FW: RE: [sdiy] Building in a DC offset

[Christopher Mullin]

Hi all,

  thanks for the replies.  I suppose my original message should have had an 
extreme Newbie warning.  However, after literally an evening of dedicated  
work, I have studied Op-Amps, and I think I might have come up a design to 
bring a signal of ±5V to 0V-10V.  I am using an analogue meter from Maplin 
with an internal resistance of 675? and a range of 0V-0.5V.  It can be seen 
here:

http://www.maplin.co.uk/Module.aspx?ModuleNo=5635&TabID=1&source=15&WorldID=&doy=15m1

  I'm after taking both CV and audio signals from A Sys/Doepfer kit, so max 
strength of 10Vss in.  There will be a simple attenuator (either external, 
or built into the input socket).  The signal will go to SPDT switch to 
select between the rectifier and offset circuits.

  For rectification, I'm using a bridge rectifier with a large capaciter 
(1000µF, 16V) for smoothing.  I'm fairly sure I've got this bit right.  I'm 
not so sure about the op-amp bit.  I'm after a non-inverting op-amp using 
the input signal to one input, and using the power rails (±12V), voltage 
split by a 10k lin pot  to provide a pos or neg offset.  However, I want to 
limit this offset to ±5V, but I'm not sure what value R1 should be.

  Would anyone be so kind as to look at a diagram I can email to them and 
see if I'm anywhere near a working circuit, and can suggest R1?

  Very much appreciated,

Stuadh


>From: "J. Larry Hendry" <jlarryh at iquest.net>
>To: "Christopher Mullin" <stuadh at hotmail.com>
>Subject: RE: [sdiy] Building in a DC offset
>Date: Thu, 13 Jan 2005 19:12:11 -0500
>
>The easiest method to do what you want can be done with almost any 
decent op
>amp.  Get a dual op amp.  Configure the first stage as a summing amp 
(also
>inverts).  You can sum in the correct amount of DC offset there. It's 
all
>about just picking the right resistor size. Configure the second stage 
to
>re-invert to get your polarity back to correct.
>
>There are many books (including some very inexpensive ones at Radio 
shack)
>that show how to wire up these little op amps.  They will work fine on
>+/-12V supply.  Get something like a TL072.  You will need only a few
>resistors and a couple of capacitors to do what you want.
>
>I am not certain how you intend to rectify or what your accuracy needs 
are.
>There are also simple op-amp circuits for accurate rectification.  A 
simple
>diode will rectify but give you the error caused by the voltage drop 
across
>the diode itself.
>
>Larry
>
>
>-----Original Message-----
>From: owner-synth-diy at dropmix.xs4all.nl
>[mailto:owner-synth-diy at dropmix.xs4all.nl]On Behalf Of Christopher
>Mullin
>Sent: Thursday, January 13, 2005 6:03 PM
>To: synth-diy at dropmix.xs4all.nl
>Subject: [sdiy] Building in a DC offset
>
>
>
>
>I am thinking about building an analogue meter like to one shown here:
>
>http://www.angelfire.com/music5/a100/sounds/DIYmodules/NET5COIL.htm
>
>However, I'm thinking about building in a rectified mode and an offset 
mode.
>   I use ±12V in a Analogue Systems/Doepfer hybrid, so I'm after being 
able
>to represent ±5V on a range of 0-10V.  I'm happy with building in a
>rectifier, but can anyone show me how to make the offset circuit?
>
>Stuadh
>
>
>
>
>