Measuring current on a Voltage Rail > 10V

[rodwatt]

I am attemting to measure the current and voltage on a 15V power rail.  (Here is a copy of my circuit attatched) I am using a USB-6225 DAQ and Lab View,

 I have my device under test connected to a power supply. I have connected two differential channels to the DAQ, one for the current and one for the voltage.

I understand that the DAQ can not handle more than 10V so I use a voltage divider to half the voltage. When I run the measurements, the current reported is incorrect. It shows 10A when it should be about 500mA.

When I reduce the voltage on the power supply to 10V, it all works fine, so I am guessing its got something to do with the 10V limit.

Please can you advise how I can connect this up correctly?

[billko]

Why are you posting a hardware issue to a software forum?

[JÞB]

R(Sense) should be within 10 volts of the DAQ chassis.  Thats all the common mode rejection you are likely to have on a DAQ device.  (But check your device specification sheet ---- don't trust me RTFM!)

if you do not have a common "Ground" between the DAQ device and the supply you are monitoring  you will need an opto-coupler (But any first year EE student could tell you that)

As an Asside.:  Who the hell ever started the convention of stuffing R (sense) on the + side of a circuit anyhow?  Use a regulated supply with "Remote sensing"  if you really care about the DUT power consumption.

[RTSLVU · Accepted solution]

Do it this way.

[rodwatt]

Great, that worked!

Thank you very much for your constructive feedback and suggestion, much apprecited!

[AnalogKid2DigitalMan]

High side current sensing does prevent ground disturbances for sensitive or high speed PWM/Inverter circuits.

I think it can be used for additional diagnostic info that low side can't provide, can't recall.

EDIT: Forgot- with high side you can use multiple sensors to monitor current of multiple loads on a common ground buss.

Never had a need to use it though.

-AK2DM