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24bit ADC + 16bit DAC ?

Thu Nov 22, 2012 5:32 pm

Hi all,
I'm considering building a temperature controller which would read temperature from 1-4 sensors and use 1-4 heaters to control the temperature with a PID loop.
For measurement I'll want a 1-4 channel ADC at 24bits (slow, something like 1-10 samples/s would be enough) and for the heater control a 16-bit DAC (also no need to go fast, 1-100 samples/s is enough).

Has anyone done something like this already? Links?


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Re: 24bit ADC + 16bit DAC ?

Thu Nov 22, 2012 6:32 pm

24 bit? do you really need a resolution of (your temp range/16,777,216) degrees? :o

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Re: 24bit ADC + 16bit DAC ?

Thu Nov 22, 2012 6:41 pm

gritz wrote:24 bit? do you really need a resolution of (your temp range/16,777,216) degrees? :o
The price for 24-bit slow ADCs isn't really that high, so why not. Most datasheets specify about 20 effective number of bits however (the rest is noise, unless you average over many samples).

It might not be trivial to design the temperature sensing circuitry to take advantage of all those 20 bits. You need to be careful with self-heating of pt100 or thermistor resistive sensors if you want to be accurate to mK or less.

For the heating side of things PWM control with digital IO might be an option. OTOH for sensitive instrumentation one does not want the kHz PWM signal showing up on the ADC measurement side, so a DAC output with a linear power-stage for the heater might be safer (but less efficient).

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Re: 24bit ADC + 16bit DAC ?

Fri Nov 23, 2012 12:37 pm

Controlling the heaters by DACs - fine but I think that implies you're going to use one amplifier per heater.

It would probably be cheaper to run the heaters via PWM. It wouldn't need to be fast PWM like you'd use for motor control, just going on and off every few seconds would probably be fine for most temperature control jobs. Unless you have some particularly demanding application in mind...

There have been a lot of posts about temperature control, everything from brewing to fishtanks to cooking, there's probably lots of useful stuff but finding it in a search can be challenging sometimes...

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Re: 24bit ADC + 16bit DAC ?

Mon Feb 06, 2017 7:38 pm

24bits? and what about calibration of your analogue part of OpAmp? (Sallen-Key Filter with Gain+wire compensation with 1-5% resistors…)

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Re: 24bit ADC + 16bit DAC ?

Sat Feb 11, 2017 12:10 am

I can't help you with the DAC.

24-bits is what I use for my home heating system. In reality I get about 18-bits after the noise is accounted for. I use a LTC2499 to read thermistor data. The LTC2499 has a 16-channel mux included, so 4 inputs would not be a problem. I sample 1/minute, process it with a rolling average and use the slope to predict when to activate the valves to provide heat to a room. The system works great. It depends upon what you really need for data acquisition that drives the ADC resolution. Don't let the naysayers take you down. 24-bits is cheap these days, but most of them require special soldering methods due to the itty-bitty packages.

I've also been experimenting with the HX711. It is a cheap ($0.50) 24-bit ADC that is used primarily in load sensors in scales (10Hz or 80 Hz sample rate), but it is fully differential and if you put a 4x1 analog mux in front of it you could have what you need. Don't expect more tha 18-19 bits of resolution due to random noise, but you can perform some data averaging to improve the result. The HX711 package is a SOIC-16 so it can be hand-soldered.

Good luck.

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Re: 24bit ADC + 16bit DAC ?

Mon Feb 20, 2017 2:00 am

You know I'm usually into things like this, but I gotta say, just put a little more work into your front-end. lol

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