[Negative to Positive CV input circuit question]

[Harry Bissell]

Harry Bissell writes: That might work. I think the gain might be a little
funny because when you put the wiper of the pot all the way to one side you
would have effectivly, 200K in series to one op-amp, and 100k to the other.
With 100K feedback resistors, that would give unity gain in the first stage,
and -1/2 gain in the other. So the circuit would divide CV by two, which could
be scaled back later (in the scanner itself). Or if the input CV was high
enough, you wouldn't care.... Nice Idea I will remember it... If noone has
objections for other reasons we can use it, so I'll wait for more comments.

The "varying input impedance" of the circuit as I drew it is usually only a
problem when it is driven from a high impedance source, or when the source
changes frequency response with different loads (like a guitar pickup,
microphone, piezo sensor etc...) Usually CV is buffered output with about a
series 1K resistor, this makes the error usually about 1-2%
:-) Harry


JWBarlow at aol.com wrote:
Hi Harry:

I've been thinking about the CV inputs for the Interpolating Scanner (the 
inputs which go negative to the left, positive to the right with zero in the 
middle) and I have a question for you. I don't know very much about 
electronics, so forgive my ignorance. About 15 years ago a friend gave me a 
little diagram of a circuit for these type of CV inputs, and I thought it was 
the type of circuit that Serge used. It works like this:

1) A basic double inverting buffer scheme (so the output is in phase with the 
input). The positive side of both opamps goes to ground and the output of the 
first opamp feeds back in to the negative input through a 100K resistor, and 
to the negative input second opamp through a 100K R. The output of the second 
opamp again feeds back to its negative input through a 100K R; i.e., a basic 
100k double inverter circuit.

2) Now, take the CV input to the wiper of a 100K pot.

3) Take one leg of the pot through a 100K R and to the summing node of the 
first opamp.

4) Take the other leg of the pot through a 100K R and to the summing node of 
the second opamp.

Does this make sense?

I've used this circuit a number of times with no problem. More CV inputs can 
be added by just adding the pot and two (pot leg) resistors to the summing 
nodes.

So my question is (FINALLY): is there a problem with this idea? I remember 
someone saying that the method of swinging an input between the differential 
inputs was "bad practice" since it changed the input impedance of the 
circuit, I don't know if that's true, but all the talk about adding more 
opamps in order to add more of these type of inputs made me think of this 
question.

Thanks
John Barlow


____________________________________________________________________
Get your own FREE, personal Netscape WebMail account today at http://webmail.netscape.com.