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# offset problem of AD8629 OPAMP

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.

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.

attachments.zip
• 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.

Thanks.

Chard

• Hi Chard,

I confirmed as for your comment to our customer.

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,

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

Please keep in touch, thanks Kaos

• 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

• 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

• Hi bcard,

No, I could not fix this trouble.

So I had recommended the another circuit for fix this trouble to our customer.

It was put the chopping circuit. It means REDESIGN for the current circuit..

In my case, thermopile have around 200kphm resistance. For AD8629, this value didnt bring a good result.

To make matters worse, SOMETIMES happen this trouble when we power on -off frequently not always.

So difficult to catch the reason why this trouble has comin....

Thanks Kaos

• Hi Bruce,

I recommend to test by a lot of qty samples AD8552s.

In fact I replaced it to another AD8629 so the trouble doesn't happen.

I also tried to replace to ADA4522-x.

But it brought the similar result.

Note zero drift AMP works tricky motion. Maybe you need enough test.

Thanks Kaos

• Did you resolve the AD8629 offset issue?  We have found a similar problem when using the AD8629 in a thermopile amplifier: the offset changes after cycling the power to the circuit.  We use 2 identical amplifier circuits in a thermopile based gas detection.  The gas concentration is calculated from the ratio of the 2 signals.  The problem is worst when one of the offset increases while the other decreases.  The decrease does not occur very often, but when it does, the calculated gas concentration changes by as much as 150 ppm.

• Thanks for the quick response Harry.

Sorry about the meager details.  I thought I was addressing Kaos.  I wanted to know if his customer resolved their problem before I started a new thread.

Application:

Gas concentration measurement using a 2 ch IR sensor

Circuit used:

Each of the 2 channels is amplified by an identical amp circuit per above, but with a reduced gain: 6700 instead of 10,000.

In our measurement range, the concentration is considered proportional to the Ln(Vch1 / Vch2)

where Vch1 and Vch2 are the voltage signals from each amp circuit.

The sensitivity of such an arrangement is low, and a slight change in the Vch1/Vch2 ratio yields a large change in the calculated gas concentration.

Problem:

After a power cycle, the output of the amp circuit [gain of 6,700] may jump either ~10 mV higher or ~10 mV lower than before.

It does not happen every time.  Once in about 10 times, the 2 output circuits jump in opposite directions causing a large change in the gas concentration calculation.

Notes:

• The offset does not merely change by up to 10 mV, but jumps by 10 mV up or down from the baseline.
• After most power cycles, the offset is at baseline.
• An AD8552 in the same circuit does not exhibit the offset jumps, but the AD8629 is cheaper...

Summary:

The problem is not the 10 mV offset which is well within the published offset [after a gain of 6700].

The problem is the uncertainty of a jump occurring and its direction after a power cycle.

Any thoughts would be welcome.

• Bruce,

It's hard to trouble shoot a problem when there is no data provide other than "...the offset changes after power

cyciling...."  How much?  If you want to start a separate thread with a schematic and some numbers, perhaps we

can help.  You could also mmeasure only the op amp as a follower and see if it changes less than 10 uV over

temperature, then it meets spec.

Harry