Single-ended vs pseudo-differential

[JimPierce]

Hi,

I need to connect a signal source to a DAQ device and I have some questions with regards to the basics of using single-ended/pseudo-differential connections.

- I have a signal source that has a small DC offset (Vcm) relative to circuit ground for the DAQ device. There's also a small AC common mode component (Vnoise) which could represent noise coupled into the wires connecting the signal source to the DAQ.

- The first diagram I have attached shows how I would connect the signal source to the DAQ using a single-ended connection. Second diagram shows how I would make a pseudo-differential connection.

Questions:

1. It's clear to me that in the case of the single-ended connection, the DAQ will measure the sum of Vsignal, Vcm and Vnoise - the signal will be indistinguishable from the noise and offset. As I understand it, this shouldn't be the case when using the pseudo-differential connection (the DAQ should measure Vsignal only), although I've read several places that pseudo-differential connections only reject common mode signals which are DC:

**broken link removed**

Why won't it reject the AC common mode component aswell? As far as I understand, the DAQ should just measure the voltage difference and any common mode component will be rejected, be it AC or DC.

2. In the case of the single-ended connection, if I only route a single wire between the signal source and DAQ device, and route no return wire close to the signal wire, won't I make a potentially huge loop for magnetic flux to penetrate and couple noise into? Should I always route a return wire, even though it's a single-ended connection, close to the signal wire and connect it to the DAQ ground? I know this will create a ground loop, my question more relates to the necessity of a return wire.

Any help will be greatly appreciated!
Thank you.

Edit:
I understand a pseudo-differential connection to be where you connect the reference of a signal source to the negative terminal of the DAQ and the signal to the postive, or the other way around depending on the polarity you want. I assume a fully differential connection would be where you, at the signal source, convert the signal to the difference between two signals 180 deg out of phase and transmit them using two wires to the DAQ inputs. I just don't understand why this is better than the pseudo-differential case.

 

[godfreyl]

....I've read several places that pseudo-differential connections only reject common mode signals which are DC:

**broken link removed**

Why won't it reject the AC common mode component aswell? As far as I understand, the DAQ should just measure the voltage difference and any common mode component will be rejected, be it AC or DC.

Your second circuit will reject any common mode component, AC or DC. The "Pseudo-Differential" circuit in the article you linked is very different to your circuit.

 

[JimPierce]

Yeah, you're right. What would you say a pseudo-differential connection is? I can't seem to find examples that agree with each other.

Also, what is great about a fully differential connection compared to my type of "pseudo-differential" connection? I understand that you can get a greater voltage swing if you transmit a signal as the difference between two waveforms 180 deg out of phase, but is that the only advantage? Seems to me that they should have the same degree of noise rejection if the noise gets coupled in as common mode.

 

[godfreyl]

Your second circuit should be as good as a fully differential connection, but be careful of impedances. Remember, the output impedances of the signal source and the input impedances of the DAQ form voltage dividers, so they have to be the same for the inverting and non-inverting inputs if you want good common mode rejection.

One easy way to get it wrong is to have a coupling capacitor at the output of the signal source.