Fast analog sw. as VC resistor

Paolo Predonzani predo at
Fri Jan 3 15:15:58 CET 1997

> > I realized last night that it would be better to use an analog switch in
> > PARALLEL with a fixed resistor, rather than without a fixed resistor like
> > in my previous post. If you use a bilateral switch like in the 4066 and two
> > resitors, you can make a three-terminal pot very easily:
> > 
> > ------+----//////-------+------//////--------+---
> >       |      R          |        R           |
> >       |                 |                    |
> >       +---------------o/  o------------------+
> >                     1/4 4066
> > 
> > This way, you can vary the resistance to any fraction of R, and just use
> > one PWM clock to control a pot.
> I'm correcting myself again: the 4066 is not a quad SPDT switch, it's a
> quad SPST switch. So to do the circuit above would require 1/2 of a 4066.
> and an inverter for the clock. I was thinking of the 4053 2:1 analog mux.
> 1/3 of it could do the circuit above with no extra inverter.
Hi Mark.
I still don't understand why you need such complicated a circuit.
Anyhow, switching resistors are an interesting topic. Look at this circuit.
It's based on a switched capacitor and is not more difficult than yours.

V1 -------------o\  o---------------V2
                -----   C

This circuit implements a variable resistor between V1 and V2.
Here is how it works:
At every cycle the capacitor C charges at V1 and discharges at V2. So
the charge Q = C*dV is transfered fron V1 to V2 (dV = V1 - V2).
Now, let's divide the above formula by T, the switching period.

Q/T = C*dV/T = C*dV*f

Q/T is, on average, equal to the current I from V1 to V2.
I = C * f * dV


R = dV/I = 1/(C*f)

R is the frequency controlled resistance you need.

There are many commercial ICs that use switched capacitors to make n-pole filters.
They usually need some external components. Probably if you don't connect
those components you can use the internal, more reliable switches.
I'll look at my databooks at home.


| Paolo Predonzani  |  email: predo at |

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