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 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 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 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.
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.
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.
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.

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