Looking for digital potentiometer hints and tips

[throbscottle]

Title says it all. I'm thinking about using digital potentiometers as presets. Would like to know if anyone has experience with these - any gotcha's, brand recommendations, usage recommendations, typical circuits, any information the mfr's don't like to tell? Or am I better getting good quality normal presets?

TIA

 

[ronsimpson]

There are many types.
Some have EEPROM so they can remember where they were last set to. (never used because I have micros with eeprom)
There are many different interfaces. I2C, SPI, etc
10k, 20k, 50k, 100k ohm.
Many are linear some are log.
Some are built for audio and are there for very different.
It is possible to get noise into the system form the micro. I have not had problems on video but audio can be a problem.
What out for where the power up default setting.
I would to to digikey.com and search for digital pot.

 

[jpanhalt]

You don't give much information about how you intend to use them. The one time I considered a digital pot was in a power supply, and the voltage limitation was a deal breaker.

John

 

[KeepItSimpleStupid]

Usually, the program-ability of the device is limited. There is one variety where you can know the value aproi.

There is an electrically programmable resistor out there, but I forget what it's called. The programming algorithm takes some time. Then there are these wierd things: Rejusters. http://www.mbridgetech.com/rejustor-products.php

Why not FRAM?

 

[ronsimpson]

Here are some examples.

 

[throbscottle]

ronsimpson - thanks for the tips!

jpanhalt - I'm looking to trim op-amp DC gain and offset, and also the value of a voltage reference - already 0.01% but the mfr (Intersil) recommend using a digital pot to trim it due to the low TC.

At the moment I'm trying to compare standard presets (which I know a little bit about) with digital pots (which I know almost nothing about) for this purpose.

KeepItSimpleStupid - I like the look of rejustors.

 

[throbscottle]

KeepItSimpleStupid - I don't need any kind of extra memory at the moment, but if I do I will bear FRAM in mind!

 

[throbscottle]

Hmmm, at the moment multiturn mechanical presets are winning...

 

[ronsimpson]

Most digital pots have a small amount of settings points. Some have 64, 128 or 256 positions.
They will not replace a 10 turn pot. I have used two digital pots to replace a 10 turn pot. The first pot sets the out put to with in 1 part in 256. The second pot then adds another 256 sub positions.

Take care where you use "pots" in precession amplifiers. Pots are not stable. I use 1% resistors (or 0.1%) to set the gain and offset to get close. Then use pots that only have a 1.5% effect on the amplifier.

 

[throbscottle]

AKA a "poor man's multiturn" I read on the interwebs when looking for fine and coarse control circuits a while ago. This must be why they put two different values in one package.

Yes - I was planning only to be able to trim out the tolerance of the resistors and any offsets.

 

[ronsimpson]

I have made products that take a human 1/2 hour to calibrate.
By using digital pots and a PC to set the pots the calibration was set in seconds. (with out opening the box)
Think about the human cost in getting the product out the door.

 

[KeepItSimpleStupid]

There you go Ron, a topic for a blog?

I left them all aout, but the result was better than average. I thought I could compute the offset and add it because I thought I had a few equations. Zero input for one of the equations wasn't easily achievable and I never manages to scale the D/A. I ran out of "time allotted by management".

 

[throbscottle]

Since this is a project for me that only I will have to calibrate, time isn't an issue - but it's a good point, if ever I feel like setting up shop I'll bear it in mind! I've actually started looking at wirewound presets now, at least for trimming the voltage reference. (And then I'll have to find a friendly place with a voltage standard willing to help me...) There are a few "new old stock" spectrol reliance ones floating around.