Hi all,

I am planning to take Shunt output which is in the range of 1-20mV range. It measures the motor Current which is approximately 2Amps. Need to tap the shunt and give to amplifier in DAS which is 10meters away. Kindly suggest which gauge twisted pair to be used or can I use Thermocouple wires.

 

10 meters isn't a long distance at all for a 4-20mA signal especially. I would NOT use thermocouple wire but a properly shielded analog cable.
A larger issue is isolation. What's the motor voltage? If you're using a shunt resistor (a resistor in series with a hot phase to the motor), it's going to be at line voltage. Your DAS will most certainly not like that.

 

Hi Joseph,

My DAS has an DATAFORTH amplifier which converts MilliVolts to Volts. Since it is a backplane amplifier I need to use shunt resistor. Motor peak current is 2A and we are using 75mV/10A shunt resistor. So, the drop across shunt is sent to the DAS Amplifier.

 

1. Cable is called STP, Shield Twisted Pair. Accept no substitute for a low level mV signal. Hopefully there is a ground connection for the shield at the analog input.

2. The issue Joseph_E2 is trying to alert you to is a common mode voltage issue. Your amplifier card almost certainly references earth ground for where it draws its power. Any voltage potential difference between the low end of the mV signal and ground on the amplifier card is common mode voltage.

If the map has a differential input, then the DAS amplifier card probably has some limited ability to deal with common mode. A low level of common mode voltage it can likely handle, a moderate common voltage will saturate the amplifier so it is full scale output (plus or minus, depending on polarities) regardless of the input, and at some point a high enough common mode voltage will fry the amplifier, that is, smoke will come out it.

To test the amplitude of the common mode voltage:
- measure the DC voltage between the AI (-) and the Low side of the shunt resistor with a DVM.
- measure the AC voltage between the AI (-) and the Low side of the shunt resistor with a DVM.

Check with the amplifier vendor to see how much common mode the amp can tolerate.