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'PIC A/D, pre-amplify small singnal more than large'
1999\02\24@101911 by Stefan Sczekalla-Waldschmidt

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Hi,

I have a Signal I like to measure with the pic onboard A/D which is
some Millivolts up to 8 Volts. Now I4m looking for something like a
VCA or something which will amplify a small signal more than a
large one.

The resulting signal should fit the range 0..FF ( 0..5 V )

Any suggestions ?

Kind regards

       Stefan

1999\02\24@111815 by Michael Rigby-Jones

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The device you describe is a compressor, used to reduce the dynamic range of
a signal.  An expander is used to do the opposite.

A transconductance amp controlled by a recitifed/filtered version of the
signal is the normal configuration.  It is possible to build a cheap and
nasty version using an LDR as part of the feedback path of a normal op-amp
circuit and using an LED driven by the input signal to vary it's resistance

If you do a search on this circuit I'm sure there will be some around on the
web

Regards

Mike Rigby-Jones
spam_OUTmrjonesTakeThisOuTspamnortelnetworks.com


{Quote hidden}

1999\02\25@065716 by Stefan Sczekalla-Waldschmidt

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Michael Rigby-Jones wrote:
>
> A transconductance amp controlled by a recitifed/filtered version of the
> signal is the normal configuration.

Can somebody eventually suggest a Voltag Controlled Amp (VCA) Circuit
which is happy with single voltage supply ?

Kind regards

       Stefan

1999\02\25@081304 by Stefan Sczekalla-Waldschmidt

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Hi,

I found that the thing I4m looking for is called varaible gain
amplifier.

Any sugestions on which one to use or how to build ?

Kind regards,

       Stefan

1999\02\25@094509 by mlsirton

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Hi,

On 25 Feb 99 at 14:14, Stefan Sczekalla-Waldschmidt wrote:
> I found that the thing I4m looking for is called varaible gain
> amplifier.
>
> Any sugestions on which one to use or how to build ?

You can buy such beasties off the shelf from Burr-Brown:
PGA102 for example lets you switch between gains of 10 and 100,
other PGA*** chips have other options. (PGA stands for Programmable
Gain Amplifier)

If you want to make your own you can use a digital potentiometer
(some are made by Dallas I believe) as a feedback resistor in an
op-amp to provide programmable gain.

Those digital pots are some form of D/A. I don't remember the exact
configuration but if you look at their data sheets and app notes you
might be able to duplicate their function with just the PIC and some
discreetes, maybe someone else can shed more light on this...

Hope this helps,
Guy - .....mlsirtonKILLspamspam.....inter.net.il

1999\02\25@102658 by Stefan Sczekalla-Waldschmidt

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Hi,

just looked up the places you4ve suggested.

> You can buy such beasties off the shelf from Burr-Brown:
> PGA102 for example lets you switch between gains of 10 and 100,
> other PGA*** chips have other options. (PGA stands for Programmable
> Gain Amplifier)

They have a binary programmable PGA 103 amp. Problem with it: price.

> If you want to make your own you can use a digital potentiometer
> (some are made by Dallas I believe) as a feedback resistor in an
> op-amp to provide programmable gain.

Looks promising, I will give this a try.

> Those digital pots are some form of D/A. I don't remember the exact
> configuration but if you look at their data sheets and app notes you
> might be able to duplicate their function with just the PIC and some
> discreetes, maybe someone else can shed more light on this...

They have a app-note about lcd-contrast control which covers also
( nobody knows why ) gain adjustable amps.

Sombody knows who also produces ee-pot4s ?

Kind regards

       Stefan

1999\02\25@132549 by John Payson

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> A transconductance amp controlled by a recitifed/filtered version of the
> signal is the normal configuration.

|Can somebody eventually suggest a Voltag Controlled Amp (VCA) Circuit
|which is happy with single voltage supply ?

Since it's easy to design an amplifier which is "gatable", one
nice trick for reasonably low-frequency signals is to PWM the
enable of such an amp and then filter the output.  This will
give decent performance if the PWM rate is sufficiently above
the highest signal of interest, and has the advantages of being
simple and working with readily-available components.

1999\02\26@013501 by Bill Arkin

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I have used the Dallas DS1806 hex digipots with both VFO's and VGA's from a
PIC with excellent results for generating wave patterns for laser sheet
scanners for entertainment.  They have 64 positions on each pot and you
shift in 48 bits in this case.

I was hoping that there might be some IC's now with I2C or 3 wire interface
for direct generation of sine or square wave as well as signal amplitude
control.  Anyone know of such devices?

Bill Arkin
**25 Years of Laser Services**
          Holo-Spectra Inc.
          7742B Gloria Ave.
        Van Nuys, CA 91406
             818 994-9577
          http://www.lasershs.com


{Quote hidden}

1999\02\26@122538 by Gerhard Fiedler

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At 22:50 02/25/99 -0800, Bill Arkin wrote:
>I was hoping that there might be some IC's now with I2C or 3 wire interface
>for direct generation of sine or square wave as well as signal amplitude
>control.  Anyone know of such devices?

i was just looking for a digitally controlled sine oscillator, but i
couldn't find any. if you come up with something, i'd appreciate a note.

ge

1999\02\26@151729 by Anderson, Roger

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I've used the ML2035 and ML2036 from Micro Linear before.
Programmable frequency 0-50kHz
Frequency resolution 1.5Hz
Harmonic Distortion -45dB max
Uses a serial interface like SPI


Roger

Roger H. Anderson
Bioengineering
The University of Iowa
54 Medical Research Facility
Iowa City, IA  52242-1183
(319) 335-8644


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