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ISO124: Problem with the offset and noise

Cesar Medrano
Prodigy 10 points
Part Number: ISO124
Other Parts Discussed in Thread: ISO122, , AMC1411, ISO224, AMC1400

To Whom It May Concern

Hello, I've got a question about the component ISO124P. I’m utilizing this component for a project, but I found that the ISO124P has a small ripple noise and a small offset voltage in the output. I attached the images taken with the oscilloscope. The first image is the output of the ISO124P at 0 V.

I added the filter that the datasheet recommends, but the offset remains.

I would like to know If TI has identified the problem and has some solution for it, I’m sure that the offset is caused by the ISO124P because I tested the module without the ISO124P (without isolation), and this offset doesn’t show. The image shows the output of the circuit with the ISO124P and the filter that’s recommended in the datasheet (blue) and the circuit without the ISO124P (yellow).

Also, I attached the actual output of the circuit (blue) and the output of the circuit with the filter (yellow). In this example, I used another ISO124P because every ISO124P has a different offset and ripple noise. The offset in the first case was 2 mV, while the offset in this ISO124P was 18 mV.

I tried a pre-gain circuit that is recommended in the ISO122P datasheet, and it attenuated the problem but did not solve it. I would appreciate a solution for this problem because the hardware is entering into conflict with the software due to this offset. At last, I attached the output with the pre-gain circuit that recommends the datasheet of the ISO122 (blue) and the output of the circuit with the pre-gain circuit and the filter (yellow).

  • 0
    TI__Mastermind 49621 points

    Hi Cesar,

    Unfortunately the ISO124 is an older device so access to design documentation is limited and I'm not entirely sure what inside the ISO124 would cause this behavior. Most likely a chopper either on the output. I do notice however, that all of these "offsets" are within datasheets limits, assuming +/-15V supplies and a load of 2kohm is present:

    If additional averaging was performed in your software or with a DMM for example, then the peaks of this "offset" would be reduced. 

    Can you share a little bit more about your application and why the ISO124 was chosen? 

    Do you have specific accuracy or timing requirements that this offset is interfering with your software in some way as you mentioned? 

    We have the newer AMC1400 and AMC1411 products in a 14mm wide-body package or the ISO224 with +/-12V input range that may be alternative solutions. 

    I would like to do my best to help you find a solution to this problem.