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RMS circuit noise w/ 2100 and 2700 DMMs

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tstusr
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Joined: November 15th, 2011, 8:50 am
Country: United States

RMS circuit noise w/ 2100 and 2700 DMMs

Post by tstusr » November 15th, 2011, 10:13 am

We are attempting to use DMMs to measure RMS circuit noise of a low-noise analog sensor circuit, and are observing very different results between 2700 and 2100 DMMs. Any comment on why the two DMMs are measuring differently and suitability of the DMMs to this application are welcomed.

The signal path is:
Signal Source -> DC to 200 Hz Low Pass Filter (8 kohm and 0.1 uF) -> DMM measuring AC Volts
The output resistance of the amplifier, followed by the 8 kohm of the LP filter, is much, much smaller than the input resistance of the DMMs, so the DMM's contribution to signal noise is basically attenuated to zero.

We are attempting to use the DMMs because:
1) Frequencies of interest, 1 to 200 Hz (will accept lower limt of 3 Hz of DMMs) are too low to be measured by common spectrum analyzers.
2) Signals are too small in magnitude to be measured on an oscilloscope, the signal we are trying to measure after the low pass filter is approx. 20 uV RMS.

With the 2700, we configure it as:
- AC coupled, SLOW AC BW (3 Hz), Filtered, 6 1/2 digits, 100 mV range.
- we first connect both leads of 2700 to circuit ground, the 2700 displays some small offset value (usually around 200 uV). We press REL to zero out the display.
- then connect the input to the low pass filter output, and we observed close to 16 uV of signal.

If we attempt the same with the 2100, we get:
- AC coupled, SLOW AC BW (3 Hz), Filtered, 6 1/2 digits, 100 mV range.
- connect both leads to ground, 2100 displays 000.0000.
- connect input to low pass filter, DMM reads 000.0000.

Why does the 2700 appear to actually measure our signal, but the 2100 does not?
Are the DMMs suitable for this application?

Thanks,
Jim.

Dale C
Keithley Applications
Keithley Applications
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Joined: June 10th, 2010, 6:22 am
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Re: RMS circuit noise w/ 2100 and 2700 DMMs

Post by Dale C » November 16th, 2011, 6:03 am

DMMs in general are not very good for this application.
If you look at the ACV specs on the Model 2000 you will see note number 2. This states specs are for slow mode with sine wave inputs >5% of range.
This basically means on the 100mV range, the spec does not start until there 5mV of signal.
So anything below 5mV in unspecified.
The best DMM Keithley has for this is the Model 2001.
But it has the same limitation but to 1mV.

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