Positive-Negative ±5V current-boosted references

ronv

Joined Nov 12, 2008
3,770
Thanks for your help.
What I mean, is that my voltmeter will start reading variations (haven't scoped it yet, but I might) in the order of at least ±20mv twice a second, and it also tends to drift... A LOT.
I'm almost sure that the problem lies in the LT1010 chip. Or the way I connected it. Either way, my sim also shows a steady -5V output, but that's not the case in real life.
I'm with the others. The caps on the output are waaay to big. They may make you think they make your problem better but that is only because the op amp can't drive that much capacitance. As an experiment you could move the 1010 outside the feedback loop by moving the right hand side of the pot to the output of the first op amp. Then the 1010 is just a voltage follower. Maybe a 0.1 ufd on it's output.
 

Thread Starter

cmartinez

Joined Jan 17, 2007
8,218
1. C1 is way too small; increase it to 100nF or thereabouts.
I increased it to 10nF (the datasheet specified only 1nF, that's why I used that value for starts) and everything's fine now.

2. Never drive heavy capacitive loads directly from an opamp output, unless you REALLY know what you're doing; put a 10Ω resistor in series with C3, C5, C7 and C8.
Those caps are there to stabilize the output. Removing the largest one (100uF) seems to have made no difference. I know, because I already scoped the thing and nothing seems to change.

3. What is the purpose of R4? I see no use for it.
R4 is there to simulate a load cell. This circuit is being used to excite a load cell using AC. That is why I'm using a 5V reference (boosted with a transistor) and a -5V source boosted with the LT1010.

4. Your trimpot value is way too small; it constitutes an unnecessarily heavy load on your +5V reference. Also, the way you've connected it is counterproductive. Replace your 1kΩ trimpot with a pair of 10kΩ resistors (preferably 1% or 0.1%) and connect a 10kΩ trimpot between +5V and -5V with its wiper connected to the OPA277 inverting input through a 100kΩ resistor. These changes will lighten the load on your +5V reference, and give you a much finer range of adjustment on your trimpot.
Just did that.... it worked like a charm... THANK YOU! :)

5. Also, make sure the OPA277's and LT1010's supply pins are properly decoupled.
already were

I suspect if you make these changes, your problem will disappear.
That is exactly what happened.... thanks again!
 
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