How to calculate offset voltage of AD628

Hi RMosig,

Thank you for considering AD628 on your application.
You're already on the right track with regards to getting the total offset voltage for AD628.
The additional error formula you see on the datasheet page 4 is just the error due to your common-mode voltage:

There are other factors that you may need to consider such as offset voltage and the effect of Bias current on your system.
For example, looking at the offset voltage of the output amplifier and the gain you set (60.2), you'll have an additional 9.03mV offset (0.15mV*60.2). 

Another is for the effect of the bias current you have 3nA*2kohm*60.2 that yields around 0.36mV :

These additional offset are on top of the one you calculated at around 46.03mV that's due to the common-mode voltage. so overall, you may have around 55.42mV total offset voltage. 

Though on your application what's the offset voltage you're seeing and what's the target accuracy? Can we see the schematic diagram in case we're missing something. If you can give us more insight on your specific application we may be able to help you out further. 

Thanks and Best Regards,
Dann

Hi Dann,
Thanks for your answer.

Do I read your answer correctly that my previously mentioned 90.3mV (caused by 0V common mode voltage) would not add up to the common mode error voltage of 46.03mV (at a common mode voltage of 43V)? With the other factors mentioned, you're right, they would need to be taken into account. But as they do not have a big impact, I would also neglect them here.

For a better understanding, here how the part is used here. Due to confidentiality and the circuitry is spread over several pages, I condensed it. The amplifier is used in a current sensing application, i.e. the 0.1 Ohm resistor shown here is the shunt.

My basic problem is that we had some devices showing an increased offset voltage under increased common mode voltage. We took affected devices from the board and measured it stand alone under several common mode voltages (input pins being short-circuited to have same voltage on both pins):

Now it's the question, if these devices are within their specifications or not. For this purpose I wanted to be sure on my calculations based on the datasheet.

Best regards,

Raphael

EDIT:

I just additionally did some error calculations to get the maximum offset voltages:

This table should include all error sources: The error due to common mode (based on the formula on page 4), the error due to output offset (based on your reply), the error due to bias error (based on your reply) and the temperature error due to 15K temperature rise.

If we compare now the calculated maximum values, they are lower than the meaured results. If we assume, I did all the calculations correctly and no other error sources are forgotten, our parts are not within in spec, right?

Hi Raphael,

The offset voltage you mentioned which is +/-1.5mV will also add up to the overall system offset, but since it mentioned that this is the RTI from the inputs for both the Differential Amplifier and output amplifier, the overall gain will only be 6 (Differential Amplifier's Gain x Output Buffer Gain = 0.1 * 60 = 6, this is shown on table 5 page 6 of the datasheet) and should only add up around 9mV, sorry I missed this one on the previous response.)

Though considering this additional offset it would seem that you're still getting higher error starting at 30V common mode. When you mentioned that only some device, does this mean that some of the AD628s you have are within spec? 

I'll continue to look into this, but on the devices that have this unexpectedly high offset, can you also measure the voltage at pin 4? This is basically the output of the differential amplifier and we'll be able to see if the unexpected offset is coming from the differential amplifier or the output buffer.

Thanks Raphael.

Best Regards,
Dann

Hi Dann,

ok, so finally the worst case error at a common mode voltage of 43V will be 56.17mV (from the table above) + 9mV (as mentioned in your last post) = 65.17mV.

So far we had this issue with devices with datecode #1935 and #1936. After replacing the problematic component with newer components, the problem seems to be solved. Might there be a problem with the batch?

We did some more measurements with a better multimeter which showed again that the offset voltage is higher than expected:

Best regards,

Raphael

Hi Dann,

did you have the opportunity yet to have a further look into this topic?

Best regards,

Raphael