I’ve once checked the input current with regard to the input voltage for several bench DMM. I thought I would show the results here, just to give you an idea what to expect from an average bench DMM in practice.

Notice that most manufacturers do not even specify input current, only input resistance, which is just a limit value that can (and should) be exceeded by far. Furthermore, the input of a DMM is not resistive anyway, but can have a rather non-linear voltage-current characteristic.

As a consequence, I would never rely on the input characteristic of a modern DMM for any critical task, just because even the best will still have a substantial input current that is not negligible for high precision measurements.

**Caution:** it should be kept in mind that the input impedance / input current of a DMM depends on the environment, like ambient temperature and relative humidity and it is subject to component tolerances. Furthermore, all of these instruments (except Picotest M3500A and Rigol DM3068) have been acquired from the 2nd hand market and their history is largely unknown. For the given reasons, the following test results can only be a guideline, only valid for that particular unit under normal laboratory conditions.

Here come the test results in alphabetical order:

1. Fluke 8842A

This venerable 5.5-digit instrument is famous for its exceptional high long-term stability, which puts many 6.5-digit meters to shame. Yet at higher sensitivities the Fluke 8842A is very noisy and requires heavy filtering, which makes especially the 20 mV range very slow – and short-term stability aka noise is still nothing to write home about.

The Fuke 8842A provides high input impedance up to 20 V; it draws a constant input current of 50 pA from 1 - 19.9 volts. The 8840A is largely identical in this regard.

Fluke_8842A_input

2. Keithley 2001

This 7.5-digit instrument certainly is not an “average” bench DMM. It is easily one of the best and combines excellent long-term stability with low noise. The Keithley 2001 provides high input impedance up to 22V and its input current is almost linear rising from 12 to 38 pA over the range of 0 – 22 volts. Its input characteristic can be approximated with 909 GΩ input impedance and 12 pA input bias current.

Keithley_2001_input

3. Keithley 2015 THD

This 6.5-digit instrument is certainly a good average lab tool. It provides high input impedance up to 12V and its input current is almost linear rising from 450 pA to 890 pA over the range of 0 – 11 volts. The input impedance remains high up to 12 volts, yet there is a steep increase of the input current beyond 11 V. The input characteristic of the Keithley 2015 THD can be approximated with 24 GΩ input impedance and 447 pA input bias current.

Keithley_2015THD_input

4. Picotest M3500A

This 6.5-digit instrument is the same as the Keithley 2100 and certainly needs not hide behind a Keithley 2000/2015. The Picotest M3500A provides high input impedance up to 12V and its input current is rather non-linear, rising from 134 pA to 370 pA over the range of 0 – 11 volts. The input impedance remains high up to 12 volts, yet there is a steep increase of the input current beyond 11 V. The input characteristic of the Picotest M3500A can be approximated with 56 GΩ input impedance and 134 pA input bias current.

Picotest_M3500A_input

5. Rigol DM3068

This 6.5-digit instrument is the one I don’t have a lot of experience with. It provides high input impedance up to 22V and its negative(!) input current is pretty linear, decreasing from -115 pA to -87 pA over the range of 0 – 22 volts. The input characteristic of the Rigol DM3068 can be approximated with 793 GΩ input impedance and -115 pA input bias current.

Rigol_DM3068_input

6. Solartron/Schlumberger 7150plus

This 6.5-digit instrument is actually only 5.5 digits where one additional digit is obtained by averaging. This is a very old design and it certainly cannot compete with the more modern DMMs. The Schlumberger 7150plus provides high input impedance up to 2V and its negative input current is all over the place, yet very low and never exceeds -10 pA over the range of 0 – 2 volts.

Schlumberger_7150plus_input

If you miss the Siglent SDS3065X here – well, I just don’t have one. Evaluating their oscilloscopes and waveform generators is sufficient to keep me from getting bored 😉

As can be seen, only the very old Schlumberger 7150plus has a really low input current (and this is not an isolated case, since I have four of them and they all behave similar). Yet this instrument is not very useful for precision tasks, as it lacks stability (both long- and short-term) and high input impedance up to only 2 volts isn’t overly useful either.

Because of all this, I’ve once built an electrometer amplifier (with <1 pA input current, voltage gain of 1 and <1 ppm/°C stability) that I used for sensitive measurements, whenever accuracy would have been ruined by the input current of a DMM.

DCA_R02_Inside_Front_01