Author Topic: uCurrent GOLD - oscillations  (Read 1215 times)

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Offline ChupacabrasTopic starter

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uCurrent GOLD - oscillations
« on: October 07, 2019, 10:19:35 am »
I'm trying to make uCurrent, it is a good project for hobbyist.
https://www.eevblog.com/projects/ucurrent/
https://www.eevblog.com/files/uCurrentRev5schematic.pdf

I have created in KiCAD, just copied Dave's schematic. And created my own PCB layout. I have used 0.1% resistors (instead of 0.05%).

Problem is I see oscillations across R8, which is at the output of second opamp (see attached oscilloscope screenshot).
I'm feeding 10mA DC to the input jacks, and expecting 10mV at the output jacks. Output of the first opamp gives 1mV (which is as expected). But output of the second opamp gives wrong values (negative millivolts - that is probably caused by those oscillations)

I tried to change opamps, just to verify whether one of them is faulty. Didn't help.
I tried to change capacitors, even tried to change values of C3 and C4 to 5pF or 30pF. Didn't help.

What could be wrong?
 

Online Kleinstein

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Re: uCurrent GOLD - oscillations
« Reply #1 on: October 07, 2019, 10:55:53 am »
Dave also had some problem with oscillations, though I thing that was before adding R10 the 270 Ohms resistor at the virtual ground.
AFAIR there was also an issue with the exact type for U2 - some where more stable than others.

R8 at the output is a little odd and could be a problem, if there is a capacitive load to the output. For a test one could bridge this.

Another point could be the layout U4 may also need some decoupling. Some Batteries can have a rather high impedance - so it may help to have an electrolytic cap at the supply somewhere (e.g. near U2 or U4). With R10 in place it could also help to have a capacitor from the VGND to the supply (e.g. negative side).
 

Offline mikerj

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Offline ChupacabrasTopic starter

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Re: uCurrent GOLD - oscillations
« Reply #3 on: October 10, 2019, 06:13:16 am »
Problem solved ;)
Thanks to mikerj for the link.

I have tried many things and finally worked Dave's fix. Adding 100nF capacitors from V+ to virtual ground and from V- to virtual ground.
« Last Edit: October 10, 2019, 06:28:09 am by Chupacabras »
 


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