Paul, OK now you're really piqued my interest and opened another can of worms so-to-speak
What's the benefit to using a slower ADC? With the MCP30008 on the SPI bus I just have a loop that reads every 5 seconds or so, takes a few readings back to back with a small delay in between, and then averages the result. I do this with both the TMP and LDRs. Does moving to a slower ADC make this reading more accurate for these type of sensors?
Look at this another way to take measurements at 5 samples/sec even aggregated up to over 16 channels is a max rate ADC of 80sps, so a 200ksps is overkill for your needs.
However the main factor you are forgetting 8bits measured AT
the sensor is more accurate than the 40ft+ round trip for sensor to A/D at any higher resolution. The unit has to be powered so your analog sensor is
- being fed by 20ft+ of cable, with its voltage drops and noise pickup,
- to retuirn down 20ft+ of cable a low voltage and low current signal, susceptible to the same voltage drops and noise pickup
No doubt most of the noise will be mains. mobile phones, spikes of nearby equipment.
If you put a low power A/D at the distal end (by the sensor) that clocks the A/D from something like the I2C clock then the analog signals can be kept electrically cleaner, simple low value caps on the analog signals and any op-amps will improve stability and range matching. If necessary you could ensure the A/D and sensors runs of 3V3 reference or similar with a cheap bandgap reference, thus negating any cable voltage drops for about 1V5 in cable runs.
By using this method helps to keep A/D and comms noise out of the signals being measured. If you want you find a low power 10 or 12 bit I2C ADC, the main point is keep the analog circuit between sensor and ADC SHORT gives better results..
Prove the design. Make a PCB, test that then make as many as you like.
I2C devices allows for simpler cabling, and are address selectable. If you want there are I2C switches so you can level translate and isolate each run until it is needed.
Regarding the 8 bit vs 10 bit. I realize that this won't affect the LDR much but won't I lose a lot of temperature resolution (i.e accuracy) with the TMP36 as I'll have 0-254 vs 0-1023 for readings?
Actually it is 0-255 vs 0-1023 to be pedantic
These are the resolution, cable runs pickup and everything else affect the accuracy, and in A/D system terms the Effective Number Of Bits (ENOB) you may have 10 bit resolution but you may not get all possible values due to range of sensor, noise and other characteristics.
However any measurement close to the sensor is going to be more a lot more accurate
whatever resolution you achieve. If you calculate what range and steps you can get from an 8 bit converter, bearing in mind the TMP36 is rated as being +/-2 deg C accuracy being able to read to 0.5 deg C steps or smaller seems pointless. To get that or better accuracy needs careful design of the thermal dynamics and thermal mass of the sensing area. Otherwise the temp sensor is affected by air currents or the temperature of its package leads compared to the die sensing area. See sections on Mounting Considerations in datasheet. Put too big a heatsink on the board and you will get wrong readings as the chip is being held at slightly lower temp, conversely if the chip self warms, your readings will be high. Seriously consider thermal paste and a piece of metal attached to the device if you want accurate results.
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