Digital Panel Meter doesnt work with a power supply.

[msngupta]

Hi,
I am using a Digital Panel meter (http://www.vellemanusa.com/products/view/?country=us&lang=enu&id=350724) which requires 9V supply, and an input voltage whose value is displayed (connections: V+, V-, 9V+, 9V_GND). The source of the input voltage is from my circuit which is powered using a linear power supply. The panel meter shows the correct reading when I power the panel meter using a 9V battery, but when I connect it to the power supply with 9V(variable power supply which is independent of my circuit) the panel meter starts showing crazy values which doesn't correspond to my input voltage. What is the difference between the power supply and the battery? How is it not working with an independent supply ?
The power supply is a variable voltage supply which I am operating in CV mode and which is independent from my rest of the circuit.

Things I have tried but failed:
1) I connected both V- and 9V_GND, still the problem persists.
2) Added a series resistance(100ohm, 1K, 10K, 1M) between supply and the panel meter to limit current but didn't solve the problem.
3) Even added a parallel resistor (shunt) but no effect.
4) Connecting/Disconnecting V- and 9V_GND not effect
5) I even tried to use the power from the supply which powers my rest of the circuit, but no luck

So, why is it working with a 9V battery but not the supply ?
Can anyone give me suggestions on whats going wrong here ?

Thanks in advance,
Sainath.

[electr_peter]

Hi,

Problem could be that power source for panel meter is not completely isolated. Problems with panel meters that require isolated power were discussed on this forum i-keep-on-blowing-up-panel-meters!

Can you describe circuit, circuit power and all connections a bit better or add a schematic? You should check that power supplies are completely isolated (not earth referenced), especially for panel meter.

Another problem could be that variable power supply is very noisy or has high leakage current to earth (which causes common mode current). What variable supply do you use? Is it properly grounded?

[Andy Watson]

The inputs V+ and V- need to be within the common mode range of the meter. A battery is an ideal floating supply - anything else will probably have sufficient leakage to drag the V+ V- inputs outside of the meter's common mode range.

These meters are very likely to be based on the 7106 - have a look at these datasheets:
http://www.intersil.com/content/dam/intersil/documents/icl7/icl7106-07-07s.pdf
http://www.intersil.com/content/dam/intersil/documents/an96/an9609.pdf