Electronics > Metrology
Scanner/Multiplexers for voltage references
dietert1:
Meanwhile i started using the USB device with the scanner. The scanner currently has three input cables on channels 9, 10 and 11. I checked the wiring with an external voltage source, yet for the purpose of calibration they have a short on them. All other channels have shorts inside the scanner.
Once more i used channel 1 for continuous recalibration of the K2182A offset. With three days of data results are similar to what i got in 2022 and the calibrated scanner with the Keithley 2182A operates with < 1 nV stdev when averaging 10 scanner rounds (800 seconds), on all 15 channels including the three channels with external short.
Next level of evolution might be substituting the USB host connection by a fiber ethernet interface, powered from the K2182A.
Regards, Dieter.
dietert1:
That ethernet fiber interface is on its way, but will need some time.
For the time being i ordered a no name USB isolator at ebay. It has a ADUM3160 inside and is capable of full speed, which is good enough. It also includes a small isolated DC-DC "BSPSU B0505S-1W". The isolator runs the STM32 based GPIB and scanner controller without any additional 5V supply.
Today i took a closer look. Capacitance between both isolated ends is 42 pF, similar to a small two chamber mains transformer. After unsoldering the converter with only the ADUM3160 left there is a coupling of 3 pF - similar to a shielded mains transformer. While running without load, the DC-DC injects about 200 mV at 50 KHz with lots of RF content into the isolated side and the 1:10 scope probe. Output voltage was measured inside the GPIB controller to be 4.7 V. Enough to work but not ideal.
Regards, Dieter
Kleinstein:
Those cheap DCDC converter blocks are often rather poor with common mode injected signal. It may be worth to just disable the DCDC and have your own 5 V supply from the meter side.
For controling your own scanner part, one should not need a high data rate and no need to go to GPIB in between. USB - UART (RS232) and than isolation for the UART should be enough.
dietert1:
The GPIB port is for collecting the nanovoltmeter measurement data, if that was your question. The USB connection works as host connection for data storage. And yes, i am currently studying a small, low common noise 5 V supply. The Kniel C5.3 i recently got is good, yet a bit large. Hopefully the ADUM3160 behaves without isolated side supply.
Maybe i will also test some USB fiber link but probably it will become an ethernet fiber connection. The fiber transceiver and the DP83822 PHY consume about 1 W. I plan to reuse that design for the Prema 6048 remake.
Regards, Dieter
dietert1:
Today i asked myself what may be the effect of the common mode signal emitted by the USB isolator resp. the built-in isolated DC-DC.
The scanner, scanner controller and nanovoltmeter form an island with one protective earth connection in the nanovoltmeter mains cable used for the meter and the controller metal enclosures, the connection between them being a 0.5 m GPIB cable. I measured the voltage across this cable, from one enclosure to the other one. I used a shorted out probe next to the USB isolator, picking up the stray field of the DC-DC converter as a trigger signal (yellow trace). As the measurement signal was about the same size as random noise, i used averaging to suppress random noise. In the beginning the GPIB cable had a 6 uH ferrite on it that i removed later. With ferrite ringing along the GPIB cable was 20 mVpp. As expected it reduced without ferrite, to about 7 mVpp.
The scope power supply was ground lifted for this measurement and the scope was grounded via the probe from the nanovoltmeter.
Then i recorded the signal associated with GPIB readout and audible scanner stepping every 5 seconds, this time without averaging. Random noise was about 24 mVpp. Then the GPIB operations (about 200 msec duration for transfer of 40 readings) stick out as four blocks with about 50 mVss and the large spikes of about 100 mVss are likely caused by the scanner operations.
My conclusion would be that the USB isolator is good enough and that one better avoids a ferrite on the GPIB cable. Don't know yet how to determine ringing on the nanovoltmeter input signal.
Meanwhile i got enough calibration data to determine the residual thermal EMF on my three external shorts to be about 1 nV/K of ambient temperature. The shorts are made by soldering the ends of the copper signal wires in the shielded cables to each other.
Regards, Dieter
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