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| 0,2V Offset on DC Measurements with Picotest M3511A DMM |
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| MaxH:
Hello Everyone, Im currently starting to debug an M3511A which has a weird constant +0,2V Offset on VDC measurements. This also affects DC and AC current measurements. AC Voltage measurement seem to go to a different ADC. Could this have something to do with the voltage reference? I will continue to try to find it out but if somebody might have a hint, it would be usefull. Thanks in advance, Max |
| David Hess:
A common cause for such an offset is damage to the input protection circuity, or high impedance input buffer. If input shunt diodes are use which is common, then one of them may have high leakage current which creates an offset though the input protection series resistor which goes to the input. If you have an electrometer, then you might be able to test this by measuring the current across the input; it should be less than nanoamps. |
| MaxH:
Thanks for the answer! Unfortunately I dont have an electrometer. Here is a picture of the input circuitry, the left relay would be the switch for the AC/DC, down is the Resistornetwork followed by a switch for the ranges and then there is the AD8638 which already has .2V diff at its Input. I will get some better probes to understand the path of those .2 Volts. Might this be a software defined offset? Greetings, Max |
| Kleinstein:
0.2 V difference at the input of the ad8638 could expain the problem (broken OP). Before replacing it, one should also check the supply to the OP - there is a good chance to have a bootstrapped supply and such a circuit is a little prone to oscillation and there could be supply problems (AC part or DC level) to cause the offset. There is very likely some offset compensation in software, but this should be more like a few µV as for the offset of the AD8638. 0.2 V of measured offset would be way too much to compensate with software, as the offset would usually not be really stable like a good voltage reference. So I very much doubt it would be a software thing. The analog part should not have that much offset to compensate. For the high impedance (e.g. > 1 Gohms) ranges one could measure the input bias current by whatching a capacitor (1-10 nF low loss like PP, PS or C0G) charge / discharge at the input. So there is no need for an electrometer. Edit: From the pictures it does not look like a bootstrapped supply, more like a meter with high Z only up to a lower voltage, like 2 V. Checking the supply to the OP is still not a bad idea. I don't see very much protection - so an ESD damage to the input amplifier is a possible cause. |
| coromonadalix:
Maybe asking Picotest for an input schematic would help, ask them under an NDA (non disclosure agreement) ?? Have you seen this page for FW updates just in case http://www.picotest.com.tw/h/Data?key=770151152829&set=8&cat=45877 |
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