Tested temp; 35,2 ⁰C.
- DCV, 10V (fixed range), AZERO OFF, FIXEDZ OFF (GOhm); 2 pA test: checks out to about -0,01 V/s
- FIXEDZ ON, 1 V range (10 MOhm): Zero reading < 1 µV; raises to about 60 µV (short removed).
These values of -2pA and +6pA for the different methods are completely fine, so the DCV input path for the 10V, 1V and 100mV is obviously
not damaged.
Remark: The AUTOZERO induces current spikes on the MUX, (which are only imperfectly compensated), therefore AZERO OFF gives the least bias current, which is reproducible and typical -2pA for different instruments.
These measurements require that the 3458A is warmed up for several hours, if I remember correctly I also found such different bias currents with the 10MOhm method when measuring 'cold' and 'warm'.
In our internal Metrology group, we compared 3458As and confirmed the findings of other authors, like Rado Lapuh in his book 'Sampling With The 3458A', where he also describes several methods for determining the bias current, typically -2pA again, and other parameters which are important for digitizing.
The bias current measurement around zero volt gives a mostly constant value, i.e. a linear voltage drift dU/dt for the capacitor method.
This voltage drift will saturate (stop) somewhere at several negative volts, in my case at about -5V.
In the end, you have to pre-charge the input capacitor to about that value, iterating from more negative or more positive values, otherwise this measurement will take forever.
This then gives you the impedance of the input, which is specified in the datasheet to be > 10GOhm.
The bias current on the other hand is only indirectly specified to be < 20pA.
This impedance is mostly determined by the impedance / leakage resistance of the input jacks, the input switch, the PCB material, whereas these -2pA are given by the leakage of the FETs of the MUX and of the FET Q103A inside the pre - amplifier.
So in my case I determined an input impedance of -5V/-2pA ~ 2.5*10
12 Ohm, or about 2TOhm, which order of magnitude Lapuh also determined for several of his instruments.
Frank