Fluctuating values in 12 bit ADC

[shaz]

hi,
i've connected a 12-bit adc with microcontroller. my problem is the values keeps fluctating. i tried adding 0.1uF cap at adc analog input pin, adc got stable to some limit but not fully. my analog input voltage is coming from an op-amp. The analog voltage is constant, i've checked that.

Please help me out guys........

 

[sankudey]

Re: fluctuating ADC

As you mentioned adding a cap at ADC input reduces the problem, then most probably the problem lies with the previous stage, i.e., OP-AMP. But, it would be nice if you first test the ADC with fixed DC voltage(s) with different values to make sure the ADC is OK.
Also, if possible, you can check the input stage of the ADC and the current requirement and cross check that if the OP-AMP is capable of doing so.

 

[maxascent]

Re: fluctuating ADC

You need to give more info regrding ADC type etc

 

[shaz]

Re: fluctuating ADC

hi i'm sending the circuit diagram which i tried. Actually i took this from this forum only.

if i ignore the op-amp stage and provide constant dc voltage to ADC analog input the output

is constant. but if i provide any dc voltage between 0-0.5v at opamp input (eg. 0.3 volts)

the output fluctuates.

Please provide me the solution to the problem.

 

[sush]

fluctuating ADC

try adding a small resistance between op-amp and adc.

 

[sunking]

fluctuating ADC

how many bits is fluctuating?if only last bit, dont care of it.
Make sure the noise of OPA output is small. And also add a cap at Vin.