Hello,
First...sorry for my English and bad grammar. I'm not native speaker.
I need to measure dc voltage signal (or get average value from slow periodic signal to 100Hz - but it's not neccesary) with biggest resolution as possible. Dynamic range is 0.2-4.8V. Let' s say 500uV resolution is neccesary and anything better like down to 100uV resolution could be awesome for me.
Measured voltage is from sensor (what has weak output buffer - max 0.5mA) and is situated in "distant" area (0.3 - 0.5 meters) where temperature reachs 120°C.
I have this design: Sensor's single-ended output voltage is converted to differental signal around middle 2.5V with differental driver (example). This differental signal is through twisted cable (long 30 - 50 cm) measured by ADC. Samples are averaged cause of better SQNR and stuff...
Considering requirements like resolution, dynamic range, dc to slow periodic signal...I would like to choose something like 24bit sigma-delta ADS1255 - full-scale is 2*Vref (Vref=2.5V), up to 30kSPS...
There will be possibility from time to time measure "offset error" of whole block (driver, filter, twisted cable, inputs of ADC) - for software correction of measured values in mcu what reads data from ADC. Correction values will be dependent on temperature value of sensor what mcu will know.
Now my dumb questions (it's first time when I have to solve "harder" analog problem).
Is it optimal way where I'm going with my design or is there something what should I be aware? I should look for op-amp with differental output with low offset and noise...Can someone recommend me specific op-amp for my application with ADS1255?
How can I make simple low-pass filter with cutoff 100Hz for reduction of noise and anti-alias function (example here) for adc input? I mean if I consider this blog post -I can increase capacity of RC filter, but I will get too much big capacity value (R = 10ohms -> C = 47uF -> 94Hz cutoff). This leads into big capacitive load for op-amp driver. Increasing value of resistor leads to bigger noise and dividing signal cause of input ADC impedance...Is better way to make low-pass filter in adding paraller capacitors to feedback resistors in op-amp driver circuit (Miller effect)?
I appreciate any advice you may have
Best regards
Longin