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Old 12th September 2012, 08:18 PM   #1
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Default Inverting rail to rail unity buffer

Hey!

I am trying to make a MCU controlled version of something similar to this Lightspeed Attenuator a new passive preamp

I have a DAC hooked up that outputs approx. 0 to +5V and is controlled by an MCU. I am currently using a MCP6002 rail to rail op amp as a non-inverting buffer for the DAC to the first LDR. This seems to work ok but I need an inverting rail to rail buffer that goes from +5V to 0 for the second LDR. I have some difficulties with getting the inverting circuit to have a similar taper to the non-inverting with the MCP6002. Anybody having good suggestions for a circuit?

Hope someone can help!
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Old 13th September 2012, 02:50 PM   #2
Itsmee is offline Itsmee  England
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The easiest way to achieve what you wish is to connect a unity gain inverting opamp input to the output of the non inverting opamp.

For unity gain, make Rf equal Rin.
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Old 14th September 2012, 07:00 AM   #3
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Quote:
The easiest way to achieve what you wish is to connect a unity gain inverting opamp input to the output of the non inverting opamp.

For unity gain, make Rf equal Rin.
Have tried. It doesn't follow the non-inverting buffers taper at all. Actually, I just came to think of that I might be able to let one of the LDR's sink then I might not need the inverting buffer (I like the idea with driving it differential though (two wires with constant 5V measured from the outside)).
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Old 15th September 2012, 02:03 PM   #4
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It may be that you need to subtract your DAC's ouptut impedance from the value of Rin for the inverting buffer circuit. For example, if Rf = 10k, and your DAC's Zout = 1k, then the value of Rin should be 9k.
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Last edited by Ken Newton; 15th September 2012 at 02:06 PM.
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Old 16th September 2012, 08:49 AM   #5
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Originally Posted by Ken Newton View Post
It may be that you need to subtract your DAC's ouptut impedance from the value of Rin for the inverting buffer circuit. For example, if Rf = 10k, and your DAC's Zout = 1k, then the value of Rin should be 9k.
Thanks Ken. Actually I think I can see the problem now in what I am trying to do;

I want to run differentiable 0-5V. I am using a rail to rail op amp with 0-5V supply. I goes fine with the non-inverting coupling but the other has an amplification of -1 if Rf=Rin which mean that 5V in makes -5V out (?). But the lowest voltage I got is 0V. The application has to be able to run from a 9V battery as well+protection diode so I have like 8.3V max. all in all. So +-5V for supply on the op amp is not possible. Hmm..any good ideas?
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Old 16th September 2012, 10:58 AM   #6
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I think I found a solution. As mentioned earlier I'm sinking one of the LDRs and sourcing the other (using two non-inverting buffers). I have to test the circuit a little more en details but it seem to behave.
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Old 16th September 2012, 02:01 PM   #7
DUG is offline DUG  Canada
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Itsme's circuit need to have the + input at +2.5V

Then it will work for your 0-5 into 5-0
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Old 19th September 2012, 07:29 AM   #8
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Originally Posted by DUG View Post
Itsme's circuit need to have the + input at +2.5V

Then it will work for your 0-5 into 5-0
Thanks Dug! That was the ticket Works like a charm..
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Old 22nd September 2012, 10:53 AM   #9
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Originally Posted by Ken Newton View Post
It may be that you need to subtract your DAC's ouptut impedance from the value of Rin for the inverting buffer circuit. For example, if Rf = 10k, and your DAC's Zout = 1k, then the value of Rin should be 9k.
I am have the same problem and wondering how to get unity gain with that calculation?

how do you find dac output impedance if not quoted in datasheet?

datasheet only gives output load resistance min 3 kohm (tda1549)

thanks
Glen
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Old 22nd September 2012, 11:10 AM   #10
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Glen: I actually have unity gain (ennough) without considering the DAC output impedance, so I haven't really paid attention to that. I can't see it in the datasheet either.
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