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Keithley 610C: Difference between revisions
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better explanation of bootstrapped amplifier principle; transimpedance amplifier circuit; accessories
(50−60 Hz from one version of the manual, 50−1000 Hz from the rear of an actual instrument) |
m (better explanation of bootstrapped amplifier principle; transimpedance amplifier circuit; accessories) |
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{{Spec|Display|Analog meter with mirror scale; positive, negative and center-zero display modes}} | {{Spec|Display|Analog meter with mirror scale; positive, negative and center-zero display modes}} | ||
{{Spec|Power|105−125 V or 210−250 V, 50−60 Hz (some units labelled 50−1000 Hz), 10 W}} | {{Spec|Power|105−125 V or 210−250 V, 50−60 Hz (some units labelled 50−1000 Hz), 10 W}} | ||
{{Spec|Accessories|<small> | |||
* Model 2503 Static Detector Probe (for measurements of charge on flat surfaces) | |||
* Model 6101A Shielded Probe (shielded low-noise cable with a needle-point probe) | |||
* Model 6103C Divider Probe (1000:1 voltage divider with a 3.8×10<sup>11</sup> input resistance) | |||
* Model 6105 Resistivity Chamber (test fixture for measurement of surface and volume resistivity) | |||
</small>}} | |||
{{EndSpecs}} | {{EndSpecs}} | ||
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==Measurement Principle== | ==Measurement Principle== | ||
When set to the "Normal" feedback, the 610C measures | When set to the '''"Normal" feedback mode''', the 610C measures an input voltage, or an input current as 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, or leaves the input open. | |||
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 only the input voltage is measured, the 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. | |||
Note that in Normal mode, 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 (for loads ≤100 μA). | |||
[[File:Transimpedance amplifier.jpg|thumb|250px|right|Principle of transimpedance amplifier ("Fast" feedback mode in 610C)]] | |||
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. | 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. | ||
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==Internals== | ==Internals== | ||
At the core of the 610C is an operational amplifier with MOSFET inputs that is constructed from discrete transistors and | At the core of the 610C is an operational amplifier with MOSFET inputs that is constructed from discrete transistors and has a ±100 V range for input and output voltages. | ||
This is done by bootstrapping – the output stage operates on ±120 V supplies, and the actual amplifier uses a floating ±9 V supply referred to that output. | |||
As a result, the instrument can measure voltages of up to 100 V in either polarity without needing a resistive input divider, thereby preserving the MOSFET gate's high input resistance of >10<sup>14</sup> Ω over the entire ±100 V input range. | |||
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 opamp has a three-level zero adjustment, with switches for coarse (at the rear panel) 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. | The 610C contains only 10 transistors and 2 MOSFETs altogether, which are all used for amplification. The power supply is regulated using Zener diodes. | ||