Products > Test Equipment
Some old school instruments showing how it's done (HP 3325A and Fluke 8506a)
SilverSolder:
--- Quote from: joeqsmith on January 30, 2021, 01:15:37 am ---After 24 hours, the capacitor discharged to 6.8V. LSD is basically flat.
--- Quote from: SilverSolder on January 29, 2021, 10:10:22 pm ---
--- Quote from: guenthert on January 29, 2021, 06:05:07 pm ---
--- Quote from: joeqsmith on January 29, 2021, 12:59:33 pm ---[..]
With that 10Gohm input impedance my little cap is not discharging very fast. After 15 hours its down to 7.4 volts. The LSD bins continue to look fairly even.
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I haven't read the specifications of the 8505/6A, but generally those long(ish) scale DMMs specify an input resistance of >10GOhm (in some ranges, which should include 10VDC). That's only a nominal U/I ratio, not to be understood as plain resistor. There will be some bias current of the FETs at the input leaking out and the AZ circuit's capacitance will want to be fed, etc. . At 10V you should expect an average current of less than 1nA (it might be less than 100pA, but peak values might very well be considerably higher), hence the nominal >10GOhm.
The bias current means, that a low-leakage capacitance on the input will actually get charged (not necessarily towards +10V), which can be used to determine the average current.
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The specification is max 5pA bias current. I measured mine to around 3pA with an electrometer. I've tried to look at the circuit diagram to figure out how they get it so low, but I'm not understanding it... analog black magic overload with stacked dual FETs etc.
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I used a new 9V transistor battery in series to try and get the noise low enough to make some sort of ballparkish measurement. With my old electrometer placed in the lowest range (20pA), standing back and let it settle looks like around 1.5pA. Any sudden movements, over 2pA. Walk away and don't twitch, 1.33pA. I can believe it's <5pA.
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I also used a 614 electrometer, and had the same user experience of having to look at it from across the room to avoid disturbing the measurement! :D
What is the principle behind putting a battery in the circuit to help noise?
joeqsmith:
I was just trying to get a somewhat close measurement. A battery is a fairly low noise source and I could place it inside the metal enclosure to minimize the lead lengths.
guenthert:
Don't know about the 614, but the 617 would be too slow to reliably catch the peaks of the input bias current of a DMM. The 617 has an analog output (amplifier output, not just a synthesized analog signal, as e.g. in the Keithley 181 nanovoltmeter) which could be used to connect a oscilloscope.
bdunham7:
--- Quote from: guenthert on January 31, 2021, 12:18:07 am --- Don't know about the 614, but the 617 would be too slow to reliably catch the peaks of the input bias current of a DMM. The 617 has an analog output (amplifier output, not just a synthesized analog signal, as e.g. in the Keithley 181 nanovoltmeter) which could be used to connect a oscilloscope.
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If it doesn't use autozero, why would there be peaks? The only meters I've seen that have issues in that area (and can cause issues when connected in parallel with other meters) are the ones with continuous autozero. My picoammeter is also too slow to catch anything but the slowest of peaks, so if there is some other mechanism that caused them I'd be interested to know what it is.
SilverSolder:
--- Quote from: guenthert on January 31, 2021, 12:18:07 am --- Don't know about the 614, but the 617 would be too slow to reliably catch the peaks of the input bias current of a DMM. The 617 has an analog output (amplifier output, not just a synthesized analog signal, as e.g. in the Keithley 181 nanovoltmeter) which could be used to connect a oscilloscope.
--- End quote ---
614 does have a "real" analog output as well, but I didn't think to trace it on the scope to look for peaks. Another interesting project...
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