6517B coulombs effective input impedance

6514, 6517 Support
jalarsonjr
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6517B coulombs effective input impedance

Postby jalarsonjr » January 5th, 2012, 6:41 pm

What is the effective input impedance of 6517B in coulomb measuring mode? Does it change with range? Does it change when autoranging? We are attempting to measure charge from an ion chamber that is intended to be terminated with very low impedance and we believe the effective voltage on the ion chamber may be reduced by the voltage drop across the 6517B input impedance. Thank you.

Dale C
Keithley Applications
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Re: 6517B coulombs effective input impedance

Postby Dale C » January 6th, 2012, 12:37 pm

Calculate input capacitance in coulombs for 6514, 6517B.
The effective input capacitance in coulombs mode is the
feedback capacitor times the open loop gain of the input stage.
In both meters this gain is at least 4xE6.
The 20nC and 200nC ranges of the 6514 use a 1nF capacitor,
giving an effective capacitance of 4 milli farads.
The 2uC and 20uC ranges use a 100nF capacitor, and the effective
capacitance is 400 milli farads.
On the 2nC and 20nC ranges of the 6517A use a 100pF capacitor
giving an effective capacitance of 400uF.
The 200nC and 2uC ranges use a 10nF capacitor, giving an effective capacitance
of 40 milli farads.

jalarsonjr
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Re: 6517B coulombs effective input impedance

Postby jalarsonjr » January 9th, 2012, 11:58 am

- Dale C -
That doesn't answer my question. What I really need is the input imedance. Is there a resistor between the input and the integrator? If so, what is the value and does it change with range? This is important because we are looking at fast current pulses and any voltage (IR drop) across the input will reduce the voltage across the device under test.
Thanks.

Nathan S
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Re: 6517B coulombs effective input impedance

Postby Nathan S » January 17th, 2012, 4:03 pm

It should be range independent and there should be approximately 1.4M Ohms.


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