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24-Bit Instrumentation ADC Recommendations

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OM222O:
just a word of reference and full scale values, the "binary" values are a lot more common than odd ball values such as 5V. maybe consider going down to 4.096V?
I'm not saying you can't find decent 5V references, just that the other values are more common and you can might have better luck finding them to your sepc.

Kleinstein:
With a 24 bit or similar ADC there is usually no simple LSB to mV ratio, so no need for special 2.56 or 4.096 V references. The scale factor is just another floating point factor.

The INA122 is a low power INA, not a low noise one. The input noise may not be lower than the ADC input noise. So one could as well use just a buffer and the ADC internal gain option. I would start with the sensor and depending on it's noise choose an amplifier with slightly lower noise. If not needed (e.g. noise already higher than the ADC noise (with gain)) it can help to avoid extra analog gain. The "gain" insider the SD-ADCs is quite stable and linear - it's quite some effort to build an external gain stage that good.

If the RTD or termistors need buffering depends on the ADC and resistance value. Chances are good 100 Ohms and maybe 1 K can work without. For 10 K it may well need a buffer (maybe ADC internal).

For ratio-metric sensors I would more like drive the sensor excitation from the ADC reference. Depending on the ADC this could very well just the voltage over the reference resistors.

max_torque:
Use your  constant current source to both excite the sensor, and to create the ADCs refference voltage across a calibration resistor!  That means you can use a low precision current source.

geo_leeman:
Ok - I already had a current source driving the thermistors, so I've put them in series along with a resistor to create the reference voltage as suggested. I'm going to just use the PGA in the ADC as well as suggested. I've decided to go with 4 of the ADS1220. At $8/per it's not insanely expensive and means no mux/settle issues. The next question is should I bother clocking them from the same source vs. let each use its own internal clock? I can put a KC2505B series oscillator buffered with some 745HC1G14s on for quite cheap, but don't know that it's worth the parts count increase?

nctnico:

--- Quote from: geo_leeman on September 04, 2019, 02:52:36 pm ---I'm working on a project that is using the "blue pill" as the microcontroller (STM32 M0 8 MHz) that reads an accelerometer, four analog sensors, and logs to an SD card. The analog inputs are pressure/temperature and need to be digitized at the highest resolution possible, so I'm using a 24-bit ADC. I started with the ADS1243, but ideally I want to log everything at 10Hz which isn't possible with the ADS1243 as it is 30 SPS max. Even 5 Hz would be pushing it I think with settling time after switching the MUX (say 1 sample settling = 8 sample periods to measure all 4 channels so 30/8 = 3.75Hz). I'm going to be doing some other revisions, so it sounds like an ADC change out is in order. I've been looking at the ADS1217, but was wondering if any of you had recommendations?

I've used some simultaneous high rate ADCs before like the MAX11040K, which seems like overkill here. So to summarize, what's your favorite 24-bit ADC that can sample 4 channels at a minimum of 10Hz each (ideally a bit more so we have some headroom).

--- End quote ---
I've used the ADS1216 for one of my designs. The downside with all these ADS12xx converters is that they need several consequtive samples on the same channel for the internal filters to work so the effective samplerate drops a lot quickly. IIRC I got to something like 100SPS for 5 or 6 channels. They are also not so nice to interface / control.

Ofcourse the surrounding design needs a lot of attention as well to keep noise out and get rid of stray currents which may cause an offset.

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