Agilent 34461A injects current into the current shunt by itself

[wraper]

So it seems that my piss poor luck on buying new test equipment might be even more hilarious than I thought. Crappy DM3068 (which Rigol finally replaced after being away for about 7 months in total. Received used one instead, with burned pcb under the replaced TVS diode and whole pcb coated in junk. But at least it works as intended now. Maybe will start another topic about it), Agilent U1272A (crappy soldering topic a year ago, but Agilent response was very good), and now Agilent 34461A.
   So it arrived today from Farnell uk. I started testing it against that used DM3068. All seemed fine and I was rather happy that that DM3068 is closer to the Agilent than I thought it would be, as it was a 1 year old repaired device without any cal certificate. So finally I started testing the current. Multimeters were already heated for about 2 hours at that time. Readings were reasonably close on both multimeters yet I decided that I need to manually null zero readings on the Agilent. Without leads readings were close to zero on all ranges. So I shorted the terminals and wow, readings became negative, ~1500 in the last 4 digits on the uA range, ~-800 on the 1mA range and within last two digits on other ranges. As long I disconnected the lead, readings zeroed, I even tried different leads, same result. So I hooked another multimeter to measure voltage across the current shunt of the 34461A and it appears that there is 12-16 uV voltage depending on the selected range. 12uV on the 100uA range which rises up to the 16uV on the 100mA range. Didn't test the amps ranges yet. Measuring positive and negative current also shows a little bit different result (up to 2 last digits). Later I gave it a break and switched off for a while, then tested again. It appears that on the first 2 minutes of operation readings without the leads connected, are negaive on all ranges (2-3 last digits) and rises up to zero while heating. Voltage across the current shunt also rises during that time. So any 34461A owners, Dave? do you have the same issue and it is a "feature". Or I got a lemon yet another time? BTW, instrument seems to be calibrated considering that "offset" current.

Without leads attached:

With shorted terminals:

[macboy]

To zero a voltage measurement, you short the leads together, ensuring zero potential between them at the contact point, and then you zero the reading. If the leads are left open, then you can't assume they are at the same potential.

To zero a current measurement, you open the circuit. That is how you ensure zero current flows. If you short the leads, you provide a path for current, and you can't assume there is zero current. Due to the low resistance of the current path through the instrument, even a small thermoelectric potential can induce a measurable current. You can't assume that this same current will flow through a series-connected load, since the load will necessarily have a much higher resistance than the shunt resistance of the instrument. So, it bears repeating: to zero a current measurement, you open the circuit.

In reality, the instrument isn't providing a 0.15 uA current offset, it is providing a 12 to 16 uV voltage offset (according to your measurements), which you can mentally account for as a source of error, along with the burden voltage caused by the shunt resistance (which is almost certainly much more than a few uV). If those microvolts of offset really affect a 'real' current measurement (not leads shorted together), then you need to consider an instrument different than a bench DMM. When measuring uA, the burden voltage will typically be much higher than a few uV, so that offset is not the largest source of error.

I suggest reading Keithley's "Low Level Measurements Handbook", now in 7th edition I believe.

[Orange]

I suggest reading Keithley's "Low Level Measurements Handbook", now in 7th edition I believe.

+1
Good reading for 6 1/2 digit meter owners 

6th edition :
http://www.keithley.com/knowledgecenter/knowledgecenter_pdf/LowLevMsHandbk.pdf

[KedasProbe]

How much do you measure if you measure the "200nA" of the DM3068 diode test?

[wraper]

To zero a voltage measurement, you short the leads together, ensuring zero potential between them at the contact point, and then you zero the reading. If the leads are left open, then you can't assume they are at the same potential.

To zero a current measurement, you open the circuit. That is how you ensure zero current flows. If you short the leads, you provide a path for current, and you can't assume there is zero current. Due to the low resistance of the current path through the instrument, even a small thermoelectric potential can induce a measurable current. You can't assume that this same current will flow through a series-connected load, since the load will necessarily have a much higher resistance than the shunt resistance of the instrument. So, it bears repeating: to zero a current measurement, you open the circuit.

In reality, the instrument isn't providing a 0.15 uA current offset, it is providing a 12 to 16 uV voltage offset (according to your measurements), which you can mentally account for as a source of error, along with the burden voltage caused by the shunt resistance (which is almost certainly much more than a few uV). If those microvolts of offset really affect a 'real' current measurement (not leads shorted together), then you need to consider an instrument different than a bench DMM. When measuring uA, the burden voltage will typically be much higher than a few uV, so that offset is not the largest source of error.


I suggest reading Keithley's "Low Level Measurements Handbook", now in 7th edition I believe.

This is not thermal EMF, that voltage is obviously supplied by some opamp circuit in the instrument. Of course it provides voltage offset across the shunt which I measured by another multimeter, however 34461A obviously measures it as current offset.  DM3068 don't have this effect at all BTW. And it affects real measurment, not too much but 2 last digits on uA range measuring 100uA current while reversing polarity with "High resistance" current source.

[wraper]

How much do you measure if you measure the "200nA" of the DM3068 diode test?

200nA, spot on (when I previously zero it with open terminals)

[macboy]

... And it affects real measurment, not too much but 2 last digits on uA range measuring 100uA current while reversing polarity with "High resistance" current source.

Did you zero the meter open circuit?

Ironically, precision instruments often reduce measurement certainty rather than increasing it.

Imagine if you used a 4.5 digit meter, like a high-end handheld. The zero offset, and the difference when switching the leads, would be 1 or 2 counts at most. This would seem acceptable. Now use a 6.5 (or higher) digit meter, and the same difference shows up as 100 or 200 counts. Somehow this seems unacceptable, even though we are talking about errors in the level or ppm or 10's of ppm.

When you get into precision and/or low level measurements (yours is both low level and high precision), sources of error are numerous and difficult to control. A few nA of error probably isn't too bad.

[wraper]

BTW, 16uV is not so small figure considering that max burden voltage is only 11mV on 100uA range.

[wraper]

... And it affects real measurment, not too much but 2 last digits on uA range measuring 100uA current while reversing polarity with "High resistance" current source.

Did you zero the meter open circuit?

Ironically, precision instruments often reduce measurement certainty rather than increasing it.
Imagine if you used a 4.5 digit meter, like a high-end handheld. The zero offset, and the difference when switching the leads, would be 1 or 2 counts at most. This would seem acceptable. Now use a 6.5 (or higher) digit meter, and the same difference shows up as 100 or 200 counts. Somehow this seems unacceptable, even though we are talking about errors in the level or ppm or 10's of ppm.
When you get into precision and/or low level measurements (yours is both low level and high precision), sources of error are numerous and difficult to control. A few nA of error probably isn't too bad.

That with zeroed with open terminals, if I'd zeroed with closed, there would be 4 digit offset. It reads little bit lower compared to the DM3068 and higher when reversed. Well, I perfectly understand that 6.5 digit meter won't show perfect figures. But 1500ppm with shorted leads (0.15% of the range) is a "bit" high for my opinion. So I asked if anyone can verify if it is a feature of this model. Moreover there is no such "feature" on the other meter. There actually is offset on 200uA range on Rigol but 6 times smaller and considering that its range is 2x higher, that would mean equivalent to 12 times compared to the agilent.

[saturation]

Agilent bench meters use a technique that carries over from prior HP designed bench meters.  There is an offset voltage applied to measurement circuits to zero the DMM and compensate for internal drift, bias currents and thermal EMF.



When you suspect your meter is off or when receiving a new device, its best to perform the performance or verification tests specified in the service manual.  These are quick tests that are separate from the internal diagnostics and simply, it tells you the limits your meter should display if the DMM is work properly, if the terminals are tested open or shorted, and its different for each function and each range.