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OPA365: Common mode gain issue

Minghan Dong
Prodigy 40 points
Part Number: OPA365
Other Parts Discussed in Thread: TINA-TI

Hi Expert,

Issue: Common mode voltage has wrong gain when debug the circuit in my system. Why?

Schematic:

Test condition:

(1)  Add voltage source to ISEN Neg=15V ISEN_Pos=15V ISEN_ADC(Vout) = 1.621V (wrong case, the right result should be 1.65V)

(2) Add voltage source to ISEN Neg=0V ISEN_Pos=0V ISEN_ADC(Vout) = 1.65V (right case)

(3) Add voltage source to ISEN Neg=0V ISEN_Pos=0.8V ISEN_ADC(Vout) = 1.65V+0.8V (right case)

Could you kindly provide you solution for high CM applicaition?

  • 0
    TI__Expert 6050 points

    Hi Minghan,

    Can you share more about your application? Are you doing high-side or low-side current sensing? Are you seeing these results across multiple devices or are all other devices outputting the expected voltage?

    One of your test conditions that you mention is ISEN Neg=15V and ISEN_Pos=15V which is a much larger input than the voltage range you outline in your schematic. Why is one of your test conditions to apply 15V at each input?

    Thanks,

    Nick

  • 0 in reply to Nick Nauman
    Prodigy 40 points

    Hi Nick,

    (1) Can you share more about your application? Are you doing high-side or low-side current sensing? 

    [Minghan]：High side current sensing in inductor current of 3PH inverter.

    (2) Are you seeing these results across multiple devices or are all other devices outputting the expected voltage?

    [Minghan]: All other device shows the same outputs.

    (3) One of your test conditions that you mention is ISEN Neg=15V and ISEN_Pos=15V which is a much larger input than the voltage range you outline in your schematic. Why is one of your test conditions to apply 15V at each input? 

    [Minghan]: I want to use non-isolated solution to sense high-side inductor current. The system add -50V~150V CM voltage to ISEN_pos ISEN_neg in the schematic above. I have simulated this solution. The IN+ IN- are in the range of （-0.1V ~ 3.4V） as shown in datasheet.

    The gain of the opa application is 1(0.01*100). The gain is too big so that the CM voltage gain is sensitive  to the resistor accuracy？

    Thanks,

    Minghan

  • 0 in reply to Minghan Dong
    TI__Expert 6050 points

    Hi Minghan,

    Thank you for providing this additional information. As you have mentioned, the mismatch in resistor could be causing this inaccuracy when inputting large voltages like your 15V input test case. Is there a specific reason you have selected 100kohm resistors or are you able to decrease the value of the resistors to 10kohm or smaller in order to be less effected by resistor variance?

    Thanks,

    Nick

  • 0 in reply to Nick Nauman
    Prodigy 40 points

    Nick,

    As I mentioned in the previous reply, I will sample the high side current signal. So I should decrease the CM voltage from -50V~150V to -0.1V to 3.3V.

    The decrement will decrease the current sample gain so the OPA should provide another gain utilized the 100k res.

    Current gain=0.01*100 I should not use OPA @ this situation?

    Minghan

  • +1 in reply to Minghan Dong
    TI__Expert 6050 points

    Hi Minghan,

    If you change both Rf and Rg resistors to 10kohm, the gain will remain as 1V/V. By decreasing the resistors to 10kohm you are reducing the effect from the bias current and therefore reducing the offset you see at the output. Attached below is a TINA-TI simulation of your schematic. You will see that the output changes in simulation depending on whether you used 10kohm resistors or 100kohm resistors.

    OPA365_CurrentSense_E2E.TSC

    Thanks,

    Nick