8846A Bogus Burden voltages? - Page 1
Forum section: electronics / testgear

author: robrenz
author_rank: Super Contributor
timestamp: December 10, 2013, 03:54:41 pm
content: I was doing some testing with my 2450 SMU and my 8846A and realized since I can source current and read voltage on the SMU I could measure the burden voltage of the 8846A directly. The SMU burden voltage is less than 100µV on all ranges so it can basically be ignored for this testing. The first 4 ranges are in spec and what I expect but what's up with 400mA and above? The same shunt and fuse are used on the 100mA and 400mA range. Am I missing something?100µA on 100µA range = 10.26mV vs 15mV in spec.1mA on 1mA range = 102.48mV vs 150mV in spec.10mA on 10mA range = 24.96mV vs 25mV in spec.100mA on 100mA range = .24999V vs .25V in spec.400mA on 400mA range = 1.0209V vs .5VWHAT?400mA on 1A range = 165mV vs 50mVWHAT?1A on 1A range = .412V vs 50mVWHAT?

author: Unknown
author_rank: Unknown
timestamp: December 10, 2013, 04:02:46 pm
content: Is the SMU using four-terminal sensing? Otherwise the voltage drop across wires and connectors might account for the difference, although that doesn't explain why the effect changes with range (400 mA on 400 mA range vs 400 mA on 1 A range), and would have to be some very long or flimsy wires.

author: robrenz
author_rank: Super Contributor
timestamp: December 10, 2013, 04:29:02 pm
content: Excellent point almbut it does not completely fix things. The first readings were triax cables out the back but 2 wire. These are 4 wire sensing from the front panel SMU banana's, sensing right at the 8846A inputs.10mA on 10mA range = 24.96mV (21.22mV 4 wire) vs 25mV in spec.100mA on 100mA range = .24999V (.21228V 4 wire) .vs .25V in spec.400mA on 400mA range = 1.0209V (.8706V 4 wire) vs .5VWHAT? Still bad400mA on 1A range = 165mV (15.016mV 4 wire) vs 50mV Ok now1A on 1A range = .412V (37.418mV 4 wire) vs 50mV OK now

author: SeanB
author_rank: Super Contributor
timestamp: December 10, 2013, 05:05:36 pm
content: Have you tried cleaning the fuse and holder, or temporarily replace the fuse with a copper bar to eliminate the fuse internal resistance.

author: robrenz
author_rank: Super Contributor
timestamp: December 10, 2013, 05:07:20 pm
content: The same fuse is on the 100mA and 400mA range so if it was fuse it should affect the 100mA but it doesn't.

author: ManateeMafia
author_rank: Frequent Contributor
timestamp: December 10, 2013, 07:40:42 pm
content: Misprint in the specs?I would guess it should be <1VComparing the other ranges and how the burden voltages are calculated across the 1 ohm shunt on the 10mA and 100mA ranges.I could be wrong though.

author: robrenz
author_rank: Super Contributor
timestamp: December 10, 2013, 08:16:20 pm
content: With every ones help I now think it is a misprint in the specs.10mA on 10mA range =(21.22mV 4 wire) This would be a 2.122 total resistance of the 1 Ohm shunt and fuse.100mA on 100mA range = (.21228V 4 wire) This would be a 2.122 total resistance of the 1 Ohm shunt and fuse.400mA on 400mA range = (.8706V 4 wire) This would be a 2.1765 total resistance of the 1 Ohm shunt and fuse.That is close enough to prove that the .5V spec for 400mA is a misprint.