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INA210: Please review the schematic

Chase Jeong
Genius 4660 points
Part Number: INA210

Hello TI experts,

My customer now consider INA21[x} op amp for their new product, here is two types of schematic.

INA21x_Case1.pdf

INA21x_Case2.pdf

- PWM_CTRL is the PWM signal which low voltage is 0, high voltage is 1, frequency is 20KHz.

- when Q1 is on, the current of VDD_5V line is 2.5A (when PWM duty is 50%), and maximum 3A about 2 seconds.

- L400 is the coil which resistance is variable from the heat.

- in case2, the lower INA21x have 3V reference voltage. they designed it because they want more gain although they got little voltage difference compared to 5V reference voltage.

anyway my customer wants they have to get more effective output from INA21x even the voltage difference is little.

what do you think that which one is proper, case1 or case2?

please let me know if you need more information to review these schematic. Thanks.

Best regards,

Chase

  • 0
    TI__Mastermind 26660 points

    Hello Chase,

    There may be some misunderstanding of how these devices function. Firstly, the INA21x is a current-sense amplifier (closed-loop), not an op amp (open loop).

    Secondly, the REF pin should not be tied to Vs if the device is sensing positive shunt voltage (Vshunt = Vin+ - Vin-). the REF voltage is meant to provide a bias to the output of the device when Vshunt = 0 (no load). The overall equals is always the following:

    Vout = Vshunt*Gain + VREF.

    So by connecting REF to VS, you are slamming the output into the Vs rail and saturating the output making the the output worthless.

    As for the difference between case 1 and case 2, there really is no functional difference because the INA21x can sense currents on low side (VCM = 0) or on high side (VCM<=26V). The only thing to consider here is the increased offset voltage error due to device CMRR. As the VCM deviates from the datasheet testing condition of VCM=12V, the input offset due to CMRR (Vos_CMRR) will become non-zero and add to the total offset. Because of this I would recommend keeping both INA21x devices on the high-side as shown in case 1 so they both will have the same error.

    Note that Vos_CMRR = |12V- VCM|*10^(-CMRR/20dB).

    The first thing to do though is to correct the REF voltages as I have pointed out. You could either connect REF to GND for both devices as this seems to be a unidirectional current sensing application. However, if the INA21x needs to measure negative current or if the INA21x needs to have stable output readings very shortly after the current is stepped in the PWM, then you will want to add a small bias voltage (~100mV) to the REF pins. The reason for this is because the best performance will come when the output is always biased into the linear operating region, or else, Vout = GND when load = 0A, thus Vout will be saturated. When Vout is saturated, there will be the overload recovery delay in output once load goes up to 2A. This delay will increase output settling time and could potentially slow down system. Figure 19 shows how the output will typically react when coming out of inverting (VOUT=GND) saturation.

    You can learn more about calculating offset error, output linear region, and more with our training videos located here:

    https://training.ti.com/ti-precision-labs-current-sense-amplifiers-gain-error

    Sincerely,

    Peter