Keithley 610C
The 'Keithley 610C Solid State Electrometer
The Keithley 610C is a versatile electrometer. While a classical electrometer is simply a DC voltmeter with very high input resistance, the 610C can additionally measure very low currents, high resistances as well as charges:
Key Specifications
input resistance >1014 Ω or selectable 10 Ω to 1011 Ω in decade steps accuracy 1% of full scale, noise 25 μV zero offset drift <1 mV per day / 150 μV per °C 28 current ranges, 10-14 A to 0.3 A full scale 25 linear resistance ranges, 100 Ω to 1014 Ω full scale 17 charge (coulomb) ranges, 10-13 C to 10-3 C (Amprere-seconds) full scale Positive, negative and center-zero display modes| Voltage | 11 ranges, 1 mV to 100 V full scale (× 1 / × 3 steps) |
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The unit has two sets of outputs on the back panel — one to attach a DVM or recorder, with +/- 3 V or +/- 1 mA full scale, using a special connector; the other output (marked "× 1") tracks the input voltage. It is therefore possible to use the 610C as an amplifier, with a bandwidth of 40 kHz at × 1 down to 100 Hz at maximum amplification.
The input connector is a PL259 socket. Newer electrometers use triaxial input connectors, with the inner shield (guard) driven by an amplifier to be at the same potential as the input, in order to minimize leakage currents. It is possible to use a guarded input connection on the 610C through an adapter, although the guard potential is only available on the back of the instrument, at the "× 1" output. (The terminal labelled "guard" is at input ground potential.)
When set to the "Normal" feedback, the 610C measures currents through the voltage drop across a shunt resistor. The large range switch selects the shunt resistance in decade steps between 10 Ω and 1011 Ω. Since only the voltage is measured, these shunts can also be used as defined input resistances in Volts mode (1 / current range), for example in the 1 nA (10-9 A) range, a 1 GΩ resistance is connected across the input.
In "Fast" feedback mode, the current range resistor is connected between the amplifier output and the input, turning the instrument into a feedback amperemeter (transimpedance amplifier) and reducing the input voltage burden to less than 100 μV. (Modern pico/nanoamperemeters work this way.)
Because there is very little (change of) input voltage, the meter's input capacitance has no effect, and the meter responds faster, hence the name.
In the Coulomb ranges (only valid in Fast mode), a capacitor takes the place of the feedback resistor, the instrument integrates input currents and thereby measures charge in Ampere-seconds (Coulombs).