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Registered Member #509
Joined: Sat Feb 10 2007, 07:02AM
Location:
Posts: 329
I'm building a high impedance buffer for my multimeter so I can measure some high value dividers in a battery monitoring system, and I just wanted to make sure I'm avoiding any 'gotchas'
I want to put the above circuit in a project box as a buffer. Since I had a DPDT switch handy, I wanted to use it to pick from unipolar and bipolar output in addition to a power switch, by moving the virtual ground from Vcc/2 to ground.
The OPA 2703 datasheet claims input range is 300mV beyond the rails, and the output is within 40mV of the rails, so that should be in spec of the chip.
But if I want my meter to go to 0V with the leads shorted, should I put pull up resistor from the virtual earth to V+ to ensure the virtual ground is always loaded more than the voltage buffer? I know the buffer is sourcing the multimeter current vs the virtual ground sinking, but to allow for manufacturing tolerance in the output resistance etc? Something like a 100k-1M pull up on the virtual earth output.
Also, as far as decoupling capacitors, just V+ to V-, or should I do V+ to virtual earth and virtual earth to V-? The capacitive load driving capability of the OPA2703 isnt really high, so I'm not sure if I should leave it be, or add a series resistor (50-100ohms) and put 10nF to V+ and V-.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
The circuit should work fine as it is. Adding decoupling capacitors to the virtual earth could make the op-amp oscillate. I've always got away with it in the past, though.
The DPDT switch you drew has the power on in both positions. Why do you need to choose between unipolar and bipolar anyway?
Registered Member #509
Joined: Sat Feb 10 2007, 07:02AM
Location:
Posts: 329
There is a middle "off" position in the DPDT I have, just not in the schematic symbol I used in eagle, so power will be disconnected there. And for what I'm doing at the moment, I guess there is no real reason for unipolar/bipolar, I inadvertently ordered a 3 postition switch instead of 2 position, so it might as well do something besides have 2 "On" position, In case it comes in handy in the future.
got the case made up.
makes some protoboard inserts for some handheld hammond enclosures, so this should come out nicely :)
Thanks for the help Steve.
EDIT: and after a bit of soldering, my meter should have an input impedance of the PCB leakage hopefully. The meter definitely all over the place with no input, but at 5.00 on my DMM check voltage reference, so I'll call it successful.
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Op-amp question on high impedance buffer
