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Low resistance when Ohm Meter channels in open circuit

Suhaila_Hisham

Before starting to use Ohm Meter on an ADALM1K board, I tested the open circuit (or channel) resistance first. Instead of obtaining a large resistance (upward of 200kOhms), the board I am using indicates measured open circuit resistance only around 700 Oh ms (at test voltage 5V). The measured Ohms stayed the same at 700 Ohms even when connected to a known resistor.

How can I fix this problem? Have I managed to short circuit some parts of the board?

  • 0
    •  Analog Employees 
    on Jun 6, 2021 2:29 AM

    Hi:

    Looking at the screen capture you included you have it set to use the internal 50 ohm resistor (connected from CH B to GND) which is OK. I'm assuming that you have nothing else connected to the 8 pin connector? You should see a high resistance value. But it looks like your board may not be (is not?) properly calibrated. With the test voltage set to 5 V the Measured voltage read back should be very close to 5 V (i.e. within a mV or two). You are getting only 4.87 which is indicative of an un-calibrated board. Also since there is nothing connected between CH A and CHB the "forward voltage" i.e voltage between CH A and CH B should be equal to the Meas voltage (CH B is internally grounded so = 0 V).

    On a calibrated Rev F board my readings are 5.0 and 4.995.

    Which revision is your board? Rev D or Rev F? If it is a rev D would not have come calibrated from the factory only the newer Rev F boards are factory calibrated. If you have a Rev F board did you by chance happen to reset the calibration to the default ("zeroed") values either using the smu command line utility or the self calibration function in ALICE (or the self calibration tool included with the Windows installer)?

    Performing a calibration on an un-calibrated board is somewhat complicated and is not something we generally expect new users to do. But here is the procedure:

    https://wiki.analog.com/university/tools/m1k/alice/self-calibration-user-guide

    Hope this helps.

  • +1 on Jun 18, 2021 5:06 AM in reply to dmercer

    1) I have found a possible reason for this issue. It happens when I connect the board to a USB hub (repeater) rather than straight to the main USB port on my laptop. I also notice that if I use the USB hub, offset voltage for Ch A and Ch B when connected to GND measured on the Volt Meter is quite high, above 0.3 V.
    Once I switched to the laptop USB, open circuit resistance is as expected (above 300KOhm) and offset voltage is about 0.0006/8.

    2) I have another question if you don't mind I continue the discussion here. I'm using a Rev D board so I do need to calibrate it. I have successfully calibrated the M1K Volt Meter using AD584 before. Now, I want to try the Self-Calibration routine in Desk Top. But I connected Ch A and Ch B after prompt, I got an error message:
    "Did not get good data from Ref V check connections"

    I double-checked the connections. I also check the Ch A and Ch B voltage reading from AD584 using Volt Meter, they are around 2.44 V, so the connections are OK.

    Is there a step I'm missing for the Self-Calibration?