high voltage differential measurement

[Blane63580]

Not sure if this is the best place but here goes.

We are trying to measure a differential voltage across a shunt in a offset voltage that is higher then +/- 10V most of the DAQ cards measure. Below is a representative schematic of our circuit,:

We also have to measure 24 of these shunts while we are moving the motor in one direction or the other. Being that in one case the voltage offset is close to 12 volts (Range 9 to 16Vdc) that eliminates all of the cDAQ cards and it looks like the PXI series also. We are presently using a SCXI 1104 attached though a USB to a portable computer for this measurement. The SCXI 1104 has a voltage range of +/- 60V and I have been unable to find anything close to that in the NI product offerings. The reason that this is going to be a problem for us is that corportation wide we are being forced to move to windows 7- 64 bit and the SCXI devices are not supported for that operating system.

I am open to any suggestion that others may have to help us correct this problem.

Thanks

Bill Lane

[crossrulz · Accepted solution]

We ended up finding some op-amps that were specifically made for high-side shunts.  Do a search on digikey for high-side current shunt amplifier and you should be able to find something.  These will turn the measurement into a single ended voltage measurement.

[johnsold]

Another possiblity is a Hall Effect device such as the Alllegro Microsystems ACS756. These are isolated between the primary current path and the secondary voltage output, have very low resistance in the primary path (130 micro-ohms at 50 or 100 A), and have bandwidths to about 100 kHz. They cost <$10 US in small quantities.

Lynn

[Process_Systems]

Another possibility, move the shunt(s) to the negative side of your 12Vdc supply, and have the neg. term of your supply be the common measurement terminal for all channels. This assumes that you use the same 12Vdc supply for all motors.

[Blane63580]

We use this method for another tester where motor direction is control though a PWM signal. The problem is that the voltage can be either positive or negative depending on the direction of the motor, so this method won't work for this product.