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INA240: Common Mode Transient Rejection Comparison (INA240 vs. INA186)

Part Number: INA240
Other Parts Discussed in Thread: INA186

Hey Team,

In the DS for the INA240, Figure 21 shows a ~500mV output response for 0 to 90V transient.

In the DS for the INA186, Figure 14 shows a ~100mV output response for a 0 to 40V transient.

  1. Does that mean that the INA240 will respond at ~250mV for a 0 to 45V transient, which is higher than the INA186 under similar conditions?
  2. Do we have data that can show better PWM rejection with the INA240? This would be for an automotive application.

  3. Should my customer be looking at a different parameter when comparing the common mode transient rejection capabilities?

Dajon McGill

  • Dajon,

    The common mode step in the INA240 datasheet shows a 0V to 80V step, not 90V. The reason this device shows an 80V step and the INA186 shows a 40V step is because these are the worst case common modes that these devices are able to handle in a high side design, respectively. Therefore, the steps exhibited in the datasheet would potentially be the worst behavior we would expect the device to exhibit as it slews up or down.

    1. You cannot really make this assumption with certainty, as CMRR is a curve over frequency, similar to PSRR and gain. The response the device exhibits will be equal to the sum of frequencies of the common mode pulse, adjusted by the CMRR curve. In short, it depends on the components of the curve, and not all common mode signals will slew at the same rate. Typically this is where the INA240 shines. Examine how quickly the device handles these transients, not just the magnitudes of the rejection. If we examine the scaling for the INA240, its 250ns/div. This DS curve shows that for an 80V step, the INA240 has rejected the common mode transients and has resumed steady state operation in about 1-1.25us.

    That said, one more thing to point out here from your question: the INA186 is rated for a 40V common mode maximum. If subjected to a 45V common mode, you would risk damaging/breaking the device.

    2. There is an app note that shows the PWM rejection of the INA240 against several of our competitors located here. Is this something along the lines of what you're looking for?

    3. The Vcm transient response is a good tool to examine when comparing CMRR, but its important to note that the PWM rejection portion is made for higher dV/dt applications. Do you know the customer use case here. i.e, what is the application, common mode they are planning to utilize, etc.?

  • Hey Carolus,

    Thank you for the detailed response, however, I am still slightly confused.

    The datasheet you mentioned here shows a similar scenario to what we are talking about. It looks like a 0-40V PWM causes a +/-250mV response on the output, which would roughly align with the customer's linear interpolation (e.g. 500mV from 0-90V => 250mV from 0-45V). '

    In this case it looks to me like the INA186 would respond with a smaller output response, at something like +/- 100mV.

    What am I missing that would cause the INA240 to respond better than the INA186 in this scenario? Is the benefit that 240 would respond in a smaller amount of time?

    Dajon McGill

  • Dajon,

    It depends on what you mean by "respond better" here. 

    First, again, you cannot "linearly interpolate" the amplitude of a CMRR response from amplifier to amplifier. The resultant amplitude would be the summation of all harmonic frequencies present in the step response waveform attenuated by the CMRR response of the device and summed together. The fact that in this example these amplitudes are roughly in line is simply sheer coincidence. This may not be the case for a device that say, steps faster (i.e., more high frequency content in the waveform).

    Second, you are correct in that the response should also be examined from a time perspective as well as an amplitude one. The INA240 is able to handle the unwanted content of an 80V common mode step in about 1200ns, where it looks as though the INA186 would need longer than this based on the time division of the scale.