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INA128: Using INA128 as current sensing. Maximum input voltage regard to ground limitation.

Jose Henrique de Melo Luna
Prodigy 185 points
Part Number: INA128
Other Parts Discussed in Thread: INA148, INA149, INA111

Hello.
I need build a current sensor for a Full bridge inverter output.
The nominal output voltage of the inverter is 127 Vrms.
At their output there will gonna be a LC filter.
The main characteristic of the circuit is that the current is low, in the order of 100 mA, at most.
The basic schematic is below.

As I have some INA128 available, I intend to use it to sensing the output inverter current through Rshunt.

So there are three questions I need to ask:

1) This is a good ideia, use INA128 to measure such low current without using a high value of Rshunt?

2) In datasheet, there are Common-Mode Voltage Range ((V-) + 2 and (V+) - 1.4 V) and Safe Input Voltage (±40 V). Since the max power supply is ±18 V, could we put ±40 V at input?

3) My main concern is about input voltage regard to ground. Despite the maximum differential voltage on IN+ and IN- be low because the low current, the voltage at input pins regard to ground will be in the order of hundreds of volts. Can this IC withstand such a voltage level?

Thanks,

Henrique Luna (BR)

  • 0
    TI__Mastermind 23050 points
    Hi Henrique,

    The INA128 is not a good fit for your application. You can apply up to +/-40V at the input without damaging the device, but the device will not be operating in the linear region. The device will certainly not be able to tolerate hundreds of volts at the inputs.

    I would suggest looking at the INA148 or INA149, which have common-mode ranges that extend to +/-200 and +/-275, respectively. Because these use a difference amplifier topology, there will be some error associated with current that passes through the input resistors instead of the shunt resistor, but this current should only be a few hundred microamps at most.

    Regards,
    Zak Kaye
  • 0 in reply to Zak Kaye
    Prodigy 185 points

    Zak Kaye said:
    Hi Henrique,

    The INA128 is not a good fit for your application. You can apply up to +/-40V at the input without damaging the device, but the device will not be operating in the linear region. The device will certainly not be able to tolerate hundreds of volts at the inputs.

    I would suggest looking at the INA148 or INA149, which have common-mode ranges that extend to +/-200 and +/-275, respectively. Because these use a difference amplifier topology, there will be some error associated with current that passes through the input resistors instead of the shunt resistor, but this current should only be a few hundred microamps at most.

    Regards,
    Zak Kaye

    Thanks. I did not know the INA148 and INA149. Actually, they gave me an idea to extend the input voltage range by placing resistive dividers at input of the INA128.
    As you mentioned, there will be an error associated with current that passes through the input resistors instead of the shunt resistor, but I think that will work.
    Of course I will not be able to work with common voltages in the house of hundreds of volts, because I believe that the resistive divider would have to work with very high resistances, being susceptible to noise, but I believe that, by adjusting the INA128 gain, I can extend rather the common mode voltage at which I can work.

    I uploaded an image of the idea.

    R1 = R2 and R3 = R4. RT is on there to correct the inevitable imbalance due imprecisions. The three capacitors are to form an input Low-Pass Filter as mentioned on FIGURE 6 of INA111 datasheet.
    If you have some comment, I will appreciate. Thanks.

    Regards,

    Henrique Luna (BR)

  • 0 in reply to Jose Henrique de Melo Luna
    TI__Mastermind 23050 points
    Henrique,

    Theoretically if you size the resistors appropriately the circuit you are proposing would work, however in practice I think you will find it difficult to maintain a high degree of accuracy with your trimpot adjustment. Trimpots tend to drift over time and you can typically only get an 1/8th of a turn precision in the adjustment, but perhaps you can tolerate this in your application. I would still recommend the INA148 because its resistors are internally matched to ensure you get good accuracy and good common-mode rejection. However, it does have a fixed gain of 1 so you may require a slightly larger Rshunt, or an additional gain stage.

    Regards,
    Zak Kaye
  • 0 in reply to Zak Kaye
    Prodigy 185 points

    Thanks Zak.
    I will take in consideration all the information that you gave me and concomitantly, I will try to adquire a INA149 to test.
    Best regards,
    Henrique Luna (BR)