Keithley DMM6500 ( Now Tektronix) - Micro Amp Measuremet  - Page 1
Forum section: electronics / testgear

author: omegaone
author_rank: Contributor
timestamp: December 07, 2018, 01:30:01 pm
content: Hello All,I have a simple problem here. Need Answers to the best of your ability.I am posting two pictures one reading obtained with a shunt selection of 10 micro Amps other 100 micro Amps.Respectively at 10 micro Amps, the internal shunt is at 10kOhms and at 100 micro Amps it would be at 1 kOhms.If I am concerned about the proper reading of evaluating the source signal between 0-10 micro Amps, which one would you recommend that I chose from the posted pictures.My main focus here is I want to measure the best I can the source signal in micro Amps using DMM6500. ( It is a calibrated Instrument, fresh from the factory and advertised as able to read even in nA and pA, but I am only testing it in micro )

author: Andree Henkel
author_rank: Regular Contributor
timestamp: December 07, 2018, 02:54:30 pm
content: higher shunt should give you the more accurate results

author: omegaone
author_rank: Contributor
timestamp: December 07, 2018, 03:11:32 pm
content: Are you able to justify your answer using electrical standards or references?I was told by Keithley Engineer to use lower shunt to get the readings.Samuel

author: Andree Henkel
author_rank: Regular Contributor
timestamp: December 07, 2018, 04:02:44 pm
content: the higher shunt leads to a higher "voltage burden" of the measurement, which can cause the circuit you measure the current in, to behave differently than with lower "voltage burden", the circuit is less disturbed with lower voltage burden, so this is probably the reason for the advice of the keithley engineer.however the current measurement itselv is at this rather low currents more accurat using higher shunt.

author: omegaone
author_rank: Contributor
timestamp: December 07, 2018, 05:25:57 pm
content: Yes it is all RELATIVE to what we are measuring and what shunt we are using.The question still remains:Which measurement is correct and in accordance with what ELECTRICAL STANDARDS? ( Are these STANDARDS AGREE AMONG METER MANUFACTURERS )

author: Brad O
author_rank: Regular Contributor
timestamp: December 07, 2018, 09:23:41 pm
content: If you want to measure between 0-10 uA with the DMM6500's DC current mode, you should use the 10uA range. That recommendation comes from the instrument specifications which give a higher accuracy for the 10uA range than the 100uA range. That accuracy is based on calibration with a Fluke 5720A or 5730A which themselves are calibrated to NIST standards.Are you sure the other Keithley engineer didn't tell you to use the lowerrange? Or did they think you were using an external shunt?Also I noticed you have a Secondary DCV measurement configured but no HI input. I just wanted to make sure you knew you can't measure voltage through the AMPS terminal.

author: HKJ
author_rank: Super Contributor
timestamp: December 07, 2018, 10:44:27 pm
content: What is precise depends on the conditions. With a device that changes current consumption depending on voltage, you need to measure the voltage at the device and compensate the power supply (Or use a 4 terminal supply or a meter with zero burden voltage).One usable solution is a power supply with a precise meter build in, I often use my 2280S for smaller currents, but it is not usable when you get down to a few uA or lower.

author: omegaone
author_rank: Contributor
timestamp: December 07, 2018, 11:18:13 pm
content: Thank you for the detail response.The Keithley Engineer suggested this solution only.I have measured 10 micro amps with Fluke 289. Then without changing the source, I measured the same source with DMM6500.Fluke 289 read: 10 micro ampsDmm6500 read: 0.1 micro ampsThe fluke 289 is sent to Fluke for testing and calibration as requested by Fluke.I have also verified the reading via another fluke 289 and it also read approximately 10 micro amps.This caused me ask the question which one is correct.Could you send me a link to the NIST standards that are used ?About the voltage. Please ignore it for the time being. As I am not able to discuss the shunt being tested at this point.We will strictly stick to the micro amp reading only. For the next test I will turn off the secondary display.

author: omegaone
author_rank: Contributor
timestamp: December 07, 2018, 11:21:56 pm
content: Thank you for your suggestions.I am not looking for a solution here. I am looking to get myself familiar with the STANDARDS everyone is following when producing the instrument.As long as they read within the tolerance specified, I am happy to use the instrument.I question the reading when it differs by a factor of 10 or 100 then I really question the readings.

author: omegaone
author_rank: Contributor
timestamp: December 12, 2018, 12:43:16 pm
content: Hello All,I like to give you an interim update on DMM6500.This is a nice instrument I have ever used and I enjoyed evaluating it to see if it will fit my purpose.I have come across some challenges from reading negative ohms to measurement issues at the micro Amp range.The latest on DMM6500 is it has encountered an LCD Touch Screen Failure.And I want to thank all who participated in answering questions.I have not evaluated all the functions on the instrument ( it would take a life time...) but rather some basic measurements only.As it stands now, I am in communication with Measurements Canada via NRC (National Research Council) and NIST in the USA.I am learning a lot and I will publish my story once I have a resolution to measurement issues I have experienced.

author: Trader
author_rank: Frequent Contributor
timestamp: October 13, 2021, 07:30:32 am
content: Quote from: omegaone on December 07, 2018, 11:18:13 pmMaybe the problem is the Burden Voltage (even more if you are using low voltage), for 10uA the Fluke 289 is 1mV and the Keithley DMM6500 is 101mV.https://lygte-info.dk/review/DMMFluke%20289%20UK.htmlhttps://lygte-info.dk/review/DMMKeithley%20DMM6500%20UK.html