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Opamp - ADC merging results in off output value

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shaz

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Dear all
i am designing a temperature controller with input range from
-8.825mV to 54.8mV . i have designed a opamp circuit in which i get a particular ouput volatge say 3.1volts. furthur i have to give this input to the adc and to 8051.


now my problem is when i test the output of the analog stage it gives me correct value. and when i apply the same volatge from a variable pot to adc to check its working the display shows the correct value.

but when i merge my analog stage with the adc , the display show huge diffreence in the value.

i tried everything like adding decoupling capacitors and rc filters on adc input but still no change.

please help me out find the mistake.
 

opamp adc

check the opamp which u r using should be very stable opamp...try using ca3130.
 

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    shaz

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Re: opamp adc merging

are the output values dc offset or are they unstable?

check
ground loops and mixing of digital and analog grounds

check
specified max output load for opamp

check
specified load impedance of ADC input

hope this helps Polymath

Added after 6 minutes:


ensure the ADC has full acquisition time
and
any filter components are not being affected
by sample charge current required by ADC

Polymath
 

Re: opamp adc merging

try this

1) Select an opamp with hi (as much as it can be) input side resistance.
2) place a current amplifier between your circuit coupler (u can built it with Opamp at the input of ADC).
 

Re: opamp adc merging

nice idea narbeh_rob
I had never considered using a current loop over a very short distance
- I must try it but it sounds a bit extreme

a guard ring could also be place around terminating resistor and ADC input pins
to minimise leakage but I do not think leakage is the problem

Once you have a stable system try
Noise cancellation in software:
Oversampling will also help to cancel random noise if that is a problem
16 samples averaged will reduce noise by √16=4 to(¼)
256 samples ..........................................√256=16 to(1/16)
according to sampling theory!!

regards Polymath
 

Re: opamp adc merging

It sounds like you have not connected together your analog and digital grounds!
Make sure these two grounds are tied together somewhere in your circuit. Ideally they will be connected together near the adc so that there will be no digital currents flowing in the analog return. If this is not practical, connect them together near the power supplies. Be sure to connect these two grounds together at ONLY ONE POINT so as to avoid ground loops.
 

Re: opamp adc merging

What if you show your circuit? Whill save lot of words and everybody will be in the picture. Could you?
 

Re: opamp adc merging

check for a stable opamp like OPA177 because it has very low offset voltage
 

Re: opamp adc merging

What kind of power supply you using?
 

opamp adc merging

try ad's opamp and read the book of ad on this aspect.i remmber it cover this
 

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