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Re: bipolar digital pot
by Mark Rages
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On Thu, May 8, 2008 at 11:45 AM, Martin <martin@...> wrote:
> I have an analog circuit that has a (normally) negative polarity DC
> feedback signal. I'd like to change the gain of the circuit digitally,
> so I'd like to use a digital pot. The problem is that a digital pot can
> only take signals between VEE and VDD - most usually probably 0v to 5v.
>
> My rails are +5/0/-5 and my feedback lies somewhere between 0 and -5.
> Has anyone encountered a digital pot that can either take a 10v supply,
> or has level shifters on the digital lines? I think I might be able to
> use a CD4052 with discrete resistors, but then it's a 16 pin chip I have
> to put on the board.
>
> Any ideas?
>
I assume the digital lines are 0->+5 V.
The digital pots I've worked with use three lines for their digital
interface. These three are all inputs.
You can use a PNP transistor for a level shifter.
Instead of ASCII art, I'll try to describe the connection: The PIC
output is connected through a 10k resistor to the emitter, the base
is connected to ground, and the collector is connected to the digital
pot's input. Also connected to the collector is a 100k pulldown
resistor to -5V.
Now when the PIC output is low, no current flows and the digital pot's
input sees -5V (low)
When the PIC output is high, current flows (the transistor saturates)
and the digital pot's input sees 0V (high). (well, maybe 0.2 or 0.3v
in practice)
The 10k could be dispensed with if you use the PIC's weak pull up and
switch from "output low" to "input", taking care to never go "output
high" with the pin.
Regards,
Mark
markrages@gmail
--
Mark Rages, Engineer
Midwest Telecine LLC
markrages@...
--
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist
> I have an analog circuit that has a (normally) negative polarity DC
> feedback signal. I'd like to change the gain of the circuit digitally,
> so I'd like to use a digital pot. The problem is that a digital pot can
> only take signals between VEE and VDD - most usually probably 0v to 5v.
>
> My rails are +5/0/-5 and my feedback lies somewhere between 0 and -5.
> Has anyone encountered a digital pot that can either take a 10v supply,
> or has level shifters on the digital lines? I think I might be able to
> use a CD4052 with discrete resistors, but then it's a 16 pin chip I have
> to put on the board.
>
> Any ideas?
>
I assume the digital lines are 0->+5 V.
The digital pots I've worked with use three lines for their digital
interface. These three are all inputs.
You can use a PNP transistor for a level shifter.
Instead of ASCII art, I'll try to describe the connection: The PIC
output is connected through a 10k resistor to the emitter, the base
is connected to ground, and the collector is connected to the digital
pot's input. Also connected to the collector is a 100k pulldown
resistor to -5V.
Now when the PIC output is low, no current flows and the digital pot's
input sees -5V (low)
When the PIC output is high, current flows (the transistor saturates)
and the digital pot's input sees 0V (high). (well, maybe 0.2 or 0.3v
in practice)
The 10k could be dispensed with if you use the PIC's weak pull up and
switch from "output low" to "input", taking care to never go "output
high" with the pin.
Regards,
Mark
markrages@gmail
--
Mark Rages, Engineer
Midwest Telecine LLC
markrages@...
--
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist
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