Hi every one, I am examining a prototype device that is designed to analyse current from an electrochemical O2 sensor (current source), The sensor will output 1.124 uV per PPM (cross 47 ohms @0.023 uA), and has acuracy of +- 2 PPM, with max 1000 PPM. it ueses 16 bit Sigma Delta ADC with 2.5 V Vref ( PIC24FJ128GC006 MCU to be exact ) with 47 ohm resistor across sensor inputs. This is my first time seeing 16 bit ADC let along sigma delta, I only know of the 12 bit successive approximation adcs, my question is, despite all the structural diffrences, is it correct to say the step size of this ADC is Vref/2^15 (bipolar) which is 76 uV, and therefore impossible to convert the O2 sensor signal? the designer of the devices has used the on chip programable gain to amplify the input by maximum 32 times, which makes it still short of the step size of the ADC. what is really puzzlig for me is, if my above calcs are correct, the RAW ADC values should only be noise (≈ 2000 counts )when i apply 20 uA ( ≈1000 PPM ), but the ADC value shoots to 5000 counts, as if it is picking up a signal, where according to my logic it really should not move that much. hence my confusion. really appreciate any bodys help on this.