Engineers

We use an AD7192 24 Bit 4.8kHz Sigma Delta ADC in our low noise design. the input mode is differential. Our external 2.5V reference produces a 0.1 to 20Hz noise of 240uV rms..

Empirically it has been found that the refernce noise is only prominent at FSD eg when the DC supplied to the differential inputs is close to 2.5V and pratically insignificant for diffenetial inputs near zero.

Disregarding ADC conversion noise, input noise etc, would it be a safe assumption to make that the ADC EXTERNAL REFERENCE noise component is proportional to differential voltage applied to the ADC inputs?

we look forward to your comments.

regards

Mark

Hi Mark,

The affects of the reference noise are dependent on the analog input voltage.

Assume the reference equals 2.5V nominally, and changes by +10% to 2.75V. - A reference will never be this noisy, but this is just for an example.

If Vin=2.5V, then the nominal ADC result is 2.5V (ADC result = 0xFFFF, which a user scales by the nominal LSB size.) When Vref changes +10%, the ADC result in codes will reduce to 0xF45d (for bipolar coding), so the calculated ADC result changes from 2.5V to 2.2727V. => A +10% change in reference causes a -227.3mV change in the ADC result.

However, if Vin is much smaller, say 25uV, then the same change in reference will cause the ADC result to change from 25uV to 22.727uV, which is only a -2.273uV change.

Therefore changes in the reference have a much smaller effect on the ADC result (in terms of Volts) for small Vin. - The relative change is the same, but since noise is calculated in Volts a larger Vin will see more noise than a small Vin.

Thanks and Best Regards,

Chris