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In all projects like that I found and in the application note of DRV8302 the shunt resistors for current measurement are placed on the ground connection of the transistors. Why not place them in between them and the motor, so that they could be used to precisely measure current in both ways?

A followup question: When using a hall-based current sensor does it make sense to place it on the motor side? It is galvanically isolated so ground referencing shouldn't make a difference.

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    \$\begingroup\$ So that they're ground-referenced? \$\endgroup\$ – Ignacio Vazquez-Abrams Jan 16 '16 at 16:20
  • \$\begingroup\$ There are differential amplifiers inside the DRV8302, so in my opinion this is not the reason. \$\endgroup\$ – Chumanista Jan 16 '16 at 16:27
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    \$\begingroup\$ Your opinion is wrong. In section 6.7 of the datasheet, it clearly states that the common-mode input range is just +/- 0.15 V. \$\endgroup\$ – Dave Tweed Jan 16 '16 at 16:49
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schematic

simulate this circuit – Schematic created using CircuitLab

Figure 1. H-bridge output stage showing feedback for first channel.

As can be seen from the sketch, ground referencing the current monitor makes it very easy to monitor the signals. They will be low voltages and easy to amplify, manipulate or feed into an ADC.

The alternative of high-side current monitoring requires differential amplifiers, high-voltage protection and possibly opto-isolation. While it may be do-able, it's more complex.

Data sheet page 4 says, "Input of current amplifier 1 (connecting to positive input of amplifier). Recommend to connect to ground side of the sense resistor for the best commom mode rejection."

I can't quite make out from the data sheet what the max voltage is for SN1, 2 and 3 but a lot of the analogs seem to be 7 V max.

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  • \$\begingroup\$ the question was about measuring not the high side current but the driven motor current. I expect that would be even harder to do well. \$\endgroup\$ – Jasen Jan 17 '16 at 3:20
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The relatively large common-mode swing w.r.t. the differential signal makes it very hard to preserve accuracy when the motor terminal currents are sensed directly. Using an opamp difference amplifier requires extremely high precision and common-mode rejection to achieve satisfactory performance. In some cases, separate amplifiers may be used to sense the current during low V and high V drive modes.

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