Stuck when using 2 LCD panel meters in parallel

[efron]

Hello,

I'm working on a DIY power supply project and I'm stuck with a problem since long time ago now.

The issue is that I want to display the output voltage and current of the power supply. To do that, I use 2 LCD panel meters.

Currently I'm using these ones:

http://www.circuitspecialists.com/pm-128e-backlit.html

These panel meters can be configured in 5Vdc with common ground. This is the option I chose, although I also tested them with 9Vdc isolated power supply.

In any case I use isolated power supply, so in practice there is should be no problem with any common ground.

Both panels are powered in parallel and connected following the schematic below.

In this configuration, LCD2 is not displaying correct measurement. However, if I disconnect LCD1, LCD2 works fine (when standalone).

From this schematic, it can be seen the Vin- of LCD1 is directly connected to the Vin+ of LCD2. My best guess is that LCD2 does not appretiate this for some reason.

How should I proceed, keeping in mind that I would like to avoid to power both panel meters with 2 separate power supply?

I understood that, when using panel meters that don't accept the comm on ground, the Vin- is somehow linked to the V+ of the power supply. Is there something similar in a common ground configuration?

Any suggestion will be appretiated.

[Neverther]

The input of the panelmeters are not "true" differential inputs.
I can assume that when you measure ohms from the voltage meter in- to the power ground, it is short or very close to it.
The current meter most likely does the same so you are basically running parellel wire with the shunt resistor, as the power grounds are tied together.

Couple options:
1) Move the in- from voltmeter to ammeter ground and suffer 0.1V/A error in the reading.
2) Run ammeter from common ground with the supply, power voltmeter from isolated source.
3) Run both meters from their isolated powersupplies (kinda overkill).
4) Run ammeter from common ground, run voltmeter trough the shunt and trim the current used by the voltmeter out of the ammeter if allows it (I'm using this method on mine as it doesnt require isolated power for either of the meters).