[sdiy] Op Amp Question (new ascii schem)

[Harry Bissell]

With the switch open, the only effective path is from opamp 1
output through the 22K. With the switch closed, the path from opamp 1
is 11K and opamp 2 is 22K so you'd get twice the signal from opamp 1.

There should be no trouble between the opamps in practice. If the circuit
was REALLY fussy you could buffer the integrator output so that it sees
a constant impedance at all times. Probably not necessary. Another solution
(if you do have an issue) is to raise the impedances of all the summing
resistors.

The opamp output impedance ~is~ finite, there will be an error but its probably
acceptably small.

One place where I did NOT find this to be true was a pseudo-summing junction
for a VCO expo converter. Opening and closing a similar analog switch caused
a slight change of impedance and threw the tuning off an annoyingly small
amount (forcing retuning). Buffers did the trick.

H^) harry


----- Original Message -----
From: Stewart Pye <stewpye at optusnet.com.au>
To: Synth DIY <synth-diy at dropmix.xs4all.nl>
Sent: Tue, 14 Dec 2010 16:27:54 -0500 (EST)
Subject: Re: [sdiy] Op Amp Question (new ascii schem)

  Hopefully this comes out better...



                 |---------////22k////------------|
                 |                                |   
                 |                                |               
OP amp 1 Output --- ////22k//// |                 |
                                |                 |         
                                |--analog switch ----Op Amp inverting input
                                |
OP amp 2 Output --- ////22k//// |



I have a circuit where I want to switch two additional inputs to an op 
amp summing circuit using an analog switch. So that I can get away with 
using just one analog switch I had the idea of joining the summing end 
of the resistors together and have an analog switch between there and 
the summing node.


My question is when the switch is in the off position we are left with a 
44k resistor between the two op amp outputs. Would this cause any bleed 
from one output to the other, or does the low output impedance of the op 
amps eliminate this?
When the switch is off I want only the op amp 1 output ( with half the 
gain of when the switch is on) but don't want any bleed from op amp 2 
output...

Cheers,
Stew.












Andre Majorel wrote:
>>     
>
> Stewart,
>
> FYI your schematic is unreadable here. Always switch to a fixed
> spacing font before attempting ASCII art.
>
>   

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-- 
Harry Bissell & Nora Abdullah 4eva