SBAS572B May 2013 – March 2019 ADS8864
The external reference source to the ADS8864 must provide low-drift and very accurate voltage for the ADC reference input and support the dynamic charge requirements without affecting the noise and linearity performance of the device. The output broadband noise of most references can be in the order of a few hundred μVRMS. Therefore, to prevent any degradation in the noise performance of the ADC, the output of the voltage reference must be appropriately filtered by using a low-pass filter with a cutoff frequency of a few hundred hertz.
After band-limiting the noise of the reference circuit, the next important step is to design a reference buffer that can drive the dynamic load posed by the reference input of the ADC. The reference buffer must regulate the voltage at the reference pin such that the value of VREF stays within the 1-LSB error at the start of each conversion. This condition necessitates the use of a large capacitor, CBUF_FLT (see Figure 46) for regulating the voltage at the reference input of the ADC. The amplifier selected to drive the reference pin must have an extremely low offset and temperature drift with a low output impedance to drive the capacitor at the ADC reference pin without any stability issues.