4
\$\begingroup\$

In my solution i have to replace an 10 turn potentiometer ( 5 K Ohms ) with a digital circuit. the potentiometer is dividing a voltage between 4v to 21v. so it seems if i could generate a voltage between 4v to 21v with a Atmega8 or Atmega128 ( indeed it must meet high accuracy ) I'm done ! I have tried to use Digital potentiometer , but got in to 2 problems :

1 - first they can not stand the current going throw the potentiometer : (21-4)/5K = 3.4mA

2 - they don't have the required accuracy (21-4)/256 = 66 mV ( but i need 0.5mV)

Is there any way i could do this?

\$\endgroup\$
  • 2
    \$\begingroup\$ Do you really need this kind of accuracy? I made this calculation: (21V-4V) / 0.5mV = 34000 Steps. Can the user currently set the pot this precise? For a 10 turn pot you have an total rotation angle of 10*360°, this is only 0.1° for a 0.5mV increment. However, you can achieve this easily with 16Bits (2^16=65536) DAC resolution. \$\endgroup\$ – 0x6d64 Aug 30 '11 at 6:55
  • \$\begingroup\$ Yes , I Do need it, actually I need more and I'm thinking that the DAC is a good idea specially when using it with an Op-Amp to bring up the voltage level to 4-21 Volts. \$\endgroup\$ – Ehsan Zargar Ershadi Sep 1 '11 at 6:50
10
\$\begingroup\$

First the accuracy. Your total range is 17V, so 0.5mV in 17V is 1/34000, just a little bit over 15 bits. As 0x6d64 notes that's a lot to get from even a multiturn potmeter. Let's see. Most multiturn potmeters are wirewound, the one I have here in front of me has a diameter of about 15mm for the wire path. If the wire is wound at a 0.5mm spacing then you can have \$10 \times \dfrac{2 \pi \times 7.5mm}{0.5mm}\$ < 1000 steps or less than 10 bits.
But let's look for a 16 bit resolution anyway, then you're safe. The same type of solution applies for a lower resolution as well.

Then there's the current. As I understand it you want to draw several mA from the potmeter's wiper, CMIIW.

A good solution may be to use a Multiplying DAC, or MDAC, followed by an opamp to give us the required output current. The LTC1595 will work:

LTC1595 app

\$V_{IN}\$ is your 4V-21V input voltage, and via the serail digital interface you can set the output voltage as a ratio of \$V_{IN}\$ like a potmeter would.

The LTC1597 is a similar device with parallel inputs instead of serial.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.