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Keithley 2100 and 175 - Problems with low AC measurements

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baswan123
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Keithley 2100 and 175 - Problems with low AC measurements

Post by baswan123 » December 13th, 2011, 12:32 pm

I am having Keithley 2100 and Keithley 175 for measuring low frequency AC voltage. The voltage used is 10Vpp Sine wave AC, 20Hz and the measurements which i am taking go down in range of 0.5-3 mV (rms). I am having some trouble with the two meters showing me different readings.

I used the same meters for measuring - DC output (voltage used AC square wave 10Vpp, 10mHz) and it gives me consistent results.

Is it rated good to make AC sine wave low frequency (20Hz) measurements down to millivolts range?

Nathan S
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Re: Keithley 2100 and 175 - Problems with low AC measurement

Post by Nathan S » December 13th, 2011, 1:15 pm

Running the specification calculations for each instrument (assuming a 3mV reading as that is your value there):

2100
+/-52.4uV

175
+/-230uV

Edit: These are for the instruments on their lowest respective ACV ranges

So this could be part of the discrepancy. However, the 175 has been obsolete for 20 years. This all depends on keeping both instruments on an annual cal cycle and assuming that none of them are in need of repair.

baswan123
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Country: United States

Re: Keithley 2100 and 175 - Problems with low AC measurement

Post by baswan123 » December 16th, 2011, 12:20 pm

If I would send in the instrument for calibration, what is the lowest voltage measurements in mV (for AC low frequency 20Hz) will it be calibrated for?

Also, does the error decreases if it reaches 10mV when compared with 3mV?

Dale C
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Re: Keithley 2100 and 175 - Problems with low AC measurement

Post by Dale C » January 17th, 2012, 6:05 am

Please look at note 1b on the Model 2100. It states the accuracy spec does not take affect on the 100mV range until 5% of the range.
This means that it is only accurate to 5mV. That is where the spec starts. Anything below that is unspecified.
Also look at the Model 175. It has a similar spec for ACV. But it states above 1800 counts. Which is 18mV.
So anything below 18mV is unspecified.
The best ACV measurement from Keithley is the Model 2001.
The Model 2001 also has this type of spec for ACV but it is 1% of range. ON the 200mV range this would be 2mV.
Anything below 2mV is unspecified.
On your 10V measurement the standard accuracy spec would apply.
But not on the lower end of your measurement.
On the 2100 the spec does not apply below 5% of the 100mV range or 5mV.
On the 175 the spec does not apply below 18mV.,
On the Model 2001 the spec does not apply below 2mV.

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