+5v supply ... but need -5v to drive a DAC

Without a DC-DC converter (such as a voltage inverter, either inductor or charge-pump based) or changing your power supply, there is no way to accomplish your goal. You ask for a method to produce a voltage when all you have is another voltage; that's DC-DC conversion.

If you mention your current requirements at -5V, someone might be able to recommend a voltage inverter or simple genereic DC-DC that will fill your needs.

With a tight interpretation of those constraints -- no.

You need to allow for a DC-DC converter of some sort. These can be done quite economically and quietly, however, so they shouldn't be ruled out. It's been a while since I've had to care, but IIRC both National and Linear Technology have easy to use switchers that can be made quite quiet just by following their app notes. In the project that I saw them used we had them co-resident on a board with a bunch of 16-bit ADCs, with no apparent problem.

You could also look at charge pumps. I couldn't guarantee that by the time you got one to be quiet in operation that it wouldn't be as big and complex as a switcher, but the actual converter part is simple and doesn't need inductors.

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Depends on the current you need. If you only need a few mA then you could use a charge pump. ICL 7660 or similar devices from Maxim. That's in deed same sort of dc/dc converter but not that kind you probably think of.

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are the inputs to the ADC floating? (is the negative input lead isolated from "ground"?) if so just reverse the leads on the +5v supplied to the input to get - 5v.

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How much current do you need from the -5V supply?

2ma should suffice

I tried this and it didnt work for me. Only +5 -5 (from another supply) worked

Point taken!

2ma should suffice. Believe it or not, I was thinking of tapping the negative voltage from a PC' serial port ... since I only need it for the DAC and opamp, and I happen to have a lot of the PC mountable db9's lying around.

Of course Ill invest in a dc converter ... perhaps for the next project. Looking forward to any and all advice.

Thanks

For instance a $1, superbly simple -5V from +5V inverting charge pump:

google "charge pump inverter" and you'll find maxim, ti, national, sipex, onsemi...

I'm reminded of the RS application note for the ancient ZN427.

That chip also needed a negative voltage, nominally -5V, but this did not need to be very accurate as it was just providing bias for the comparator tail. See page 16 of this:

for a circuit that generates a continuous -5V rail.

It was actually only needed during the A to D conversion, so a simplified circuit can just use the !CS signal to generate a single pulse of -5V.

I can't find the circuit diagram right now but I do have a board that used that circuit. I can buzz the connections for you if you wish. It's just a transistor, a diode and a capacitor IIRC.

I think I might have used a ZN448 See page 10 of:

I used circuit (b): One tranny, one cap, one diode, three resistors.

In article , John_H wrote: [...]

He may be able to do it with a marking pen.

Relabel the +5V as ground and the ground as -5V. Now he doesn't have a

+5V. He may not need one if the part he is using is assuming ECL supplies and signalling.

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kensmith@rahul.net   forging knowledge

Do you also want an RS232 I/O? If so add a LT1081 and a 3 pin regulator.

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kensmith@rahul.net   forging knowledge

Depending on the amount of current you need you can derive + and - voltages from as low as 4 VDC supply using an LT1026 -- I have a couple tubes of these if you need

go 1/2 way down the page. It's a good idea to postfilter the output, and they like to work with a load of several ma.

You can also derive an artificial ground with an opamp -- if you need more current you can use one of the huskier varieties from Burr-Brown.