8 input digital POT

[Colin]

I have an application where I need to replace a mechanical potentiometer
with a digital one. The problem I have is I want to use 8 5v inputs to
control the resistance (eg. 8 control pins on a pc parallel port). Is
there an IC that will address this, or is there a circuit that can be
built? I can find 256 tap digi pot IC's but they all have an increment/
decrement thing going on. Please Help TY

 

[exxos]

I have an application where I need to replace a mechanical potentiometer
with a digital one. The problem I have is I want to use 8 5v inputs to
control the resistance (eg. 8 control pins on a pc parallel port). Is
there an IC that will address this, or is there a circuit that can be
built? I can find 256 tap digi pot IC's but they all have an increment/
decrement thing going on. Please Help TY

Why don't you settle with 7 volume chips, use the 8th line for UP/DOWN
select, and then just pulse the data lines to get the volume you want.

Chris

 

[Colin]

Why don't you settle with 7 volume chips, use the 8th line for UP/DOWN
select, and then just pulse the data lines to get the volume you want.

Chris

Sorry, I should have been a little more specific. It is for a motor
speed control. I was hoping to use just a combination of high/low binary
outputs to control the motor with 256 different settings. Thanks for the
quick response though.

 

[Vidar Løkken]

I have an application where I need to replace a mechanical
potentiometer
with a digital one. The problem I have is I want to use 8 5v inputs
to
control the resistance (eg. 8 control pins on a pc parallel port). Is
there an IC that will address this, or is there a circuit that can be
built? I can find 256 tap digi pot IC's but they all have an
increment/ decrement thing going on. Please Help TY

I'm sure there is a chip that does this. Otherwise, you can use a
Digital to analog converter, and apply it as base current to a
transistor... Depending on you application, that might be the
best/easiest solution.

 

[Colin]

---
Since you say you're concerned with the resistance change I assume
you're going to be using this thing like a rheostat, no?

Do you need it to be able to handle bipolar signals?

What's the application?

Yes. A rheostat seems to be the thing I am looking for. But I need one
that is controllable with 8 signals. Low signal is 0v and high is 5v

 

[exxos]

Sorry, I should have been a little more specific. It is for a motor
speed control. I was hoping to use just a combination of high/low binary
outputs to control the motor with 256 different settings. Thanks for the
quick response though.

ah right, I ws thinking audio, but why not do that anyway ? instead of
outputting a binary number, just pulse D0 either up or down 256 times, and
use D1 to set speed up or speed down... since your output is a PC controled
code, just alter the program to output pulses instead. A fraction more
complex program, but a lot easier than trying to find IC pots to fit the
job.

Chris

 

[scada]

I have an application where I need to replace a mechanical potentiometer
with a digital one. The problem I have is I want to use 8 5v inputs to
control the resistance (eg. 8 control pins on a pc parallel port). Is
there an IC that will address this, or is there a circuit that can be
built? I can find 256 tap digi pot IC's but they all have an increment/
decrement thing going on. Please Help TY

Analog Devices (as well as others) make a few nice packages that are serial
controlled (I2C). That way you can call a setpoint serialy directly to the
chip, without the step - up, step - down increments. If you need parallel,
of course, you only need to add a serial to parallel converter.

I designed with the AD5232 (1024 position) and the folks at AD were very
helpful by forwarding examples of VB code.

 

[Vidar Løkken]

Sorry, I should have been a little more specific. It is for a motor
speed control. I was hoping to use just a combination of high/low
binary outputs to control the motor with 256 different settings.
Thanks for the quick response though.

Depending on your circuit, a DA-converter and a transistor should do
this. Simply feed a fet-transistor or bipolar one (depending on the
motor) from the DA. The DAC can have 8 bits in, which will give 255
discrete levels.

 

[Jeff Thon]

Think R-2R ladder. This will be most suitable for your 8 bit parallel port
requirement. Google for it. There are many devices with varying levels of on
chip interface logic. I am sure you can find something that fits the bill.

Jeff

 

[Colin]

Think R-2R ladder. This will be most suitable for your 8 bit parallel
port requirement. Google for it. There are many devices with varying
levels of on chip interface logic. I am sure you can find something
that fits the bill.

Jeff

Ah, this R-2R thing looks like exactly what I'm after. One thing I am
having a hard time understanding is why there is an op-amp after all of
the resistors in the circuits I am finding. So I guess my questions
would be, what is this op-ampo for, and is this a special type of op-amp,
I can't find one anywhere that looks like that, with 3 connections, all
the ones I see have 5, and I don't know what the other 2 are for.
Thanks for getting me on the right track.
Colin

 

[Jeff Thon]

http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/dac.html
http://www.learn-c.com/experiment8.htm
http://www.electronicproducts.com/print.asp?ArticleURL=sepana1.sep2003

Here are a few links to some online tutorials that may shed some light...

The R-2R ladders can be used in a current mode or a voltage mode.
The opamp is used for two reasons depending on how it is used. In both cases
it acts as a buffer. In current mode it is a current to voltage convertor.
In both cases the opamp can provide gain with respect to the reference
voltage applied to the ladder. You may have a precision 4.096V reference,
but with gain you can get 10V out, for example.

If you were to use an 8 bit R-2R ladder in a voltage mode and you had no
load connected to the output tap you would read 0 to the reference voltage
in 256 steps. Without considering the impedance of the reference voltage,
this would be true for all values of R > zero. Making R=10K and adding a 20K
resistor as the load to the output tap would cause the output voltage to
drop somewhere around 6.8V when all taps were switched high. 0-10VDC is a
typical motor drive speed command and 20K would be a typical input impedance
for a motor drive. Under these conditions you would not get full speed by
connecting the ladder direct to the drive. With the high input impedance of
an opamp the 20K drive impedance would not have an effect on the ladder
allowing it to output full reference to the opamp wiith all taps high and
allowing the opamp to buffer it and also output the required output voltage
to the motor drive.

The opamp is just an opamp. Nothing special. I'd be interested in the opamps
you have seen with 5 connections. I suspect the other two may be power rail
connections.

Jeff