I am using the AD719x in my system. When I look at the output of the ADC, I don’t get the full resolution; a few of the LSBs are flickering. Why is this happening?
While the AD719x has a resolution of 24 bits, the accuracy of the part varies with output data rate and gain. The datasheet lists the rms noise and achievable accuracy for different output data rates and gains. For example, the AD7190 has a peak-to-peak resolution of 20.5 bits when the gain is 1 (analog input range of ±VREF), the output data rate is 50 Hz, the sinc4 filter is selected, chop is disabled and bipolar mode is selected. If the output data rate is changed to 4800 Hz, the peak-to-peak resolution is reduced to 16.5 bits. If the gain is now changed to 128 (±39 mV input range), the peak-to-peak resolution is reduced to 15.5 bits.
The accuracy of an ADC is specified as effective resolution or peak-to-peak resolution. The effective resolution is calculated using the rms noise that is given in the datasheet. The effective resolution equals log (input span / rms noise) / Log 2. The peak-to-peak resolution is the number of bits which do not flicker and is calculated using the peak-to-peak noise which equals 6.6 * rms noise. Therefore, the peak-to-peak noise equals log (input span / (6.6 * rms noise)) / Log 2.
The datasheet rms noise values are measured with the chosen analog input channel shorted to some voltage such as AVDD/2 (both terminals of the analog input channel are connected to AVDD/2). Therefore, the user should short the analog input on their system board using a similar method to ensure that the best performance is being obtained from the part. After performing a calibration, commence conversions. Using several thousand samples, the rms noise can be calculated and, from these,.the peak-to-peak resolution can be determined. The obtained value should be compared with the values given in the datasheet. If the accuracy specified in the datasheet is not obtained, this is due to noise on the circuit board, for example, ensure that ground loops do not exist and ensure that the power supply is adequately decoupled using a 10uF tantalum capacitor in parallel with a 0.1uF ceramic capacitor from each power supply to its respective ground. The capacitors should be placed as close as possible to the ADC’s pins.