8.2.1.2.1 ADC Reference Driver

The external reference source to the device 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 100 µV_RMS. Therefore, in order 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 from the reference source, the next important step is to design a reference buffer that can drive the dynamic load posed by the reference input of the ADC. At the start of each conversion, the reference buffer must regulate the voltage of the reference pin within 1 LSB of the intended value. This condition necessitates the use of a large filter capacitor at the reference pin of the ADC. The amplifier selected to drive this large capacitor should have low output impedance, low offset, and temperature drift specifications.

To reduce the dynamic current requirements and crosstalk between the channels, a separate reference buffer is recommended for driving the reference input of each ADC channel.

The application circuit in Figure 55 shows the schematic of a complete reference driver circuit that generates a voltage of 2.5-V dc using a single 5-V supply.

The 2.5-V reference voltage is generated by the high-precision, low-noise REF5025 circuit. The output broadband noise of the reference is heavily filtered by a low-pass filter with a 3-dB cutoff frequency of 160 Hz. The decoupling capacitor on each reference pin is selected to be 10 µF. The low output impedance, low noise, and fast settling time makes the OPA2350 a good choice for driving this high capacitive load.