Burden voltage behaviour across range

[user-one]

Hi

How does the burden voltage of a feedback Ammeter behave accross a a measurement range? Let's say the burden voltage is <200uV of the Keithley 6485. Can I assume that the device most likely has a maximum burden voltage towards the upper end of the range and goes down to 0 closer to 0 current? Or is it a constant over the range?

I am setting up a test to characterise a current source (voltage source with resistor) going from several mA downto 0 current. I am trying to figure out the how to take care of the error causesd by the burden voltage.

Thanks,

Markus

[Norbert W]

Hello,
for low current measurements with your 6485 the input amplifier offset voltage will be quite independent on actual current level as the
amplifier gain is high.
For higher currents cabling will show its contribution - just think of 2 mA current at 0.1 Ohm.

If you have access to an electrometer like 6514 or 6517B you can measure the burden voltage simultaneously - and if you want even
correct your voltage source for this amount.

To get some coarse information about voltage burden you can connect some resistors (with suited values) to the input connection. The burden voltage will
be converted in a current which in turn will be measured by the unit and displayed.

[user-one]

Thank you for the response Norbert.
I do not yet have a 6485, I am trying to figure out if it is worth to get that or one of the 61xx devices. I curently use and Agilent DMM with a shunt. It is fairly easy to deal with a known shunt (same for known cable resistances etc). I am curious to know how the burden voltage will influence my measurement, especially while sweeping over 0 current. It is very important for me to know where 0 current in relation to the voltage I am sweeping over is.
Thanks,