offset problem of AD8629 OPAMP

Dear Sir/Madam,

Our customer have met one trouble as following.

(This discuss came from another sled cause my miss understood)

On my enclosed schematic Fig-1.pdf,

step-1 power off and keep waiting around 30 sec

step-2 power on

step-3 around 90msec offset voltage appear at out-1 pin

step-4 power off and on but the 90mV offset is not cleared

Also refer to my enclosed time chart "tme chart when power off and on.pdf".


*1 ; 30sec is not certainly.

*3 ; in some cases, it return to expected value

After once this trouble happen, out-2 is up and dowm on offset voltage 900mV(x10 gained value)


When we work with above action, does your AD8629 do such a motion?or not.

If he replace the thermopile(TP) to fixed resister 175kohm, then the result was same.

Please advice us before your holiday!!

Thank you in advance Kaos
  • 0
    •  Analog Employees 
    on Jan 11, 2016 7:55 AM

    Hi Kaos,

    Apologies for the delay of response. You mentioned that in the circuit you gave, the TP was replaced by a 175k fixed resistor but how about the 5V and resistor divider connected on the inverting input, is it still present when the customer is testing the circuit? With the current configuration, any voltage between 10k and 910 resistors will be reflected on the output with a gain of 1 as previously mentioned on the older thread. It would be helpful if you could provide the updated circuit that the customer is using.



  • Hi Chard,

    I confirmed as for your comment to our customer.

    Your understanding is correct.

    The voltage that is divided by 10kohm and 910ohm for 5V should be outputed gain1, and the potential difference between + and - pin of AD8629 should be amplified 3000 times and be outputed he think.

    So here is 175kohm on TP, then the voltage that is divided by 10kohm and 910ohm for 5V should be outputed simply he think.

    This problem didnt bring the expected result.

    Why this 90mV is added??? Circuit design mistake??

    step-3 around 90msec offset voltage appear at out-1 pin

    ===>sorry typo! 90mV not 90msec I correct.

    Thanks Kaos

  • Thanks for your good comment, Chard,

    I already forwarded your comment to our customer.

    When I got the comment from our customer then I will put here soon.

    Please keep in touch, thanks Kaos

  • 0
    •  Analog Employees 
    on Jan 27, 2016 12:20 PM

    Hi Kaos,

    There are factors that could generate that 90mV offset voltage. One of which is having a very high gain where it will amplify your input signal but also the noise. Another factor is having a large feedback resistor which can generate a voltage error depending on the bias current of the device. You can change the value of the 1.3M to a lower resistor value because this makes up some of the offset voltage on your circuit.

    Some additional questions, how do you mean by "Never Return"? is the offset still present even when the circuit's supplies are cut off then turned on back in a specific time or the offset remains? Does this happen everytime they try the circuit whenever you try powering it up? What is your expected value?



  • Hi, Chard

    Very sorry to my late response  to here.

    Here is customer response.

    After power off, the offset voltage doesn't remain there.

    Customer tried to make change R35 from 1.3Mohm to 130kohm, but the offset voltage still remain there.

    After around 20 sec, there is not the offset voltage  so return to expected value in some cases.

    Sorry not never return I correct this.

    So again customer explained this trouble.

    In some cases, there is the offset voltage become plus 90mV than expected value

    when the stable condition after power on.

    After power off , the output of Amplifier (the offset voltage )fall to 0V.

    After around  a couple of minutes

    , then the offset voltage fall the offset value become plus 90mV than expected value.Si the plus 90mV value is kept to power off. But this appear sometimes. Not always.

    Any Comment?

    Thanks Kaos