Keithley 610C

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The Keithley 610C Solid State Electrometer can measure voltages (with a very high input resistance), very low currents, high resistances and charge.

Key Specifications

Voltage	11 ranges, 1 mV to 100 V full scale (× 1 / × 3 steps)
Input resistance (volts mode)	>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
Current	28 ranges, 10–14 A to 0.3 A full scale
Resistance	25 linear ranges, 100 Ω to 1014 Ω full scale
Charge	17 ranges, 10–13 C to 10–3 C (Amprere-seconds) full scale
Display	Analog meter with mirror scale; positive, negative and center-zero display modes

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 PL-259 ("UHF") 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.)

Measurement Principle

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).

Internals

At the core of the 610C is an operational amplifier with MOSFET inputs that is constructed from discrete transistors and works with +/- 120 V supplies, allowing it to measure voltages of up to 100 V in either polarity without needing the resistive input divider that is commonly found in electronic meters.

Note that the "× 1" output on the rear follows the input voltage within the full range, i.e. when the input is at +100 V, so is that output.

The opamp has a three-level zero adjustment, with switches for coarse and medium levels, and a 10-turn potentiometer for fine nulling. Zero drift is fairly low once warmed up, especially compared to the predecessor units like the Keithley 610B which still relied on electrometer vacuum tubes in the input stage.

The 610C contains only 10 transistors and 2 MOSFETs altogether, which are all used for amplification. The power supply is regulated using Zener diodes.

Pictures

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