Hi! This is my first post, so I'll try to keep this brief as possible.

I have a small (~uV or lower) square wave signal (f~500Hz) which is on top of a much larger DC voltage (~50mV). I need to determine the amplitude of the square wave signal to high accuracy as part of my experiment.

Currently I am using a lock-in amplifier, which works well for strong signals (~uV). However the signal becomes undetectable below around 100nV. I have a DC amplifier (adjustable 100x to 100,000x gain) to amplify to the signal before I put it into the lock-in, however because of the large DC offset (~50mV) it overloads immediately.

So the question is: What is the best way to remove the background DC voltage without affecting the amplitude of the 500Hz signal? I have a very low-ripple computer controllable voltage source available. So if I measure the offset to be 50mV I can for example set it to output -50mV. I just do not know the correct way to apply the offset to the signal

Because of the low frequency of the signal, I guess a low-pass filter is not really an option here?

Thanks in advance!

 

hi SS,
Welcome to AAC.
One method that is used with higher level signals is to use a OPA to apply a cancelling DC Offset voltage signal to the source signal.
Which type of amplifier do you use on the nV signals.?
Do you have a diagram that you could post.?
E

 

Thanks for the quick reply!

The amplifier I use is this:
https://www.femto.de/en/products/voltage-amplifiers/variable-gain-100-khz-dlpva.html

The lock-in is this:
https://www.thinksrs.com/products/sr865a.html

Sorry, I am not familiar with the acronym OPA. I do not have a diagram to hand I am afraid.

 

I believe Eric means using an operational amplifier (OPA) as a differential amplifier

https://www.allaboutcircuits.com/textbook/semiconductors/chpt-8/building-a-differential-amplifier/

I am not sure how you can do it with a so miniscule signal

 

The amplifier I use is this:
https://www.femto.de/en/products/voltage-amplifiers/variable-gain-100-khz-dlpva.html

That amplifier has switch-selectable AC/DC coupling. Why not just set it for AC coupling? That would allow your 500 Hz AC signal to be amplified, and the ≈ 50 mV DC component to be ignored.

 

Thanks Doros. Got it. I am sure I could buy a differential amplifier of some sort which could do it if necessary. I just thought there may be some easy way to apply this offset to the signal.

Thanks OBW0549 as well. The AC coupling has a cut-off frequency of either 1kHz or 100kHz so is not going to work with my low-frequency signal unfortunately.

Basically I got the wrong amplifier for this application.

I realize in my original post I said "low-pass" filter when I meant "high-pass". I also now realize I may be able to it with a really low cut-off frequency high pass filter (like 0.1Hz). As long as I leave my experiment running for a minute or so before I measure I think this may work...

 

Why do you think you need such a low frequency cutoff?
For example, a simple 8Hz HP RC filter will be down ≈1mdB @ 500Hz (see below)
It has a settling time-constant of 20ms at start-up.

 

Thanks OBW0549 as well. The AC coupling has a cut-off frequency of either 1kHz or 100kHz so is not going to work with my low-frequency signal unfortunately.

Ummm...no. You are misinterpreting the specs for this amplifier: it has a high-frequency cutoff of either 1 kHz or 100 kHz; the lower end of its frequency response is 1.5 Hz. See the specs for "Lower Cut-Off Frequency":

 

Thanks for all the help! I think I know what to do / try.