Hi,

I am using a panel mounted digital voltage meter to measure the output of a op amp. The op amp is powered by a 12v linear power supply. When I use a 9v battery to power the voltage meter, everything works fine. When I power the voltage meter with a switch mode 9v power supply, the meter reading goes haywire.

I think this is related to some common reference between the two power supplies. Is there anything I can do to solve this problem? I don't want to need to use a battery to power the meter.

Thanks,

 

Did u try decoupling the supply lines

 

I don't know what you mean

 

the two power supplies are not connected to each other in any other than through the 110 VAC side and their mutual connection to the voltage meter.

 

Connect a 100n ceramic caps at the output of the SMPS.

 

Connect a 100n ceramic caps at the output of the SMPS.

I should just connect it across the DC output on the supply? Can you please explain what this does?

 

Connect it across the SMPS out put.
it will ground the HF noise in the circuit. It could be the switching frequency that is coupling into the meter

 

Thanks, I read up on the concept. I will try to connect it tomorrow and let you know how it works out.

 

cool.
IMHO DMM's do not like switching noise in the supply. It interferes with the ADC, the sampling , u know what I mean

 

Are the 2 PSU grounds commoned? If not they should be. The supplies could be floating. The same would be true for the battery as well but you may have accidently grounded the case of the battery effectively commoming the grounds so it would work fine.

 

I don't think I should connect the grounds on the two supplies. I am fairly sure that the meter's power source cannot share a common ground with the signal that it is measuring. In fact, I suspect that some relationship between the two signals is somehow causing the problem I definitely didn't have the battery grounded--and it worked perfectly.

Another notable fact is that when I leave the SMPS connected as the source to the meter, and I apply voltage to the meter with a lab bench power supply, it seems to work.

Would this be consistent with the coupling issue? It doesn't seem like the coupling issue would cause problems if the SMPS was connected regardless of the measured signal. I only have a problem with the combination.

 

U need a common ground in either cases

 

Is this one of those inexpensive eBay meters that requires a separate isolated power supply? If so that's your problem, they can't share a common ground.

I bought a few by accident, just got a few isolated DC-DC converter modules from Mouser and that solved it.

 

Is this one of those inexpensive eBay meters that requires a separate isolated power supply? If so that's your problem, they can't share a common ground.

I bought a few by accident, just got a few isolated DC-DC converter modules from Mouser and that solved it.

I don't understand. I am using separate power supplies. Do they communicate through their 120 side somehow?

It is a fairly expensive meter from Newark. I called them and told them that I wanted a meter that could run on a power supply that has a common ground with the signal it was reading--because originally I wanted to do it that way, but they said that they couldn't promise that any of their meters could be used that way.

 

Earth and ground are two separate things.

U need a ground connection to measure the voltage .
A ground is needed in the same way as you need a negative and a positive to measure the voltage

An earth is used for protection

 

Clearly, they both have their respective grounds. The problem is that if I measure between the 9v smps and earth, I measure half its voltage. The op amp signal is also referenced to earth.

Is there a way to isolate the smps from ground? This is happening through the 110 vac side somehow. Maybe I need a different ps.

 

Does the opamp use dual supplies?

why don't u show us ur work

 

Clearly, they both have their respective grounds. The problem is that if I measure between the 9v smps and earth, I measure half its voltage. The op amp signal is also referenced to earth.

Is there a way to isolate the smps from ground? This is happening through the 110 vac side somehow. Maybe I need a different ps.

Could be, your current PS may be one of those transformerless designs that simply reduces the voltage. They're generally sufficiently isolated to be safe and work fine for things like charging cell phone batteries but their output related to actual earth ground may be floating in some manner.

In theory you should be able to connect everything into a common ground situation but your current PS may not be as noise free nor function as expected in producing a pure DC output.

 

Here is a very interesting idea......

POST A SCHEMATIC.

That way we can see whats happening. I think you are confusing the grounds. Ignore the mains side of the circuit. Pretend it doesn't exist.

You have 2 DC power supplies that have a positive and negative output. In MOST circuits you would conect the 2 negatives together. Without seeing your schematic we don't know if there is some strange connections going on.

 

Attached is a block diagram of the circuit. I didn't think it was necessary to include the specifics of the amplifier. The voltage meter clearly states that the measured signal cannot share a ground with the meter's power supply. The negatives of the power supplies are not connected directly--although I can measure a voltage between them, and I assume that this relationship is the cause of the problem.