When I switch into 2W resistance I notice a high initial reading when shorted in the 0.500Ohm to 2 ohm range that gradually decreases to around 10mOhm range after about 10-60mins depends on how high it starts at. it drops off fast then as it gets closer to zero. Actuating the Front/Rear switch seems to have effect on this. same behavior in 2W mode is on the rear as well but not as promounced. ie. it seems to start in the <50mOhm range and goes down and settles within 5minutes.
4W mode does not seem to be effected. readings front and rear are in the <1mOhm range
I'm thinking the Front/Rear Switch is contaminated but the 4W behaving differently puts doubt on it. I would expect 4W to behave the same if it was the F/R switch.
If I null out the value and read a 50Ohm 0.1% resistor I have it reads 49.997.x on 2W and 49.999.x in 4W
so if nulled it reads very accurately (I do have a Rigol DM3068 which reads the same resistor at 49.987.x in 4W mode, just to see what another meter shows)
I checked the F/R switch and while most terminals show 10mOhm a few are much higher.
5-6 0.456Ohm
17-18 0.374 ohm
22-23 0.423Ohm
Trying to understand the test points and these reading to me say I should see this on the rear inputs not the front
unless I'm reading things wrong.
Sandra,
a contaminated switch very well explains the malfunction in 2W Ohm. It is usually contaminated if the instrument is older than about 10 years.
My 34401A showed exactly this behavior, and even my 3458A from 2001 has such a switch, which already shows an offset in 2W mode.
4W Ohm mode explicitly removes any residual resistances of the Input High and Low paths, as it measures the resistance (or the created voltage) directly at the DUT.
Therefore, the observed difference between 2W and 4W mode is correct, more than that, it has to be that way.
2W measurements includes all resistances of the external and internal cabling and components, up to the internal sense points, which you may identify in the schematics, pages 9-8 and 9-9.
The switch pins you mentioned are all NOT involved. Anyhow, these too high resistance values indicate a contamination of the whole switch.
Instead, pin 21-20 is the front Input Low path, pin 19-20 rear Input Low, to AGND.
Pin 3-2 and 1-2 are the respective ones for Front / Rear - Input High.
This latter path can be traced to the point between K101, pin 3 and L101, which is the High reference point, as there the reference current is injected, and the High voltage is tapped off, towards the input multiplexer U101-B.
So only pin 21-20, pin 3-2 and K101 are inside the instrument , and also the involved PCB traces and solder joints, which may effect your 2W measurement, for the FRONT setting.
Using your Rigol, please measure these components, or the complete paths from the Input jacksto the reference points (AGND, DCV_Low), if they show high resistances and/or changes after actuation of front/rear switch.
A fraction of this error is removed during instrument zero calibration, so reading of the 34401A and of the Rigol may be different.
Frank