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Out of curiosity, can anyone explain to me why the input filters for the current sense amplifiers in the old ODrive schematics are unbalanced (i.e., the external resistor is only on the negative CSA input)? All the examples I have seen online have resistors on both inputs. Is there some sort of offset/compensation factor to consider? Here is a screenshot from the schematic:

enter image description here

These old ODrive boards (e.g., see the v3.5 schematic) use the CSAs within the DRV8301 motor driver IC. A simplified block diagram for the CSAs can be seen here (via datasheet): enter image description here

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It's hard to say without having a link to the chip spec, but I suspect it's because SP1 and SP2 are already connected to ground. R25 and C36 (for instance) probably serve to filter high frequency noise from the input. The inputs are likely high impedance and won't notice 120 ohms. Another resistor would be used on the SP1/2 inputs if they were on any other node, but if they're really ground there's not much point in it and leaving it out helps filter SN1 better.

I'm more curious as to why the SN* goes to the high side of the resistor and the SP* to the low side, but until I know what the chip is I can only guess.

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  • \$\begingroup\$ To clarify -- The chip is mentioned in the post with the datasheet linked: DRV8301; the current sensors are included as part of the motor driver. The shunt amplifier specs are on page 9 (section 6.6) and there is an application note on page 16 (section 7.3.2) with a block diagram. The application note mentions that "SPX should connect to the ground side of the sense resistor for the best common mode rejection." \$\endgroup\$
    – Murphy
    Commented Nov 7, 2023 at 23:22
  • \$\begingroup\$ Sorry, didn't notice the link first pass. The datasheet doesn't show any resistors between the sense resistor and amplifier inputs, further reinforcing the idea that this is just a filtering circuit, not a significant part of the amplifier tuning. \$\endgroup\$
    – Cristobol Polychronopolis
    Commented Nov 8, 2023 at 14:11
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can anyone explain to me why the input filters for the current sense amplifiers in the old ODrive schematics are unbalanced

Short answer - they are already balanced by internal resistors, Figure 6 (the one you posted) shows 5k resistors between each opamp input and the external IC pins. So, no need to add any other external resistors.

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  • \$\begingroup\$ Hmm…doing a quick calculation with ideal properties, the formula in section 7.3.2 seems to roughly match the figure shown (fig 6). Did I misunderstand your comment or am I missing something else? Quick Calcs \$\endgroup\$
    – Murphy
    Commented Nov 7, 2023 at 23:25
  • \$\begingroup\$ What I did not find in the datasheet is whether the output at SO1 is in phase or 180 degrees out of phase with the signal at the top of the sense resistor. With no current, SO1 will be at Vref/2. The question is: when positive current flows, does SO1 go up or down? Assuming SP1=0, SN1 +ve the formula gives SO1 going down (below Vref). So does Figure 6. OK, they are in agreement - my bad. Got so say that "-(SN - SP)" is a strange way to express the input voltage of an opamp, why not just write: +(SP - SN) ? I will edit my answer accordingly. \$\endgroup\$
    – Fabio Barone
    Commented Nov 7, 2023 at 23:47
  • \$\begingroup\$ Anyhow, I hope you see that I answered your main question re: balance resistors. Cheers. \$\endgroup\$
    – Fabio Barone
    Commented Nov 7, 2023 at 23:49

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