We are using AD9826 adc for sampling CCD sensor data @ single channel 2.5MHz. We are doing short circuit test for noise calculation wherein we've shorted input with 50 ohm register.
When input range selected is 4V, gain = 1V/V: We are getting noise (standard deviation) of 4.8 LSBs rms (285uV rms).
When input range selected is 2V, gain = 1V/V: We are getting noise (standard deviation) of 8 LSBs rms (245uV rms).
According to AD9826 datasheet, noise performance specification is 3 LSBs for gain = 1v/v.
It should result in 185uV rms for 4V input range, and 95uV rms for 2V input range.
We are measured AC noise on ADC pins such as AVDD, DRVDD, OFFSET, CML, CAPA, CAPB using 6.5 digit multimeter and we are getting noise lesser than 20-30uV.
Please let us know what we are missing for achieving required calibrated noise data as given in data-sheet.
Our requirement is 200uV rms for 4V input range, and 100uV rms for 2V input range.
The AD9826 input referred noise is constant, so in 2V range you will see twice the output noise (in LSB) as the 4V range.