Current Sensor

[simon400]

Thanks for the reply, but I didnt understand "matching 4 resistors to 0.1%" for building from conventional opamp. Is the connection different from the normal noninverting amplifier?

 

[MikeMl]

Thanks for the reply, but I didnt understand "matching 4 resistors to 0.1%" for building from conventional opamp. Is the connection different from the normal noninverting amplifier?

Yes, quite different, pun intended. A differential amplifier is simultaneously an inverting and a non-inverting amplifier at the same time.

Also look at this spec sheet for a TI INA118. It is far better than one you can build...

The big problem for you is you are trying to amplify a mV difference between the ends of the current-sensing resistor at the same time that the resistor is being switched from near ground to tens of volts above ground. This requires a differential amplifier with the ability to amplify the difference signal while ignoring the common-mode switching signal. To get this degree of common-mode rejection, the resistors have to matched to better than 0.1%. The INA118 referenced above does this by laser trimming the resistors inside the IC.

 

[simon400]

Thanks MikeMI, will try that out

 

[Mr RB]

Ummmm.... Put the sense resistor from the bottom of the h-bridge to ground, so one end of the resistor is always grounded.

Unless you are using pwm or regenerative braking??