# Fast analog sw. as VC resistor

Paolo Predonzani predo at dist.dist.unige.it
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
|
|
GND

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.
So:
I = C * f * dV

and

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.

Bye.

--
+-------------------+----------------------------------+
| Paolo Predonzani  |  email: predo at dist.dist.unige.it |
+-------------------+----------------------------------+

```