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'[EE] Charge pump as analog negative supply'
2005\11\08@100505 by Chris Emerson

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Hello all,

I have a project which will have both a PIC and some op-amps to
amplify/filter the signal from a 40kHz ultrasonic transducer.

It would be convenient to have a negative supply for the analog side,
and since I've got a PIC handy I wondered about generating the negative
supply with a simple capacitor/diode charge pump.  That would give me
roughly -3.6 (or -4.3 with Schottky diodes) to +5V which would be just
fine.  I would imagine I'd want to avoid running the charge pump at,
say, 40kHz.

The question is how much of a problem would the relatively poor
regulation of the negative supply likely to cause me?  The op-amp
circuits will all have feedback and will be referenced to 0V.

Best regards,

Chris

2005\11\08@101623 by Mchipguru

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Depending on signal characteristics and the power supply rejection ratio of the Op Amps it may give no problems. I have done this before without problem. If you need to you could use a zener to regulate the negative supply.
Larry
{Quote hidden}

> --

2005\11\08@101655 by Neil Baylis

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It's pretty common to do this using a max-232 or similar device to
generate +/- 10 Volts. But in addition, there are plenty of good
single supply op-amps available..

Neil

On 11/8/05, Chris Emerson <picspamKILLspamnosreme.org> wrote:
{Quote hidden}

> -

2005\11\08@102936 by Mike Hord

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> I have a project which will have both a PIC and some op-amps to
> amplify/filter the signal from a 40kHz ultrasonic transducer.
>
> It would be convenient to have a negative supply for the analog side,
> and since I've got a PIC handy I wondered about generating the negative
> supply with a simple capacitor/diode charge pump.  That would give me
> roughly -3.6 (or -4.3 with Schottky diodes) to +5V which would be just
> fine.  I would imagine I'd want to avoid running the charge pump at,
> say, 40kHz.

Actually, running the charge pump at 40 kHz may not be a bad idea.
If the ultrasonic and the charge pump are in phase, it'll reduce the
noise during the time you're interested in- the few microseconds
after the audio pulse when you're looking for the reflected signal.

> The question is how much of a problem would the relatively poor
> regulation of the negative supply likely to cause me?  The op-amp
> circuits will all have feedback and will be referenced to 0V.

I've made some fairly good circuits using charge pumps to create
a negative bias, although I typically use something like the LMC7660,
which is an 8-pin part using two external 10 uF caps to provide a
pretty good -V from a +V input.  It also pays to watch the PSRR of
your op-amps and be smart about bypassing.

Mike H.

2005\11\08@105337 by Dave Tweed

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Chris Emerson <.....picKILLspamspam.....nosreme.org> wrote:
> The question is how much of a problem would the relatively poor
> regulation of the negative supply likely to cause me?

The "regulation" will simply be the regulation of the positive supply
as seen through the impedance of the charge pump. You can control the
latter through the size of the pump capacitor and the switching
frequency. For example, a frequency of 100 kHz and a capacitor of
1 uF will yield an effective impedance of 1/f*C = 10 ohms.

Whether the variations in the negative supply are a problem for the
opamps is determined by their power supply rejection ratio (PSRR),
which is generally pretty good for most modern chips.

-- Dave Tweed

2005\11\08@122628 by Spehro Pefhany

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At 10:53 AM 11/8/2005 -0500, you wrote:
>Chris Emerson <EraseMEpicspam_OUTspamTakeThisOuTnosreme.org> wrote:
> > The question is how much of a problem would the relatively poor
> > regulation of the negative supply likely to cause me?
>
>The "regulation" will simply be the regulation of the positive supply
>as seen through the impedance of the charge pump. You can control the
>latter through the size of the pump capacitor and the switching
>frequency. For example, a frequency of 100 kHz and a capacitor of
>1 uF will yield an effective impedance of 1/f*C = 10 ohms.

Which would be thoroughly swamped by the resistance of the PIC port drivers,
of course. This sort of thing is most useful when the amount of current
to be drawn is very small, say in the scores of uA or so, and it doesn't
matter much if it is +/- 0.5V or whatever. It can come in handy where you
can't control the ground reference and need to accommodate negative
voltages-- otherwise it's probably best to design your circuit to work
with a single solid supply.

>Whether the variations in the negative supply are a problem for the
>opamps is determined by their power supply rejection ratio (PSRR),
>which is generally pretty good for most modern chips.

Provided you don't run out of "headroom" (or is it footroom, when it's
negative? ;-) ). That includes common mode range and output swing. The
PSRR is also much better at DC than at high frequencies, so best to filter
the supplies well (unless you're using them as outputs or something really
clever like that).

>Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
speffspamspam_OUTinterlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
->> Inexpensive test equipment & parts http://search.ebay.com/_W0QQsassZspeff


2005\11\08@142544 by Richard Prosser

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Or if you're already using a MAX232 etc. you may be able to get the
negative supply from that.
RP

On 09/11/05, Spehro Pefhany <@spam@speffKILLspamspaminterlog.com> wrote:
{Quote hidden}

> -

2005\11\08@144912 by David Minkler

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PSRR at DC is usually pretty good but it falls off with frequency (check
the data sheets on your favorite op amp).  The real problem here is not
likely to be regulation but rather ripple at the charge pump's switching
frequency.  By the time you get to 40 - 100kHz, PSRR on even fairly good
op amps is down to 40dB or so.  If you have significant ripple at your
switching frequency, it'll bleed right into the signal path.

You can reduce ripple by having two 180 degrees out of phase pumps
charging a single supply capacitor.  No theoretical reason this can't be
extended to say, 4 pumps 90 degrees out of phase.  It'll start to look
silly though and you can do a fair bit of good with a simple filter on
the output of the supply.

Dave

Dave Tweed wrote:

{Quote hidden}

2005\11\08@162852 by olin piclist

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Chris Emerson wrote:
> I have a project which will have both a PIC and some op-amps to
> amplify/filter the signal from a 40kHz ultrasonic transducer.
>
> It would be convenient to have a negative supply for the analog side,
> and since I've got a PIC handy I wondered about generating the
> negative supply with a simple capacitor/diode charge pump.  That
> would give me roughly -3.6 (or -4.3 with Schottky diodes) to +5V
> which would be just fine.  I would imagine I'd want to avoid running
> the charge pump at, say, 40kHz.
>
> The question is how much of a problem would the relatively poor
> regulation of the negative supply likely to cause me?  The op-amp
> circuits will all have feedback and will be referenced to 0V.

You're probably fine.  I'm thinking of doing something similar in a current
project.  The opamp shouldn't draw that much current, so you should be able
to put a resistor in series with the opamp supply and a capacitor to ground
if needed.  Look up the power supply rejection ratio of the opamp and figure
out how much ripply you can tolerate on the supply.


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Embed Inc, Littleton Massachusetts, (978) 742-9014.  #1 PIC
consultant in 2004 program year.  http://www.embedinc.com/products

2005\11\09@083542 by Chris Emerson

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On Tue, Nov 08, 2005 at 03:05:04PM +0000, Chris Emerson wrote:
> The question is how much of a problem would the relatively poor
> regulation of the negative supply likely to cause me?  The op-amp
> circuits will all have feedback and will be referenced to 0V.

Thanks to everyone who given interesting and informative replies.  I'll
certainly try it, but compare to a single-supply alternative.

Regards,

Chris

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