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Original question:

INA500: can this part works out good precision for 16-bit system

INA500: Accuracy becoming worse when input > 3V

LIU QINGFENG
Prodigy 70 points
Part Number: INA500
Other Parts Discussed in Thread: REF2025, OPA365, REF2030

Tool/software:

Dear supporter:

In our design, We are trying to using INA500C to measure battery voltage,  with 1.25V reference..

and found it accuracy performed quite good, up to 0.02% after calibration, when battery voltage is from 0.5V to 2.5V.

However when the battery voltage higher than 3V, accuracy drops,  even worse when bat voltage goes higher such as 4.7V, the offset is more than 20mv, and cannot be calibrated because offset is not linear..

DC source was used to simulate battery, but we also tried with actual battery (100AH), same performance.

the reference is from REF2025, whose 1.25V o/p is buffered by OPA365.

Is there limitation for INA500C ? any suggestions?

Regards.

Qingfeng

  • 0
    TI__Mastermind 33393 points

    Hello Qingfeng,

    I am not able to read the schematic you have provided, could you please attach a higher resolution photo?

    What is the supply voltage of the amplifier?

    Best,
    Jerry

  • 0 in reply to Gerasimos Madalvanos
    Prodigy 70 points

    Hi, Jerry,

    Supply voltage is 3V3

    and I wish this has high resolution.

  • 0 in reply to LIU QINGFENG
    TI__Mastermind 33393 points

    Hello Qingfeng,

    Thank you for providing higher resolution schematic.

    I believe you are running into the upper end of the input common-mode range for this device, and seeing the crossover distortion. I will confirm this if you confirm the following information.

    Could you please provide what is the max battery voltage you are trying to measure with this device.

    Also could you please clarify the following differences between schematic and description.

    Supply is said to be 3V, schematic says 3.3V. Please say which value is correct

    Reference is said to be 1.25V, schematic says 1.5V. Please say which value is correct.

    If you are able to lower the reference voltage this may resolve your issue.

    Best,
    Jerry

  • 0 in reply to Gerasimos Madalvanos
    Prodigy 70 points

    Jerry:

    We are debugging the circuit, the schematic says 1.5V because REF2030 is planned,  when we found this nonlinear issue we tried REF2025 as well..

    When we are using REF2030, the ADC reference is 3.0V and INA500C reference input is 1.5V 

    when REF2025, the ADC reference is 2.5V and INA500C reference input is 1.25V.

    but the VCC to INA500C, always 3V3.

    The Battery voltage range, supposed to be -5V to 5V (to measure reversely connected battery as well).

    As I said,  when battery input is from 500mv to 2.5V, the ADC reading matches the instrument reading very well..

    But when the battery input goes  higher than 3V, the errors becoming much bigger. 

    Example here:

    ADC Reading Actual Meas. Error
    477.3 477.3 0.0
    1000.2 1001.2 1.0
    1500.0 1502.1 2.1
    2000.1 2002.6 2.5
    2500.0 2502.3 2.3
    3000.0 3001.1 1.1
    3500.1 3497.9 -2.2
    3940.0 3932.5 -7.5
    4000.1 3991.3 -8.8
    4500.0 4482.6 -17.4
    4900.0 4870.7 -29.4
  • 0 in reply to LIU QINGFENG
    TI__Mastermind 33393 points

    Hello Qingfeng,

    Could you provide what ADC you are using, as well as what sampling frequency? Are you seeing this error only on the ADC, or are you seeing this nonlinearity when probing the output of the amplifier directly?

    Best,
    Jerry

  • +1 in reply to Gerasimos Madalvanos
    Prodigy 70 points

    Jerry:

    Since battery voltage will not change dramatically, the sample frequency is quite low and s quickly there is digital filter to filter out possible spikes, also you can see in the schematic there are 3 stages hardware filters.

    We did notice nonlinearity in the OPA output when battery voltage beyond 4.8V, 

    The  other channel of the ADC has been used to measure other signals which is also quite good....

    Sorry I can not tell the ADC here but it is also Ti part (Leave a email address here then I can share you more)...

    Same after we removed the TVS before the AMP 

    Whether there were nonlinear circuits inside the INA500C chip, such as diodes o clamp circuits to cause this kind of nonlinearity?

    Best.

    Qingfeng.

  • 0 in reply to LIU QINGFENG
    TI__Mastermind 33393 points

    Qingfeng,

    I have added you as a friend on E2E. Could you please send me a message through there to share the ADC and the sampling frequency you are using? This will be a private message and will not be seen on public forum.

    Best,
    Jerry

  • 0 in reply to Gerasimos Madalvanos
    TI__Mastermind 33393 points

    Hello Qingfeng,

    I have provided a message to you directly with an answer to improve the 1.5V ref circuit.

    Best,
    Jerry

  • 0 in reply to Gerasimos Madalvanos
    Prodigy 70 points

    Thank you very much, Jerry.