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Keithley 610C: Difference between revisions
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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.) | 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.) | ||
==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 10<sup>11</sup> 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<sup>-9</sup> A) range, a 1 GΩ resistance is connected across the input. | 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 10<sup>11</sup> 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<sup>-9</sup> A) range, a 1 GΩ resistance is connected across the input. | ||
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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). | 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 MOS FETs altogether, which are all used for amplification. The power supply is regulated using Zener diodes. | |||