I need to implement low-side current sensing for a H-bridge motor driver. While there are dedicated current sense amplifiers such as TI’s INA180, can I use a simple difference amplifier for this with equivalent gain of the INA180? What are the specific advantages of dedicated CS amplifiers?

There are issues with stock availability of the INA180 variant with specific gain in my part of the world. However opamps such as the MCP6001 are widely available and inexpensive.


What are the specific advantages of dedicated CS amplifiers?

Current sense amplifiers are designed to operate from a low source impedance, so they can have lower input impedance than opamps, which allows topologies that offer features not available to opamps.

Among these the most interesting is the ability to operate with input voltages way beyond the power rails of the amplifier. This is extremely useful for high-side current sense.

You also get convenience and cost features, like having good offset and CMRR without having to buy expensive matched precision resistors.

These features are very useful for a high-side current sense, but not for a low-side current sense. In low-side, you don't have much common mode so CMRR is not that important, and input voltage is close to ground, so you don't need an amp that can take inputs beyond the rails.

  • \$\begingroup\$ Thank you. Used in conjunction with a current sense resistor, an opamp should be sufficient then. \$\endgroup\$ Jan 21 at 8:19
  • \$\begingroup\$ many op-amps don't like having the inputs close to the power rails. \$\endgroup\$
    – Kartman
    Jan 21 at 9:18
  • \$\begingroup\$ Yes, you have to check the opamp datasheet for allowed input common mode range. Either rail to rail input, or input common mode includes negative supply. \$\endgroup\$
    – bobflux
    Jan 21 at 11:46
  • \$\begingroup\$ Was thinking of using the MCP6001 which has rail-to-rail input and output. common mode range is between Vdd + 0.3V and Vss - 0.3V. Absolute maximum Vdd - Vss = 7V. \$\endgroup\$ Jan 21 at 14:43
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    \$\begingroup\$ That's good. Note input offset max +/-4.5mV so take that into account. If offset is on the positive side, you get permanent offset on the output. If offset is negative, you get output stuck at 0V until current sense voltage is high enough to compensate for offset. Should not be a problem for motor current, this is not a super precision application. \$\endgroup\$
    – bobflux
    Jan 21 at 14:51

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