How measure differential uV signals on larger bias/offset voltage?

[userx2]

Hello,

I am looking for a recommendationon how to measure these signals on the bench:

I have a bridge sensor with a tolerance offset/bias of up to +/-200mV.
But the actual user induced signal is in the range 20uV to 800uV.
This then sits on top of the offset.

I have a Fluke multimeter with a 600uV range, ideal for this, but because of the large DC bias, it is out of range.
My oscilloscope has the same issue, even with a differential probe.
AC coupling is no an option since the signals are DC basically.

Best regards
X

 

[KlausST]

Hi,

I´m a bit confused.

You say "differential" .. thus I expect two signal lines.

How to understand the given values?
You say the bias voltage is DC +/-200mV. If it is DC, how can it be +/- then? Does it fluctuate? If yes, with what frequency?
Is this bias voltage "common mode" or "differential mode"?
And like any signal .. also the bias signal will contain some noise and drift. What about this?

You say the signal of interest is DC, too. Obviously this signal needs to be "differential mode".
Again here there will be noise.
And - since it is to be measured - it will not be stable. It will change with time (somehow in the range of 20uV to 800uV). But if it changes the signal is not pure DC .. there needs to be a frequency .. for it to change. You may call it time constant. What is known about the signal timing. And btw: it is positive only?

****
My first thought was:
Use a difference amplifier to get rid of common mode noise.
So that you have now only the differentail one.
Then if there is some offset: Subtract a constant voltage.
But if both voltages fluctuate ... and the "unwanted" is much bigger than the signal of interest .. you can never be sure where this fluctuation comes from.

****

A sketch surely would be helpful.
Also what kind of sensor it is and where the bias comes from and where the signal of interest comes from.

****
Since just a day ago a member was annoyed that I had a question about his application (While he claimed he is the only one to ask questions) .. please don´t respond at all if you are annoyed by my post. ;-)


Klaus

 

[userx2]

Hello,

Yes, 2 outputs, differential.

DC offset depending on tolerances of manufacting for resistive bridge.
Sensor has a Wheatstone configuration internally and it is a strain gauge.

The signal of interest for testing in this case is the total differential voltage down to uV range, including any bias/offset it has.

To rephrase my question,
How does one measure 200mV differential accurately to uV range?
Or for argument sake, 1V voltage to uV accuracy?

In this case, I do not need to have the absolute value accurate, just how much it changes from a starting point.
I can also probably get away measuring 1 of the 2 diff. signals if that is easier to achieve.


My latest thinking is that one may need an external offset generator which can be added serially into one of the test signals. That then to be manually adjusted to compensate the actual circuit offset enough so a uV meter can be used in range. Such a circuit will however need to be very clean and stable.

Is there a suitable meter out there that can already do this?
It would need a zeroing button to compensate offset and then measure uV with that setting.


Best regards
X

 

[KlausST]

Hi,

a strain gauge in wheatstone?
--> Simply add a resistor with a suitable value across two sensor pins (one input, one output) to compensate the offset to below 1mV or so.
You may use a pot.

Klaus

 

[FvM]

Is there a suitable meter out there that can already do this?
It would need a zeroing button to compensate offset and then measure uV with that setting.

Most bench top multimeters with sufficient resolution can do this, e.g. 6.5 digits Keysight 34460A.

 

[userx2]

Most bench top multimeters with sufficient resolution can do this, e.g. 6.5 digits Keysight 34460A.

Thanks.
I just had a look at that meter datasheet. It has a 100mV scale which is too small a range for the measurement at hand. But the 1V range looks like it may do it.

Keysight 34461A Meter datasheet

Best regards
X

 

[userx2]

I have now persuaded management and just placed an order for a 34465A meter.
I am moderately excited again as I love test equipment
Oh yes and also because I think it will do the trick here with this project.

Thank you FvM. I have not ever personally used such a meter nor though of it.

Best regards
X