[sdiy] digitally controlled potentiometers

[Ingo Debus]

Am 06.05.2007 um 19:01 schrieb Heiko van der Linden:

> Another option was suggested by Ian Fritz: to use to mosfets  
> connected together
> by the sources. http://www.zetex.com/3.0/appnotes/design/dn3.pdf
> And a furtherl method was suggested by ASSI. He suggested to use  
> control gates like
> the DG4xx series. I can see problems with both methods. The first  
> method will result
> in a voltage loss due to the forward diode drop over the substrate  
> diode

I don't think so. When the switch is on, both FETs conduct, thus  
bypassing both diodes. You only need two FETs because when a FET is  
off, the diode would still conduct in one direction.
I once built such a switch. It worked well, and could handle several  
amperes.

AFAIK there are switch ICs that have an on-resistance of only a few  
ohms. I think there are also optocouples that can be used as analog  
switch with quite low on-resistance.

> The lowest resistor is R, than 2R, 4R, 8R etc. I want to use  
> digitally controlled
> switches to bypass the resistors and thus increase or decrease the  
> resistance
> according to a digital value.

This way you get a variable resistor. For a true potentiometer you  
would need this circuit twice, no?

Ingo