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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.

- jalarsonjr
**Posts:**2**Joined:**January 5th, 2012, 6:31 pm**Country:**United States

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.

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.

- Dale C
- Keithley Applications
**Posts:**2117**Joined:**June 10th, 2010, 9:22 am**Country:**United States

- 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.

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.

- jalarsonjr
**Posts:**2**Joined:**January 5th, 2012, 6:31 pm**Country:**United States

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

- Nathan S
- Keithley Applications
**Posts:**256**Joined:**February 28th, 2011, 10:17 am**Country:**United States

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