# 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".

note

*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

attachments.zip
Parents
• 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?

Thanks

Chard

Reply
• 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?

Thanks

Chard

Children
No Data