LTC2400 SPI interface

[HenrikOlsson]

Hi,
I'm sure you have your reason but can you explain why you need 24bits?

I mean, 100bar across 24bits gives you a resolution of 0.000006bar. Does a transducer capable of measuring 100bar really resolve to that kind of precision?

If you already have your 4-20mA converted to 0-5V (and not 1-5V) you'd get ~0.1bar resolution with the built in 10bit ADC. You could do some oversampling to "emulate" a 12bit ADC giving you ~0.025bar resolution.

Just some ideas.

/Henrik.

[PUGALENTHI.P]

Hi,
I'm sure you have your reason but can you explain why you need 24bits?

I mean, 100bar across 24bits gives you a resolution of 0.000006bar. Does a transducer capable of measuring 100bar really resolve to that kind of precision?

If you already have your 4-20mA converted to 0-5V (and not 1-5V) you'd get ~0.1bar resolution with the built in 10bit ADC. You could do some oversampling to "emulate" a 12bit ADC giving you ~0.025bar resolution.

Just some ideas.

/Henrik.

THANKS FOR YOUR SUGGESTION MR.HENRIK

Actually as you said the 10 bit adc gives a 0.1 resolution on 0~100bar range which is as equal to a "primary master" and thats fine,but i want to use the 24bit adc because i want learn how to communicate a external adc through "spi" to the pic and particularly the 24bit adc is because thats the one i have in my hand to give a try.can you please explain me about the "oversampling to "emulate" a 12bit ADC to give a ~0.025bar resolution"
THANKS A LOT FOR SPENDING YOUR VALUABLE TIME FOR ME.

[HenrikOlsson]

Hi,
I'd say learning qualifies as a (very) valid reason ;-) By all means try using the 24bit ADC but perhaps you should start with interfacing to something "easier" like a EEPROM to begin with so you learn the basics about how the SPI interface works. I haven't looked at the datasheet for your ADC though, perhaps it's pretty straight forward.

As for the oversampling, all you really do is take a bunch of readings and then average them. If you take 16 readings with your 10bits ADC, add them all together and divide the result by 4 you'll get a value ranging from 0-4096.

You can read a bit more about here. Note, you can not use that code with Oshonsoft, the theory behind it obviously applies to any language/compiler though.

Good luck!
/Henrik.

[PUGALENTHI.P]

Hi,
I'd say learning qualifies as a (very) valid reason ;-) By all means try using the 24bit ADC but perhaps you should start with interfacing to something "easier" like a EEPROM to begin with so you learn the basics about how the SPI interface works. I haven't looked at the datasheet for your ADC though, perhaps it's pretty straight forward.

As for the oversampling, all you really do is take a bunch of readings and then average them. If you take 16 readings with your 10bits ADC, add them all together and divide the result by 4 you'll get a value ranging from 0-4096.

You can read a bit more about here. Note, you can not use that code with Oshonsoft, the theory behind it obviously applies to any language/compiler though.

Good luck!
/Henrik.

hai Henrik,

Thanks for your suggestions.is that the communication between the eeprom and pic is very easy?what i thought is am trying to make use of the spi communication only and
no matter whether it is ADC or EEPROM (am i right).

when i used the internal adc for example
i injected 1.234v where the range is 0~5v and the related 10bit adc value is 252.7232 but the pic reads only 252 i took a bunch of readings and calculated the mean value (asume the value is same 252)
then i multiplied the value by 10000 then the value is 2520000 again divide it with 2048 and the answer is 1230.46 where the pic read only 1230
that means we get 1.230
as you said (252*16)/4=1008 /819.2=1.230

what is the difference between these two.

thanks a lot for clarifying my doubts

PUGAL

[HenrikOlsson]

Hi,
It's because the reading you get from the ADC most likely won't be stable at 252 across several consecutive samples, it will probably "tip over" towards 253 a couple of times.
Let's say you get the following values:
252, 253, 252, 253, 253, 252, 252, 253, 251, 253, 252, 253, 253, 252, 252, 252

The sum is 4039. Averaging them across 16 samples results in 252.4375 but you'll only "see" 252 which is ~0.29% off. Averaging them across 4 instead gives you 1009.75, you'll only "see" 1009 which is ~0.19% off.

Had the result been 1040 instead, averaging them across 16 samples would give you the same result, ~0.29% off while the oversampling method would only be 0.009% off.

Try it out and make sure you read thru the page I linked to, also look up the term oversampling. Here's a quote from Wikipedia:

In practice, oversampling is implemented in order to achieve cheaper higher-resolution A/D and D/A conversion. For instance, to implement a 24-bit converter, it is sufficient to use a 20-bit converter that can run at 256 times the target sampling rate. Averaging a group of 256 consecutive 20-bit samples adds 4 bits to the resolution of the average, producing a single sample with 24-bit resolution.

/Henrik.

[PUGALENTHI.P]

Hi,
It's because the reading you get from the ADC most likely won't be stable at 252 across several consecutive samples, it will probably "tip over" towards 253 a couple of times.
Let's say you get the following values:
252, 253, 252, 253, 253, 252, 252, 253, 251, 253, 252, 253, 253, 252, 252, 252

The sum is 4039. Averaging them across 16 samples results in 252.4375 but you'll only "see" 252 which is ~0.29% off. Averaging them across 4 instead gives you 1009.75, you'll only "see" 1009 which is ~0.19% off.

Had the result been 1040 instead, averaging them across 16 samples would give you the same result, ~0.29% off while the oversampling method would only be 0.009% off.

Try it out and make sure you read thru the page I linked to, also look up the term oversampling. Here's a quote from Wikipedia:

/Henrik.

HAI HENRIK,
thanks for your support.as you said i tried the oversampling method and it gave me a better result than the previous method i used.but i have a transduser which range from 0~60000 psi on 4~20 mA,i used a 250 ohm resistor and i got 1~5v difference.

in this case do i need to use a external ADC more than 14bit resolution or can able to use the oversampling method on internal ADC by sampling 4096 times and divide by 64 to get a 16bit resolution.is it possible? or do we have any other ways to achive it.
THANKS FOR YOUR KIND SUPPORT,
PUGAL.

[HenrikOlsson]

Hello,
You still haven't said what kind of resolution you actually NEED, just that you wanted to use 24bits becaue that is what you had. I also asked you earlier if you had the 4-20mA signal converted to 0-5 or 1-5V. With 1-5V it means that you are basically "wasting" 20% of the ADC range.

Now, with a 1-5V signal I'd configure the ADC to use the external Vref inputs instead of the VDD and VSS. That way, if you feed a stable 1V signal into Vref- and a stable 5V into Vref+ you'll use all the available resolution and you'll get 0 for 1V and 1023 for 5V. That way you get a basic resolution of 0.1bar as I outlined earlier and can increase that by oversampling.

If you try the aproach of 1024 samples remember that you need to add them into a 32bit variable as the result may not fit in a 16bit variable.

/Henrik.