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[ts-7000] Re: max sample rate?

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Subject: [ts-7000] Re: max sample rate?
From: "Phil" <>
Date: Fri, 12 Oct 2007 10:46:09 -0000
--- In  "j.chitte" <> wrote:
> Hi Phil,
> 
> thanks for picking this up.

No problem. We're here to help!


> kernel is ts11

I don't think this is the issue now.


> In my last post I was confused further by adc_logger channels being 
> 0-7 , that was a large part of the problem. Beg your pardon.

Yeah, I thought something like that may have been part of the problem.
There were some reasons why I numbered the device files adc1 -> adc8.
Part of it was due to how the device minor numbers had to be
allocated. Feel free to change the code to name the devices adc0 ->
adc7, look at line 97 in adc_main.c and remove the "+ 1".


> How can I decrypt the std module output?

The numbers that you are getting from the driver (even at +/-10V
range) seem ok, other than /dev/adc3 but I have a theory on that too.
But to convert it should just be a case of value / 2048 * 10V (e.g.
for /dev/adc4 you have 263 / 2048 * 10V = 1.284V)

Firstly though, can you try Jim's adc_logger with the range set to
+/-10V ("-r 3" I think) and see what output you get.

My theory is that you should get about 0.6V for that 3rd value even
with adc_logger. If this is the case, I think the ADC itself, plus the
driver/Jim's app is all working ok. What is the hardware setup that
you have going in to the ADC inputs? You mentioned using thermistors,
are they buffered by say an opamp or something? The MAX197 chip has
reasonably low input impedance PLUS this input configuration changes
with different uni/bipolar setups. Have a look at the MAX197 datasheet
to see what I mean. When set to unipolar mode it wouldn't matter what
impedance input you used the input is at 0V anyway, so inputting 0V
works fine. However, when set to bipolar there is a reference voltage
applied at the input and if your input isn't strong enough to drive it
to 0V then there may be a slightly higher voltage still on the pin
(the 0.6V that I mentioned or the 123 numbers in the cat /dev/adc3
output).

If this is the case, you will need to provide buffering for all your
inputs, because you would find that the ADC loading should be
affecting all the inputs, it is just not as obvious in its effects.


> Thanks for your help.

Not a problem. I hope this all does help and you are able to get
everything working.


Cheers
Phil





 
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