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INA138 high side current sensing issue

BALKRISHNA TULSYAN
Intellectual 850 points

Other Parts Discussed in Thread: INA138, MSP430G2553, TLV272

Hi,

I am trying to measure high side current and the schematic used is same as given in the INA138 datasheet (page no. 11).

1) Rsense used is 0.005 ohm (two 0.01 Rsense in parallel) connected between Vin+ and Vin-

2) Rl = 120 kohm, thus gain = 120k/5k = 24 (27.7 db)

3) Series resister connected between TLV272 opamp and ADC input of MSP430G2553 is = 220 ohm AND capacitor is 0.1uF

4) Supply voltage to INA138 = 12 Volt

I am measuring current flowing from the DC adapter of 12 volt to the LED load connected at the output of  synchronous buck regulator (MOSFET of buck regulator is driven by MOSFET DRIVER which in turn is driven by 25 khz frequency generated by MSP430 controller).

Load current = 330 milliamp


Since the load current is 330 mamp, voltage across the Rsense = 0.005*.33 = 0.00165

Gain = 24, so voltage across RL = 0.00165*24 = 0.0396 Volt

But to my surprise,i am getting voltage across Rl = 0.2 volt (same voltage across 0.1 uf Capacitor) because of which current measurement goes wrong.

Can anyone suggest me about the possible cause and solution for this issue.

Thank you..!!

  • 0
    Intellectual 850 points
    I want to update on my own question..!!

    I am testing the whole circuit on bread board. Since the Rsense resister is very low i.e. 0.005 ohm, I tried adjusting the Rsense in the bread board. Earlier it was inserted deep in the socket of bread board but i removed Rsense and plugged it again keeping a caution this time NOT to insert deep into the socket of bread board . The moment i did this, i got a change in reading...so, the reading is changing based on the Rsense placement (insertion) on the bread board. Is this effect because of breadboard's parasitic component or this behavior is abnormal.

    The output reading is changing based on placement of Rsense resister. At one particular position i am getting the correct reading but don't know whether it's really correct or not..!!

    Can anyone guide me on this too??
    what should i do to counter this problem??
    Any expert opinion ??

    Thank you..!!
  • 0 in reply to BALKRISHNA TULSYAN
    TI__Genius 15770 points

    Hi AMIT,

    With that configuration you should see 39.9mV at the INA output. My first recommendation is to eliminate the parallel connection of the sense resistors. This might be creating the error, use a 5m Ohm resistor instead.

    Parasitic component can make a huge difference in your readings. Be aware of that. You need to make sure you are connecting the sense resistor in a Kelvin configuration.
    training.ti.com/getting-started-current-sense-amplifiers-session-13-layout-considerations

    Regards

  • 0 in reply to Mayrim Verdejo
    Intellectual 850 points

    Hi,

    Thank you for your valuable suggestion. Link really helped me to get better understanding.

    .