC/64 12 TAD Result: 1019
FRC 16 TAD Result: 1019
FOSC/64 16 TAD Result: 1019
FRC 20 TAD Result: 1020
FOSC/64 20 TAD Result: 1019


************************************************** ***
39k resistor in series with input (TACQ=9.2us)
************************************************** ***

Input voltage: 25.7mV
FRC 6 TAD Result: 7
FOSC/64 6 TAD Result: 7
FRC 8 TAD Result: 7
FOSC/64 8 TAD Result: 7
FRC 12 TAD Result: 7
FOSC/64 12 TAD Result: 7
FRC 16 TAD Result: 7
FOSC/64 16 TAD Result: 7
FRC 20 TAD Result: 7
FOSC/64 20 TAD Result: 7

Input voltage: 1.650V
FRC 6 TAD Result: 511
FOSC/64 6 TAD Result: 511
FRC 8 TAD Result: 512
FOSC/64 8 TAD Result: 511
FRC 12 TAD Result: 511
FOSC/64 12 TAD Result: 511
FRC 16 TAD Result: 512
FOSC/64 16 TAD Result: 511
FRC 20 TAD Result: 511
FOSC/64 20 TAD Result: 511

Input voltage: 3.286V
FRC 6 TAD Result: 1019
FOSC/64 6 TAD Result: 1019
FRC 8 TAD Result: 1019
FOSC/64 8 TAD Result: 1019
FRC 12 TAD Result: 1019
FOSC/64 12 TAD Result: 1019
FRC 16 TAD Result: 1019
FOSC/64 16 TAD Result: 1019
FRC 20 TAD Result: 1019
FOSC/64 20 TAD Result: 1019

Pretty consistent. It shows that waiting longer than the minimum doesn't get you anything, but it also doesn't change anything either.
The interesting thing is as you look through the raw data you can see the effect of not using the proper TAD setting for the clock freq you're using, or not waiting/setting TACQ to at least the minimum time.

[richard]

it might be interesting to connect the "dummy conversion" pin to ground and then in another pass vcc , and give it a long acquisition and then take your reading . I would expect the real effect of a too short acquisition time would become more evident (ie the previous reading sample cap charge will have a effect on the current reading)

[tumbleweed]

I wondered about that too. As you say, connecting it to a floating input would probably just leave the charge on Chold since all you have is the leakage current to discharge it.

I noticed in figure 19-4 that they show a 2TAD "discharge" time between measurements, and they say this:

19.2.3 DISCHARGE
The discharge phase is used to initialize the value of
the capacitor array. The array is discharged after every
sample. This feature helps to optimize the unity-gain
amplifier, as the circuit always needs to charge the
capacitor array, rather than charge/discharge based on
previous measure values.


The PIC18 reference manual has the same sort of text, but is also has the statement
The charge holding capacitor (CHOLD) is not discharged after each conversionso it sounds like they're talking about something to do with the internal ADC structure and not Chold.

It WOULD be interesting to see the same measurements without any averaging and with alternating input voltages for the in between "idle" cycles.

[HenrikOlsson]

Hi guys,
OK, doing a dummy conversion on a floating input wasn't the greatest idea....

I took out the averaging and set up my arb gen to generate 3.3Vpp noise and injected that into AN1 (the dummy channel).



25.6mV input, 10k source resistor, 3.3V noise on "dummy channel":
FOSC/2 2 TAD Result: 512
FOSC/8 2 TAD Result: 120
FOSC/32 2 TAD Result: 23
FRC 2 TAD Result: 6
FOSC/4 2 TAD Result: 184
FOSC/16 2 TAD Result: 54
FOSC/64 2 TAD Result: 7
FOSC/2 4 TAD Result: 190
FOSC/8 4 TAD Result: 59
FOSC/32 4 TAD Result: 14
FRC 4 TAD Result: 13
FOSC/4 4 TAD Result: 117
FOSC/16 4 TAD Result: 21
FOSC/64 4 TAD Result: 14
FOSC/2 6 TAD Result: 127
FOSC/8 6 TAD Result: 32
FOSC/32 6 TAD Result: 11
FRC 6 TAD Result: 11
FOSC/4 6 TAD Result: 74
FOSC/16 6 TAD Result: 7
FOSC/64 6 TAD Result: 10
FOSC/2 8 TAD Result: 512
FOSC/8 8 TAD Result: 15
FOSC/32 8 TAD Result: 15
FRC 8 TAD Result: 12
FOSC/4 8 TAD Result: 56
FOSC/16 8 TAD Result: 10
FOSC/64 8 TAD Result: 6
FOSC/2 12 TAD Result: 512
FOSC/8 12 TAD Result: 11
FOSC/32 12 TAD Result: 16
FRC 12 TAD Result: 13
FOSC/4 12 TAD Result: 31
FOSC/16 12 TAD Result: 11
FOSC/64 12 TAD Result: 14
FOSC/2 16 TAD Result: 512
FOSC/8 16 TAD Result: 13
FOSC/32 16 TAD Result: 11
FRC 16 TAD Result: 11
FOSC/4 16 TAD Result: 18
FOSC/16 16 TAD Result: 11
FOSC/64 16 TAD Result: 9
FOSC/2 20 TAD Result: 49
FOSC/8 20 TAD Result: 15
FOSC/32 20 TAD Result: 10
FRC 20 TAD Result: 8
FOSC/4 20 TAD Result: 14
FOSC/16 20 TAD Result: 10
FOSC/64 20 TAD Result: 13

***************************************

3.285V, 10k source resistor 3.3V noise on "dummy channel":

FOSC/2 2 TAD Result: 511
FOSC/8 2 TAD Result: 861
FOSC/32 2 TAD Result: 1008
FRC 2 TAD Result: 1022
FOSC/4 2 TAD Result: 812
FOSC/16 2 TAD Result: 963
FOSC/64 2 TAD Result: 1014
FOSC/2 4 TAD Result: 511
FOSC/8 4 TAD Result: 964
FOSC/32 4 TAD Result: 1021
FRC 4 TAD Result: 1023
FOSC/4 4 TAD Result: 888
FOSC/16 4 TAD Result: 1004
FOSC/64 4 TAD Result: 1015
FOSC/2 6 TAD Result: 511
FOSC/8 6 TAD Result: 994
FOSC/32 6 TAD Result: 1019
FRC 6 TAD Result: 1023
FOSC/4 6 TAD Result: 911
FOSC/16 6 TAD Result: 1018
FOSC/64 6 TAD Result: 1019
FOSC/2 8 TAD Result: 511
FOSC/8 8 TAD Result: 1006
FOSC/32 8 TAD Result: 1019
FRC 8 TAD Result: 1021
FOSC/4 8 TAD Result: 964
FOSC/16 8 TAD Result: 1022
FOSC/64 8 TAD Result: 1016
FOSC/2 12 TAD Result: 511
FOSC/8 12 TAD Result: 1015
FOSC/32 12 TAD Result: 1019
FRC 12 TAD Result: 1019
FOSC/4 12 TAD Result: 991
FOSC/16 12 TAD Result: 1023
FOSC/64 12 TAD Result: 1015
FOSC/2 16 TAD Result: 511
FOSC/8 16 TAD Result: 1014
FOSC/32 16 TAD Result: 1018
FRC 16 TAD Result: 1018
FOSC/4 16 TAD Result: 1005
FOSC/16 16 TAD Result: 1016
FOSC/64 16 TAD Result: 1023
FOSC/2 20 TAD Result: 983
FOSC/8 20 TAD Result: 1020
FOSC/32 20 TAD Result: 1017
FRC 20 TAD Result: 1023
FOSC/4 20 TAD Result: 1010
FOSC/16 20 TAD Result: 1017
FOSC/64 20 TAD Result: 1019

Then I connected the dummy channel to GND:


25.6mV input, 10k source resistor, "dummy channel" connected to GND:

FOSC/2 2 TAD Result: 8
FOSC/8 2 TAD Result: 6
FOSC/32 2 TAD Result: 8
FRC 2 TAD Result: 7
FOSC/4 2 TAD Result: 6
FOSC/16 2 TAD Result: 7
FOSC/64 2 TAD Result: 7
FOSC/2 4 TAD Result: 8
FOSC/8 4 TAD Result: 7
FOSC/32 4 TAD Result: 8
FRC 4 TAD Result: 7
FOSC/4 4 TAD Result: 6
FOSC/16 4 TAD Result: 7
FOSC/64 4 TAD Result: 7
FOSC/2 6 TAD Result: 8
FOSC/8 6 TAD Result: 7
FOSC/32 6 TAD Result: 7
FRC 6 TAD Result: 7
FOSC/4 6 TAD Result: 6
FOSC/16 6 TAD Result: 7
FOSC/64 6 TAD Result: 7
FOSC/2 8 TAD Result: 8
FOSC/8 8 TAD Result: 7
FOSC/32 8 TAD Result: 7
FRC 8 TAD Result: 7
FOSC/4 8 TAD Result: 7
FOSC/16 8 TAD Result: 8
FOSC/64 8 TAD Result: 7
FOSC/2 12 TAD Result: 8
FOSC/8 12 TAD Result: 7
FOSC/32 12 TAD Result: 7
FRC 12 TAD Result: 7
FOSC/4 12 TAD Result: 8
FOSC/16 12 TAD Result: 7
FOSC/64 12 TAD Result: 7
FOSC/2 16 TAD Result: 8
FOSC/8 16 TAD Result: 7
FOSC/32 16 TAD Result: 7
FRC 16 TAD Result: 7
FOSC/4 16 TAD Result: 8
FOSC/16 16 TAD Result: 7
FOSC/64 16 TAD Result: 7
FOSC/2 20 TAD Result: 8
FOSC/8 20 TAD Result: 7
FOSC/32 20 TAD Result: 7
FRC 20 TAD Result: 7
FOSC/4 20 TAD Result: 8
FOSC/16 20 TAD Result: 8
FOSC/64 20 TAD Result: 7

************************************************** *****

3.285V input, 10k source resistor, "dummy" channel connected to GND:

FOSC/2 2 TAD Result: 511
FOSC/8 2 TAD Result: 714
FOSC/32 2 TAD Result: 996
FRC 2 TAD Result: 1019
FOSC/4 2 TAD Result: 610
FOSC/16 2 TAD Result: 892
FOSC/64 2 TAD Result: 1018
FOSC/2 4 TAD Result: 511
FOSC/8 4 TAD Result: 862
FOSC/32 4 TAD Result: 1018
FRC 4 TAD Result: 1018
FOSC/4 4 TAD Result: 687
FOSC/16 4 TAD Result: 991
FOSC/64 4 TAD Result: 1019
FOSC/2 6 TAD Result: 511
FOSC/8 6 TAD Result: 955
FOSC/32 6 TAD Result: 1020
FRC 6 TAD Result: 1019
FOSC/4 6 TAD Result: 761
FOSC/16 6 TAD Result: 1010
FOSC/64 6 TAD Result: 1019
FOSC/2 8 TAD Result: 511
FOSC/8 8 TAD Result: 983
FOSC/32 8 TAD Result: 1020
FRC 8 TAD Result: 1018
FOSC/4 8 TAD Result: 836
FOSC/16 8 TAD Result: 1018
FOSC/64 8 TAD Result: 1019
FOSC/2 12 TAD Result: 511
FOSC/8 12 TAD Result: 1010
FOSC/32 12 TAD Result: 1020
FRC 12 TAD Result: 1019
FOSC/4 12 TAD Result: 944
FOSC/16 12 TAD Result: 1019
FOSC/64 12 TAD Result: 1020
FOSC/2 16 TAD Result: 511
FOSC/8 16 TAD Result: 1017
FOSC/32 16 TAD Result: 1019
FRC 16 TAD Result: 1018
FOSC/4 16 TAD Result: 981
FOSC/16 16 TAD Result: 1020
FOSC/64 16 TAD Result: 1019
FOSC/2 20 TAD Result: 511
FOSC/8 20 TAD Result: 1017
FOSC/32 20 TAD Result: 1019
FRC 20 TAD Result: 1019
FOSC/4 20 TAD Result: 998
FOSC/16 20 TAD Result: 1021
FOSC/64 20 TAD Result: 1020

And finally, dummy channel to Vdd (3.3V):

25.6mV input, 10k source resistor, "dummy channel" connected to Vdd:

FOSC/2 2 TAD Result: 512
FOSC/8 2 TAD Result: 344
FOSC/32 2 TAD Result: 29
FRC 2 TAD Result: 8
FOSC/4 2 TAD Result: 432
FOSC/16 2 TAD Result: 192
FOSC/64 2 TAD Result: 9
FOSC/2 4 TAD Result: 512
FOSC/8 4 TAD Result: 262
FOSC/32 4 TAD Result: 9
FRC 4 TAD Result: 8
FOSC/4 4 TAD Result: 383
FOSC/16 4 TAD Result: 46
FOSC/64 4 TAD Result: 8
FOSC/2 6 TAD Result: 512
FOSC/8 6 TAD Result: 180
FOSC/32 6 TAD Result: 8
FRC 6 TAD Result: 7
FOSC/4 6 TAD Result: 350
FOSC/16 6 TAD Result: 17
FOSC/64 6 TAD Result: 7
FOSC/2 8 TAD Result: 512
FOSC/8 8 TAD Result: 99
FOSC/32 8 TAD Result: 8
FRC 8 TAD Result: 7
FOSC/4 8 TAD Result: 312
FOSC/16 8 TAD Result: 10
FOSC/64 8 TAD Result: 7
FOSC/2 12 TAD Result: 512
FOSC/8 12 TAD Result: 25
FOSC/32 12 TAD Result: 7
FRC 12 TAD Result: 8
FOSC/4 12 TAD Result: 229
FOSC/16 12 TAD Result: 8
FOSC/64 12 TAD Result: 7
FOSC/2 16 TAD Result: 512
FOSC/8 16 TAD Result: 12
FOSC/32 16 TAD Result: 7
FRC 16 TAD Result: 8
FOSC/4 16 TAD Result: 147
FOSC/16 16 TAD Result: 7
FOSC/64 16 TAD Result: 7
FOSC/2 20 TAD Result: 512
FOSC/8 20 TAD Result: 9
FOSC/32 20 TAD Result: 7
FRC 20 TAD Result: 8
FOSC/4 20 TAD Result: 66
FOSC/16 20 TAD Result: 7
FOSC/64 20 TAD Result: 7

*********************************************


3.285V input, 10k source resistor, "dummy channel" connected to Vdd:

FOSC/2 2 TAD Result: 1022
FOSC/8 2 TAD Result: 1021
FOSC/32 2 TAD Result: 1020
FRC 2 TAD Result: 1020
FOSC/4 2 TAD Result: 1022
FOSC/16 2 TAD Result: 1020
FOSC/64 2 TAD Result: 1019
FOSC/2 4 TAD Result: 1022
FOSC/8 4 TAD Result: 1019
FOSC/32 4 TAD Result: 1019
FRC 4 TAD Result: 1019
FOSC/4 4 TAD Result: 1019
FOSC/16 4 TAD Result: 1021
FOSC/64 4 TAD Result: 1020
FOSC/2 6 TAD Result: 1023
FOSC/8 6 TAD Result: 1021
FOSC/32 6 TAD Result: 1019
FRC 6 TAD Result: 1020
FOSC/4 6 TAD Result: 1020
FOSC/16 6 TAD Result: 1020
FOSC/64 6 TAD Result: 1019
FOSC/2 8 TAD Result: 1022
FOSC/8 8 TAD Result: 1020
FOSC/32 8 TAD Result: 1019
FRC 8 TAD Result: 1019
FOSC/4 8 TAD Result: 1020
FOSC/16 8 TAD Result: 1021
FOSC/64 8 TAD Result: 1019
FOSC/2 12 TAD Result: 1022
FOSC/8 12 TAD Result: 1020
FOSC/32 12 TAD Result: 1019
FRC 12 TAD Result: 1020
FOSC/4 12 TAD Result: 1020
FOSC/16 12 TAD Result: 1021
FOSC/64 12 TAD Result: 1020
FOSC/2 16 TAD Result: 1021
FOSC/8 16 TAD Result: 1021
FOSC/32 16 TAD Result: 1019
FRC 16 TAD Result: 1021
FOSC/4 16 TAD Result: 1021
FOSC/16 16 TAD Result: 1019
FOSC/64 16 TAD Result: 1019
FOSC/2 20 TAD Result: 1020
FOSC/8 20 TAD Result: 1021
FOSC/32 20 TAD Result: 1019
FRC 20 TAD Result: 1021
FOSC/4 20 TAD Result: 1018
FOSC/16 20 TAD Result: 1019
FOSC/64 20 TAD Result: 1020

Henrik.

[tumbleweed]

Thanks Henrik. So again, removing the ones where the ADCS setting is invalid/too fast gives:


25.6mV input, 10k source resistor, 3.3V noise on "dummy channel":

FRC 2 TAD Result: 6
FOSC/64 2 TAD Result: 7
FRC 4 TAD Result: 13
FOSC/64 4 TAD Result: 14
FRC 6 TAD Result: 11
FOSC/64 6 TAD Result: 10
FRC 8 TAD Result: 12
FOSC/64 8 TAD Result: 6
FRC 12 TAD Result: 13
FOSC/64 12 TAD Result: 14
FRC 16 TAD Result: 11
FOSC/64 16 TAD Result: 9
FRC 20 TAD Result: 8
FOSC/64 20 TAD Result: 13

*********************************************

3.285V, 10k source resistor 3.3V noise on "dummy channel":

FRC 2 TAD Result: 1022
FOSC/64 2 TAD Result: 1014
FRC 4 TAD Result: 1023
FOSC/64 4 TAD Result: 1015
FRC 6 TAD Result: 1023
FOSC/64 6 TAD Result: 1019
FRC 8 TAD Result: 1021
FOSC/64 8 TAD Result: 1016
FRC 12 TAD Result: 1019
FOSC/64 12 TAD Result: 1015
FRC 16 TAD Result: 1018
FOSC/64 16 TAD Result: 1023
FRC 20 TAD Result: 1023
FOSC/64 20 TAD Result: 1019

*********************************************

25.6mV input, 10k source resistor, "dummy channel" connected to GND:

FRC 2 TAD Result: 7
FOSC/64 2 TAD Result: 7
FRC 4 TAD Result: 7
FOSC/64 4 TAD Result: 7
FRC 6 TAD Result: 7
FOSC/64 6 TAD Result: 7
FRC 8 TAD Result: 7
FOSC/64 8 TAD Result: 7
FRC 12 TAD Result: 7
FOSC/64 12 TAD Result: 7
FRC 16 TAD Result: 7
FOSC/64 16 TAD Result: 7
FRC 20 TAD Result: 7
FOSC/64 20 TAD Result: 7

*********************************************

3.285V input, 10k source resistor, "dummy" channel connected to GND:

FRC 2 TAD Result: 1019
FOSC/64 2 TAD Result: 1018
FRC 4 TAD Result: 1018
FOSC/64 4 TAD Result: 1019
FRC 6 TAD Result: 1019
FOSC/64 6 TAD Result: 1019
FRC 8 TAD Result: 1018
FOSC/64 8 TAD Result: 1019
FRC 12 TAD Result: 1019
FOSC/64 12 TAD Result: 1020
FRC 16 TAD Result: 1018
FOSC/64 16 TAD Result: 1019
FRC 20 TAD Result: 1019
FOSC/64 20 TAD Result: 1020

*********************************************

25.6mV input, 10k source resistor, "dummy channel" connected to Vdd:

FRC 2 TAD Result: 8
FOSC/64 2 TAD Result: 9
FRC 4 TAD Result: 8
FOSC/64 4 TAD Result: 8
FRC 6 TAD Result: 7
FOSC/64 6 TAD Result: 7
FRC 8 TAD Result: 7
FOSC/64 8 TAD Result: 7
FRC 12 TAD Result: 8
FOSC/64 12 TAD Result: 7
FRC 16 TAD Result: 8
FOSC/64 16 TAD Result: 7
FRC 20 TAD Result: 8
FOSC/64 20 TAD Result: 7

*********************************************

3.285V input, 10k source resistor, "dummy channel" connected to Vdd:

FRC 2 TAD Result: 1020
FOSC/64 2 TAD Result: 1019
FRC 4 TAD Result: 1019
FOSC/64 4 TAD Result: 1020
FRC 6 TAD Result: 1020
FOSC/64 6 TAD Result: 1019
FRC 8 TAD Result: 1019
FOSC/64 8 TAD Result: 1019
FRC 12 TAD Result: 1020
FOSC/64 12 TAD Result: 1020
FRC 16 TAD Result: 1021
FOSC/64 16 TAD Result: 1019
FRC 20 TAD Result: 1021
FOSC/64 20 TAD Result: 1020


Probably the ones of most interest are where Chold has to swing the full range between samples:


3.285V input, 10k source resistor, "dummy" channel connected to GND:

FRC 2 TAD Result: 1019
FOSC/64 2 TAD Result: 1018
FRC 4 TAD Result: 1018
FOSC/64 4 TAD Result: 1019
FRC 6 TAD Result: 1019
FOSC/64 6 TAD Result: 1019
FRC 8 TAD Result: 1018
FOSC/64 8 TAD Result: 1019
FRC 12 TAD Result: 1019
FOSC/64 12 TAD Result: 1020
FRC 16 TAD Result: 1018
FOSC/64 16 TAD Result: 1019
FRC 20 TAD Result: 1019
FOSC/64 20 TAD Result: 1020

*********************************************

25.6mV input, 10k source resistor, "dummy channel" connected to Vdd:

FRC 2 TAD Result: 8
FOSC/64 2 TAD Result: 9
FRC 4 TAD Result: 8
FOSC/64 4 TAD Result: 8
FRC 6 TAD Result: 7
FOSC/64 6 TAD Result: 7
FRC 8 TAD Result: 7
FOSC/64 8 TAD Result: 7
FRC 12 TAD Result: 8
FOSC/64 12 TAD Result: 7
FRC 16 TAD Result: 8
FOSC/64 16 TAD Result: 7
FRC 20 TAD Result: 8
FOSC/64 20 TAD Result: 7


For no averaging those numbers seem pretty reasonable.