Unwanted capacitance in current measurement setup - Page 1
Forum section: electronics / projects

author: sean87
author_rank: Regular Contributor
timestamp: January 16, 2012, 10:00:21 am
content: I have a current measurement setup (which supposed to measure < 1nA). It has 20 channels and switched using a relay matrix.Using a LCR meter I measured for one channel (In the schematic below, removing capacitor and putting LCR probes in series), and it happend in open mode, the whole setup capacitance is6pFand if a chanel is closed (Obviously when taking a measurement, the respective channel is closed) the capacitance is around50pF.I really would like to know how this capacitance (which I beleive comes accumulated from 22 coaxial cables, roughly 20 meters and count the internal wiring of test setup) can affect/contribute to my measurement? What is the theory behind it (I want to study it further) and possible workarounds?Here is the schematic of my setup:Thanks!

author: IanB
author_rank: Super Contributor
timestamp: January 16, 2012, 10:11:55 am
content: Capacitance introduces time varying effects (dynamics) in current flow. You could visualize it like interconnected tanks with different levels and current sloshing around between the tanks.What it means in practice is that if you want to measure such small currents accurately, one thing you will need to do is establish steady state conditions where all dynamics have settled out.Imagine your conceptual equation like this:(input) - (output) - (leakage) = (accumulation)Input is the source of current you are trying to measure. Output is the current at the point of measurement. In order to get an accurate measurement you need to reduce both leakage and accumulation as close to zero as possible. Leakage is going to involve stray current paths and conductance where you don't want it. Accumulation is going to result from capacitance in the system.With larger currents these error terms are insignificant compared to the size of the current being measured. However, once your currents become small enough, stray leakage and stray capacitance will increasingly affect the measurements.

author: Unknown
author_rank: Unknown
timestamp: January 16, 2012, 10:16:54 am
content: A higher source capacitance will increase the noise from a current source (lower output impedance at noise frequencies), though the anti-parallel series diode should help with this. See the Keithley low level measurements handbook (free download from their website) for details.

author: sean87
author_rank: Regular Contributor
timestamp: January 16, 2012, 10:24:39 am
content: Quote from: alm on January 16, 2012, 10:16:54 amWell I am following the keithley handbook (I am using 6485 picoammeter and since it does not it's own supply, I use 2400 SMU). I built the setup exactly the same. BUT I did not use the back to back diodes at the output, neither I used diodes in series with resistor and capacitor (capacitor is the DUT which it's leakage current should be checked).Would you mind telling based on what thoery those back to back diodes would help? I read that they help with noise reduction...But I was afraid that they introduce theire own problem in the setup. Please tell me more.

author: Unknown
author_rank: Unknown
timestamp: January 16, 2012, 10:37:58 am
content: Quote from: sean87 on January 16, 2012, 10:24:39 amThey just add a current-dependent series resistance, increasing the source impedance the ammeter sees, especially at higher (noise) frequencies. Without them, the source impedance goes to zero at higher frequencies. Disadvantage is obviously the increased burden voltage the source sees. If noise is not an issue for you, I guess you might as well leave them out. As long as the charge current through the cap is limited to a reasonable value by the resistor.It's explained quite well in section 2.3.2 (noise and source impedance) of the 6th edition of the Keithley handbook in my opinion. If you short the input terminal of a feedback ammeter to ground, the gain of the internal noise will be infinite (theoretically). Adding series resistance between the input and ground decreases this gain, at the cost of a higher burden voltage.

author: sean87
author_rank: Regular Contributor
timestamp: January 16, 2012, 11:10:49 am
content: Thanks,Though the term higher frequencies confuses me. My application is measuring leakage current of capacitors, How to determine if I am doing something in high frequency or not?Without those diodes I am using 1k 0.1% resistors, in the handbook it says adding the diode, much lower resistor values can be used. Still a big confussion about the resistance value to use if I am going to add the diodes in the setup.