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INA146: Programable GAIN and RISK of degrading CMMR

Mike_3007
Expert 2055 points
Part Number: INA146

Hi everyone

This is Mike, thanks for all your support

I have some questions about the INA146

I designed this using the INA146 but I read at the datasheet ina146.pdf  in page 9 says that if the source impedance is bigger than 800 ohms can cause  mismatch at the resistor ratios

The Voltage is Line to Line, and It will feed the adc of C200 mcu. I used the resistor configuration shown at page 8 of ina146.pdf where the total gain is 1 and A2 gain is 10

Also I'm a little be concerned about a temperature rising that my pcb cannot achieve.

If you can help me with this.

Thanks for all your help.

  • 0
    TI__Mastermind 23050 points
    Hi Mike,

    Using a difference amplifier with the source impedance as large as you have it is going to result in very poor CMRR and significant gain error because as the datasheet mentions, the internal resistors are not trimmed to absolute values, but rather to balanced ratios and a large source impedance will degrade this matching. If you are dividing the input as shown, you would be much better off using an instrumentation amplifier and splitting R5 between two resistors with a ground connection in the middle to evenly divide the common-mode and provide a path for bias current.

    I'm also not sure why you have chosen to use an INA at the end of your signal chain. The signal out of the isolator should already be referenced to B GND. I think it would be better to just use an op amp there instead.
  • 0 in reply to Zak Kaye
    Expert 2055 points
    Hi Zak
    Thanks for your asnwer. Yep, that about the source impedance will be a problem, I was thinking about use a voltage follower between the ina and the iso, but I think that would be more hardware and it can be not helpful as I think, the divider resistor at the input was designed to achieve 3Vrms line to line then using the ina because of the differential measurement, then using the iso to isolate, then using the other ina to reduce voltage as 0.1V out of iso, and mounting an offset because the Output of the signal chain will feed the adc of a mcu. Please if something can be done in order to use some of this IC (actually I have other op amps as AMC) or some guidelines would be appreciated
    Best Regards
    Mike
  • 0 in reply to Mike_3007
    TI__Mastermind 23050 points

    Hey Mike,

    It's really not necessary to use a device like the INA146 if you do not have a high common-mode voltage, but if you are limited to using the INA146 here then you can try to do something like attached to keep the effective source impedance low. I'm not sure how large you require the input resistance to be but you may need to increase the input resistors and add some extra gain.

    INA146 Divider.TSC

  • 0 in reply to Zak Kaye
    Expert 2055 points
    Zak
    Thanks for the advise and the simulation circuit, I think I will use other IC such as the AMC familiy or the new ISO. I was trying this INA146 because here, at the lab, we have many of them, I don't for which project they were using it. In fact here is another question. I already known this is a Programmable OpAmp, but, in your expertise , Which are certain applications you've seen this OpAmp is used?, I will take some to put them work..
    Regards
    Miguel
  • 0 in reply to Mike_3007
    TI__Mastermind 23050 points
    Hi Miguel,

    This device is most useful in applications such as measuring a current through a shunt in the presence of a high common-mode voltage, Like doing a high side measurement on an 80V bus for example. The input divider scales everything down by 10:1 to accommodate a high common-mode or differential voltage outside of your op amp supply range. Typically, you then want to gain up the actual signal once the common-mode has been stripped off by the diff amp and you have a single ended signal. Most high common-mode devices operate in unity gain or less, so an extra stage is usually required.

    You could use this for other things as well though, such as building a high precision voltage controlled current source.
  • 0 in reply to Zak Kaye
    Expert 2055 points
    Thanks for the reply Zak
    Just one more question?
    Is there any pdf file or some guide lines to design with this Op Amp or with those kind of Op Amp
    Regards
    Miguel
  • 0 in reply to Mike_3007
    TI__Mastermind 23050 points
    Hey Miguel,

    There are a couple of documents in the technical documents tab for the device, but I don't think there's anything that really goes into detail about designing with it. The applications section of the datasheet is a good reference though for circuit ideas.
  • 0 in reply to Zak Kaye
    Expert 2055 points
    Thanks Zak
    Regards
    Mike