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

[pronforall]

I have a circuit thats driven by a +5v supply ... but I need -5v
(I could also use values between +-18v but since I have 5v ... I want to
see what I can do with that)

Does anyone have an idea how I might rig this up without having to buy
a dc converter or changing my power supply?

Thanks

 

[John_H]

I have a circuit thats driven by a +5v supply ... but I need -5v
(I could also use values between +-18v but since I have 5v ... I want to
see what I can do with that)

Does anyone have an idea how I might rig this up without having to buy
a dc converter or changing my power supply?

Thanks

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.

 

[Tim Wescott]

I have a circuit thats driven by a +5v supply ... but I need -5v
(I could also use values between +-18v but since I have 5v ... I want to
see what I can do with that)

Does anyone have an idea how I might rig this up without having to buy
a dc converter or changing my power supply?

Thanks

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.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Posting from Google? See http://cfaj.freeshell.org/google/

 

[Peter Heitzer]

I have a circuit thats driven by a +5v supply ... but I need -5v
(I could also use values between +-18v but since I have 5v ... I want to
see what I can do with that)

Does anyone have an idea how I might rig this up without having to buy
a dc converter or changing my power supply?

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.

 

[no_one]

I have a circuit thats driven by a +5v supply ... but I need -5v
(I could also use values between +-18v but since I have 5v ... I want to
see what I can do with that)

Does anyone have an idea how I might rig this up without having to buy
a dc converter or changing my power supply?

Thanks

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.

 

[John Fields]

I have a circuit thats driven by a +5v supply ... but I need -5v
(I could also use values between +-18v but since I have 5v ... I want to
see what I can do with that)

Does anyone have an idea how I might rig this up without having to buy
a dc converter or changing my power supply?

 

[pronforall]

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

2ma should suffice

 

[pronforall]

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.

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

 

[pronforall]

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.

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

 

[John_H]

For instance a $1, superbly simple -5V from +5V inverting charge pump:
http://www.maxim-ic.com/quick_view2.cfm/qv_pk/2050

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

 

[Kryten]

I have a circuit thats driven by a +5v supply ... but I need -5v
(I could also use values between +-18v but since I have 5v ... I want to
see what I can do with that)

Does anyone have an idea how I might rig this up without having to buy
a dc converter or changing my power supply?

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:
http://www.web-ee.com/Electronic-Projects/data/zn427.pdf
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.

 

[Kryten]

I think I might have used a ZN448
See page 10 of:
http://www.datasheets.org.uk/datasheet.php?article=3618516

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

 

[Ken Smith]

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.

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.

 

[Ken Smith]

2ma should suffice

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

 

[jack]

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 http://www.tech-diy.com/smpschips.htm 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.