CIRCUIT060007: Bidirectional low-side op amp configuration questions

Hi Lucas,

why not running a TINA-TI simulation? A TINA-TI reference design of OPA313 can be found here:

By the way, has this thread to do with this one?

Kai

Hi Lucas,

I have a few questions concerning the configuration of the op amp in this application as well questions surrounding the choice of op amp.

1. Would there be consequences of swapping the inputs to U1? (positive input to ground side of the sense resistor and negative input to high side of resistor)

Yes. The feedback resistor would be connected from the OPA313 U2 output to the non-inverting input; hence, positive feedback. And the +1.65 V common-mode voltage established by OPA313 U2 would be summed into the inverting input, which would attempt to cause the output voltage to be below 0 V.

2. How does one go about choosing an op amp with proper bandwidth, slew rate, ect for this application? Say for low-side bidirectional current sensing in a motor control application where one wants to use an op amp for cost purposes. 

Certainly the ac characteristics of the op amp would be based on the input and output signal performance requirements. Many motor control applications are relatively slow in speed compared to the bandwidth and slew rate of the op amp. Nevertheless, once the input and output signal requirements are fully defined, then an op amp having the necessary ac characteristics to support the circuit performances can be determined.

Regards, Thomas

Precision Amplifiers Applications Engineering

Kai,

Thanks for the advice to simulate it. I did exactly that. Yes this does have to do with that other question.

Thomas,

I've simulated my circuit with the inputs to op amp connected as I have described. The op amp is sensing the voltage across a 0.5mOhm resistor with a 40A 40kHz sinusoidal current being injected through it. The feedback resistor doesn't change location. It's only how the op amp connects to the sense resistor that is swapped. So does this seem ok?