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Original question:

INA191: Negative terminal measurement

INA191: Ac current sense 400VAC 50Hz

deepak naiknavare
Prodigy 230 points
Part Number: INA191
Other Parts Discussed in Thread: INA199, INA2191, INA241A

Tool/software:

Hello,

In existing design we are using AZ-0500 1:500 (Talema) current transformer and TLV9002ID* to measure 400VAC 50/60Hz

Primary current range: 62.5mA(min) and 9A (max in failure state) which reflects to min secondary current of 125uA and maximum of 20mA (burden resistor is 30 ohms)

To increase the accuracy i was thinking to use INA199 as per the note TIDA 00753.

The issue i see is the high biasing current of 28uA, comparable to secondary current mentioned above(125uA).

We have 2 channels, so INA2191 looks closer to the our requirements considering dual channel and low input bias current. We will connect Ref pin to 2.5V.

Query: 1  The figure 7.2 INA2191 Diagram shows capacitive input in series which is slightly confusing how it process DC input?

Query: 2  Do you think, we can go with INA2191 for AC current sensing or some other better solution.

 

  • 0
    TI__Expert 5995 points

    Hi Deepak,

    1. The inside capacitive-coupled circuit of INAx191 has capacitors that are being constantly charged and discharged to transfer charge (the signal) to the second stage, which is the difference amplifier. The overall process of this switching is very complicated and can’t be shared in any straightforward way. 

    2. What is the maximum common mode voltage? If it is 400V then this device will not work as it is only spec'd to operate up to 40V. If the voltage is within the common mode range then this device should be ok to sense the currents you have listed.

    BR,

    Holly

  • 0 in reply to Holly R
    Prodigy 230 points

    I have done the simulation and just found out in the fault conditions we can reach upto 28mA across the burden resistor, which results in a Sine voltage of around +-500mV peak to peak. Please find below waveform. i have also attached the same.

    The datasheet says maximum recommended negative voltage allowed is GND-0.2. In actual i can find more than recommended

    Do you have any inputs/suggestions for me? The 30R burden resistor is selected for the sensing typical current which is 330uA

  • +1 in reply to deepak naiknavare
    TI__Expert 5995 points

    Hi Deepak,

    If you have 28mA across a 30R resistor this means your sense voltage is ~840mVpk-pk (as shown in photo). If you gain this up by the 50V/V of the device you are going to saturate the output. 0.84V * 50 V/V = 42V

    The snippet below from the DS specifies how you are able to go beyond the voltages listed if the current on the pins are limited to 5mA.

    I would recommend simulating with your common mode voltage set like it will be in the application as shown here. This will allow you to know the real voltage seen on the input pins with respect to GND. 

    BR,

    Holly

  • 0 in reply to Holly R
    Prodigy 230 points

    Thanks Holly. Now i am sure to proceed with this IC. Else i was thinking to go for INA241A.

    1) Yes at 840mV it will and should saturate my output as i this is a fault scenario. Just it should not damage the IC. I will ensure it by adding 1k resistor in series for IN+ and IN-. This should suffice the negative voltage issue upto -5V right?

    In general i will have maximum +-60mV across the burden resistor, which should be safe.GND-0.2V 

    2) Related to simulation:-> I am having input from the current transformer (1:500), In this scenario what should be the voltage (as shown by you in above picture) to consider for simulating current of 28mA?  

  • +1 in reply to deepak naiknavare
    TI__Expert 5995 points

    Hi Deepak,

    1. Yes this should suffice.

    2. The voltage I show in the picture is the common mode voltage you expect to see at the nodes. This is application specific. You will also have the current source as I show in green and that will ensure you are pulling 28mA. It might be unclear in the photo but the current source is flowing to GND.

    Best,

    Holly