Digital Panel Meter Confusion

[Kevin Walton]

Hi

I have purchased a couple of Lascar Digital Panel Meters, see the data
sheet here:

http://www.lascarelectronics.com/data/pdf/V 125.pdf

http://www.lascarelectronics.com/PRODUCTS.CFM?STOCKNO=V 125&CFID=1412915&CFTOKEN=65281098#

I was trying to treat them as a standard analog meter, at 200mv FSD
and a 2v max input signal, simply a 10:1 resistor network to achieve
the result I needed. However, I didn't realise the confusion that the
supply voltage would cause, issues with a single ended supply etc.
Basically I have played with them for hours and am now very confused!

My application is measuring the RSSI signal from an LPRS radio module,
powering the panel meter from the same supply as the module. The LPRS
module uses 0v and 5v direct from the USB, I have made up a double
circuit (using a step up reg) which gives me 0v and 10v for the
module.

I connect InLo to 0v, InHi to the middle of the resister network (1M,
100K), the bottom of the resistor network to 0v and the top of the
resistor network to RSSI. I get a nice 0.5v on RSSI, 0.05v on InHi
but the meter seems to float around 0.008 to 0.011 regardless of InHi
connected to 0v or the middle of the resister network!

L1 is open, for a single ended voltage.

What am I doing wrong? All comments and suggestions welcome.

Cheers
Kev

 

[nospam]

Hi

I have purchased a couple of Lascar Digital Panel Meters, see the data
sheet here:

http://www.lascarelectronics.com/data/pdf/V 125.pdf

http://www.lascarelectronics.com/PRODUCTS.CFM?STOCKNO=V 125&CFID=1412915&CFTOKEN=65281098#

I was trying to treat them as a standard analog meter, at 200mv FSD
and a 2v max input signal, simply a 10:1 resistor network to achieve
the result I needed. However, I didn't realise the confusion that the
supply voltage would cause, issues with a single ended supply etc.
Basically I have played with them for hours and am now very confused!

My application is measuring the RSSI signal from an LPRS radio module,
powering the panel meter from the same supply as the module. The LPRS
module uses 0v and 5v direct from the USB, I have made up a double
circuit (using a step up reg) which gives me 0v and 10v for the
module.

The spec says inputs must be no closer to the supply rails than 1v. Your
input is almost at the -v supply rail.

Simplest solution would be to change your voltage doubling supply circuit
for a voltage inverting circuit and run the meter from + and - 5v supplies.

 

[Kevin Walton]

The spec says inputs must be no closer to the supply rails than 1v. Your
input is almost at the -v supply rail.

Simplest solution would be to change your voltage doubling supply circuit
for a voltage inverting circuit and run the meter from + and - 5v supplies.

Ahhhh, ok, I was thinking about the 10v rail and didnt think about -v
being 0v. One voltage inverting circuit on it's way....

Thanks
Kev