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TOPIC: SPI controlled Digital potentiometer that does not have a 5v limit!

SPI controlled Digital potentiometer that does not have a 5v limit! 5 years 1 month ago #16859

  • GM_Mike
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For a project I am working on I have had to put together a variable resistance ladder that had to support at least 16V top to bottom. BUT it was something that is going to be used in a high speed production environment, so I could not use normal potentiometers (trimming 4 pots to get voltage drops at 5 testpoints correct in order to do a system level calibration. I finally got everything working, and I wanted to share. I finally have it to the point where it is humming away. If you are working with audio tuning systems, the ability to use +16/-16V is really nice as well.

Biggest issues are that it does not have a good handle on ESD, and if you do not control the power on sequence perfectly it will not handle it well. Programming partner blew a few of them before we figured them out. But no issue with the current layout. Below is the main thrust of what I did to get the power setup correct.

The 5V into the board is gated by a 24V relay. 5V hits the DC converter, pumping out the 30V VDD reference, that has to hit the pot first. When it gets to the relay coil, I have a voltage split with the 470 resister(relay is 2k ohms, so 30V /5 is 24 ish) when the voltage exceeds 18V across the relay coil, it closes and the 5V then hits the 3.3V regulator, which is the logic voltage on the board.(Part will work with a range of VCC from 2.7 to 5.5 i believe) Also the constant load with the relay helps a lot to stabilize the DC converter. Add some varistors for ESD clamping at the outputs, and a diode to protect the relay, and we have not had a board die in more than a month (there are some other things on the comms side to clean up the pinout using an I2C - SPI master, because 4 wires versus the 9, and noise filtering there too)

Any questions feel free to ask. Upload as PDF so you can read the part numbers/values.
Last Edit: 5 years 1 month ago by GM_Mike. Reason: Quick edit because grammar is fun.
The following user(s) said Thank You: Graham Mitchell, Jon Chandler

SPI controlled Digital potentiometer that does not have a 5v limit! 5 years 1 month ago #16860

  • Jon Chandler
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The AD5293 is a good part to know about – there aren't many digipots that work over an extended range.
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