MPXA4115A picbasic code

[bitmaniac]

Hi! Alain

Sorry my friend but I hav not completely followed your idea.
Do you mean sth like the attachment one?

(ie: I have to 'expand' the range 200 , 50 and 32 'between' 0 - 3000 m I want to calculate?)

I have to follow your idea, so help me out , as I can see you have a point here!

[Acetronics2]

Hi, Alexandros

As a good drawing is worth ...

see this XL sheet ...

shows how things go on ...

And Overall shows a differential amplifier is needed between Sensor and ADC, to get a good resolution ( see the very, very limited ADC result range ... if direct connexion ).

Note @ sea level the 300 m altitude is less than .1 v change @ sensor Output. Not that good for noise impact on signal ...

also note the mean value for altitude meters vs pressure change is precise enough for our use and so, we can consider it only depends on reference pressure @ ground level ... or even, that variation is linear ...

From the datasheets, It looks the ADC ref ( V+ref - V-ref ) must be, at least 3 v ... no hope from the Vref- side.


Another way could be to use the MS 5534 Intersema sensor, a NUMERIC output sensor whose resolution is ... 15 bits. Paying attention to noise would be cancelled too ...

Note particular calibration data from factory is included in the chip memory ... so, no further calibration would be needed ...

For the moment, I let you play with the calc sheet ...

Alain

[ScaleRobotics]

You may want to use Darrel's A/D Oversampling routine to get some more precision. It allows you to go all the way up to 16 bit, but you may want to stick to 14 bit. That would get you accuracy of about half a meter.


http://sites.picbasic.net/component/...,70/pageid,73/

[bitmaniac]

So... Acetronics

*your XL was perfect for make me understand many things!


After a lot of experiment and according to your very usefull XL I finally decide to two different senarios

Acording to your XL (and 10 , 12 and 14 bit results attached to xl file below)
the 'resolution' is at 10bit -> around 8m, 12bit -> around 2m and 14bit -> around <1m

So I am thinking of
1st. just use simple pic 10 bit adc and Darrel's oversampling method (thank you scalerobotics for the link)

2nd use an LM385 for example dual amp and take just 'a window' od the mesurements of MPX4115 between 0-2300 meters

3rd compine LM385 and MAX187 (12bit) to get better resoloution

My onle concern is that as I want to keep pcb at minimum of costs / and parts count if Darrel oversampling method works , it would be Great!

I let you know.

I am in the progress of schematic and pcb design BUT I have to decide the method first!!

[ScaleRobotics]

Acording to your XL (and 10 , 12 and 14 bit results attached to xl file below)
the 'resolution' is at 10bit -> around 8m, 12bit -> around 2m and 14bit -> around <1m

Not that it matters much, but in your graph, you are breaking it down to measurements every 0.10 mBar (not A/D conversion resolution). For every 0.10 mBar, your altitude changes by about 0.82 meters. However, this is not the resolution of your analog to digital converter at 14 bits. You will notice that from 0 to 22.13 meters altitude, your a/d conversion reading changes by 40. So ... 22.13 meters/40 = 0.55325 meters per 14 bit increment of your a/d converter. A lot closer to half a meter of resolution with a 14 bit conversion.

[bitmaniac]

@scalerobotics

Of course you have not notice that in the last attached XL I upload there are three XL sheet in the file 10,12,14bit

Test it

My only concern is what method to choose (to start building) pic A/D with oversampling tehnicue , 12bit MAX and pic or DIF AMP + MAX + pic?

I cannot decide yet.

[Acetronics2]

Hi, Alexandros

I'd recommend you to read the Freescale AN 1646 ...

Interesting matter !

take care to the LM 358 output voltage range ... I think it's not the good choice ...
LMC662 or AD 623 would be better ...

Have a good Evening

Alain

[ScaleRobotics]

@scalerobotics

Of course you have not notice that in the last attached XL I upload there are three XL sheet in the file 10,12,14bit

Actually, I did notice, and it looks to be 100% correct. I am only pointing out that the increments with the 14 bit is 0.553 meters (near sea level), and not the ~1 meter that you mentioned in your previous post.

Thanks,

Walter