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Author Topic: Little op-amp help/chalange  (Read 963 times)
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Wizpic
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« on: August 13, 2011, 04:42:46 16:42 »

Attached is a circuit ot of a fluke volt meter and I've tried to simulate it in ISIS but keep getting strange results and not sure if ISIS can. Has I found sometimes is ISIS it does not always work has it say it does.

The question is I don't fully understsand how op-amps or how the calculations are made so I thought I'd ask the question here, In the attached PDF is the schematic with some REF points on there.
What would be the voltages be at REF1,REF2,REF3,REF4,REF5 say if I had an input voltage say 35V

This should help me get a better understanding on how the circuit works, Plus would I have to use the same op-amp's has there labled in the schematic, I did go to order them but found they where very costly and this is another reason I had not built it up in real life did not want to waist money to find out it does not do what I thought it did.

The idea is to use a PIC chip and to make a custom built voltmeter I only need to measure dc voltage upto 70v really, I'm thinking of adding a built in data logger, and a daul axis level sensor and may be a hieght sensor.
I think the way it works on the input side it does not matter which way round the leads get connected from the battery am I thinking correctly.
any info will help
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solutions
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« Reply #1 on: August 14, 2011, 01:24:57 01:24 »

The first op amp is a clamp to keep from overdriving the following stages

The second is a variable gain/attenuator stage (can't tell where those switches go to say which for sure, but most likely attenuator)

Third and fourth stage are low pass filters. DC voltage on your Ref nodes here is a silly concept.

Each stage is independent, so you should be able to simulate each one individually in SPICE. But not just DC....you'll need to do an AC analysis to fully understand it.

If it were me, I'd clone the whole thing and replace the X1, X2,and X4 range selects with the PIC. Assuming you need variable gain/attenuation.

Please post the rest of the Fluke schematic.  Could be useful to some of us at some point.

==========
Looking at it again, the second stage varies gain. 1, 2, 4. 
« Last Edit: August 14, 2011, 05:50:15 05:50 by solutions » Logged
hosmis
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« Reply #2 on: August 14, 2011, 02:20:32 02:20 »

single supply rail to rail ....

http://www.futurlec.com/ICLinearOpAmps1.shtml

http://www.futurlec.com/cgi-bin/search/searchold.cgi?Q=rail+to+rail&E=1&X=0
« Last Edit: August 14, 2011, 02:23:10 02:23 by hosmis » Logged
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« Reply #3 on: August 14, 2011, 09:24:49 09:24 »

Nice one that helps and nicely explained,
Here is the complete service manual for the fluke meter 39/41b

I shall do a bit of playing and break it down like you said and see how I get on

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