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Discussion in 'The Projects Forum' started by AetherNZ, May 7, 2008.

1. ### AetherNZ Thread Starter Member

Mar 23, 2008
21
0
Im building a dual-slope ADC for a university project which needs to run on +12V and 0V supply.

Vin can range from 100 uV to 2 V.

I have read up about the integrator circuit and think I know how it works. Using the Vout = 1/RC|Vin.dt formula have calculated the RC values.

I have a couple of problems though:

How do I create the negative Vref voltage with only 0 and 12V? Charge pump type circuit? or maybe bias the voltage regulator?

How do you determine what size R and C should be? eg 1uF & 22k or 1nF & 220k ect does it even matter?

How do I build the digital switch shown in the diagram? Multiplexer from a few FETs?

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2. ### mik3 Senior Member

Feb 4, 2008
4,846
70
You can use a battery with its positive terminal connected to ground and its negative to -Vref. Otherwise search in google for inverting DC-DC converter.

3. ### AetherNZ Thread Starter Member

Mar 23, 2008
21
0
Cheers, battery wouldn't be too elegant for this design. Will have a look into inverting DC-DC converters, thanks

4. ### Papabravo Expert

Feb 24, 2006
12,285
2,724
Either use a bipolar supply, or create a virtual ground at V/2 = 6V and set -Vref at ground. Then AC couple the input via a capacitor.

5. ### AetherNZ Thread Starter Member

Mar 23, 2008
21
0
Thanks for the reply, can't use a bipolar supply as its designed to run on a 12V lead acid battery system.
Yea I was thinking that a virtual ground would work alright as I only need 5V out of the 12, cheers.

6. ### Papabravo Expert

Feb 24, 2006
12,285
2,724
If you were going to use a DC-DC converter for -Vref anyway, what's the problem with having a bipolar supply? You certainly don't want to have single supply amplifiers with a negative reference that won't work at all.

With +12V in you have a -12V out DC-DC converter. Now you have a bipolar supply. What's the problem?

You do know that single supply amplifiers often have trouble going all the way to the rails -- right?

7. ### AetherNZ Thread Starter Member

Mar 23, 2008
21
0
Ah I see what you mean by having a bipolar supply. I thought you meant just get a dual rail power supply haha.

yea I was wondering if that was going to work. So you think I should set myself up with a solid -12V +12V system using a DC-DC converter and use dual supply op-amps?

8. ### Papabravo Expert

Feb 24, 2006
12,285
2,724
Yes. The DC-DC converter is an easily obtained module. Once you get it working, you can branch out and try alternatives.

9. ### AetherNZ Thread Starter Member

Mar 23, 2008
21
0
Thanks for the help, will give it a go. Any suggestions for the digital switch?

Mar 24, 2008
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