Supplying near perfect DC signals from Arduino UNO

Question

My problem seems very simple but has been impossible to find a straight forward answer searching online. I need my Arduino to output different DC voltages from six ports (probably PWM ports), like 2 V from port 3, 2.1 V from port 5 and so on. And then be able to quickly change to like 4 V from port 3, and 4.1 V from port 5 and so on. But these DC signals need to be near perfect for the highly sensitive sensor I am working with. Everywhere I've searched says to use the PWM outputs and then an RC filter but the ripple from this process is concerning, can I get the the ripple to be under a few mV amplitude? Is there more smoothing I can do beyond a simple RC lowpass filter?

TLDR: I basically need a DC voltage generator that can select between 1 and 5 volts at 0.1 V intervals, and I need to be able to do it with output from the Arduino and some external hardware that will provide a near perfect DC signal to a sensor.

Answer 1

Would 0-4.095V in 1mV (or 0.5mV 0-2.047V) steps do for you? Then why not use some MCP4822 SPI SAC chips? You get 2 channels on each 8-pin chip (DIP chips are available, yes), and if you need a different voltage range you could always amplify it with a simple op-amp buffer.

So 6 ports would be 3 chips, which would entail using SCK, MOSI and three CS pins, so 5 pins in all.

Answer 2

(1) A multipole active lowpass filter works wonders.
Something like a 5 pole Bessel filter will probably meet your need. That needs two active stages (1 input RC pole +2 +2 ). You can use transistors in emitter follower mode with some reservations but using 2/4 of a quad opamp per filter is easier overall. I can't spare the lookup time just now (later maybe) but AD & LT and NatSemi all have good free active filter design programs.

(2) A Simple sample & hold per channel will give you steady output limited only by the drift of the opamp used. You can buy commercial S&H ICs or use std opamps. I've used even the venerable LM324 for this but there are much better - you apply the voltage via a switch to a cap[acitor at a unity gain buffer input and then turn off the switch. Again, the "switch can be as simple as an ancient CD4016 or CD4051 but much better more modern ICs are available.

With 1 x CD4051 and 2 x LM324 (or better) you can feed 8 x S&H and update them occasionally top compensate for drift. If you add another CD4051 with identical addressing but working backwards you can read back the current capacitor voltage and "nudge" it with a simple digital on signal via a diode and high R resistor if it is slightly off. You can even do ALL signal setting this way if required with NO PWM (or you are effectively implementing adaptive PWM without formally doing so). You eg use 4 driver lines - a fast and slow drive high and a fast and slow drive low. Each is just a digital pin + a resistor and a diode. Select one output channel with the output mux (CD4051 or whatever, and the return mux connects it to your ADC. Pump it up to about right with fast-high, top it off with slow-high and correct with slow-low if needed, then step the mux to the next channel and the S&H cap looks after it. Note that the return channel to the ADC must not load the cap significantly while reading.

If this answer attracts significant interest I can add detail and suggestions re more modern components etc. If not, the above is all a keen person needs to look into the concept.