Fluke 5500A DC Current Problem

[Bryan]

OK, have 5500A and fine wire in a 20 turn loop, looking at it with Tek AM503B/P6302 probe & Scope.  Apply 4 mA and I meaure about 1.6 div deflection @ 50 mA/Div-close enough.  (4ma x 20=80 mA or 1.6 div/50 mA/div)
Bringing down the DCI level of the 5500A there a range switch point from 3.3 to 3.2 mA.  When it goes into the lower range @ 3.2-blammo! I got a big a-- sinewave at about 1.04 MHz riding on the current.  I figure it to be about 8 mA p-p when I factor out the 20 turn loop.  It's there all the way down to 0 and on to -3.2 mA, then it ranges again @ -3.3 mA and goes away.


I should also add if I take off the coil and use a single pomona cable that will fit thru my current probe I can knock down the scale to  1 or 2 mA/Div where 8mA p-p should be very visible but it is not there, so I am inclined to think the 5500A is probably OK but cannot explain why this is happening, shouldn't theoretically.  Grabbed a 5520A and it performed as expected.
Thanks for any input you may have.

[jwilley127]

I checked with one of my "go-to" calibrator guys and he stated the following:

I did a little testing. I think it is loop compensation with the transconductance amplifier in the 5500 for that range. It is probably working just as it should. Adding the 20 turn loop of small wire is adding substantial inductance across the output into low impedance. An easy test for them would be to simply add either a 100 ohm resistor in series with the coil, or add a capacitor across the coil and see if the frequency changes or the oscillation goes away. Adding a small cap, say <0.1uF will actually tend to destabilize the loop as most amplifiers are not designed to drive capacitive loads. Adding 10uF-47uF is well beyond the range where this matters, will not affect stability, and should reduce the loop bandwidth to the point that it should be obviously free of any oscillation. The 5520 has higher bandwidth than the 5500, probably limiting this particular combination of factors from inducing oscillation.

Hope this helps and if you need anything else I'll be glad to dig deeper anytime.

[Bryan]

I sure appreciate it, I was thinking it had to be something setup related throwing things out of kilter.  I give that a shot.