Keithley 220 with driven Guard Triax? - Page 1
Forum section: electronics / general

author: Echo88
author_rank: Frequent Contributor
timestamp: October 24, 2017, 01:56:36 pm
content: I own a Keithley 220 Current Source (rear lug of the two-lug-triax-connector filed off, to connect standard 3-lug-triax-cables) and would like to know if theres a possibility to connect the driven guard as the inner triax-shielding. This would avoid current loss in the triax-cable, if one would source current in the pA-range to very high resistors (say 100GR or 1TR).As far as i can tell from the schematic its possible to connect the driven guard to the output common, but then the current return path would have to be an extra cable, which has to connect to Analog common in the K220 somehow...which isnt available from the rear-panel.So: Wheres my thought error? Is the current error due to the finite cable-impedance compared to the basic current-accuracy negligible or was the K220 not planned to be able to connect the guard to the triax-inner shield from the start?

author: Zucca
author_rank: Unknown
timestamp: October 24, 2017, 02:25:25 pm
content: https://www.eevblog.com/forum/testgear/keithley-220-output-triax-connector-mod/msg397850/#msg397850The answer is there, but read everything. It will help you a lot.

author: Echo88
author_rank: Frequent Contributor
timestamp: October 24, 2017, 03:33:03 pm
content: I must be blind, since the only thing i see is the conversion from 2-lug to 3-lug-triax in this thread. The connection between guard and inner triax-shield is not discussed further in the mentioned thread. Or did you solder the guard to the inner triax and i overlooked it?

author: Zucca
author_rank: Unknown
timestamp: October 26, 2017, 08:13:21 am
content: Quote from: Echo88 on October 24, 2017, 03:33:03 pmThe Keithley EE RnD guys wuold have done that before us.... there is a reason why a K220 is like this.If you connect the J1014 Guard with the J1015 Output Commen with a banana-banana cable, do you not solve your problem?Moreover... the K 220 is a current generator and it spits out from the HI terminal J1013 (triax center) and the return path is the LO termin J1013 (middle coax triax).As long the voltage diffecence beetween HI-LO is small I would not consider a leckage current probem in the Triax cable.If you have to deal with 100GR or 1TR beasts then put then in a metal case and connect it to guard (that´s why there is a J1014 on the back the unit), you can read the calibration procedure of the K220 in the service manual for more details: it's not so easy.PS: Take also a look on the calibration procedure of a K 617 electrometer, there you will find that the guard is connected in the middle coax triax cable. Form my memories the middle coax in a triax in not always the guard, it depends on what you are doing.

author: VintageNut
author_rank: Frequent Contributor
timestamp: October 26, 2017, 12:09:50 pm
content: Quote from: zucca on October 26, 2017, 08:13:21 amElectrometers are different than current sources or sourcemeters.Electrometers measure current on the LO side. Sourcemeters/sources measure currrent on the HI side.So, if you have a resistor as a load for an electrometer, any leakage from the resistor HI somewhere other than electrometer LO will not be measured by the electrometer.On the other hand, for a sourcemeter/source, you have to make sure that there is no path for leakage from the resistor HI to ANYWHERE because that leakage will be indistinguishable from current flowing through the resistor.In an electrometer, the GUARD is passive. It is connected to an input of an opamp that cannot source current other than the bias current of the opamp. The GUARD of an electrometer is above LO by the amount of the input offset voltage of the input opamp.For a sourcemeter/source, the GUARD will be at the voltage of HI.