The OPA365 is used to drive the 16-bit analog-to-digital converter (ADS8326). The RC filter at the output of the OPA365 is used to reduce the charge kick-back created by the opening and closing of the sampling switch inside the ADC. Design the RC filter such that the voltage at the sampling capacitor settles to 16-bit accuracy within the acquisition time of the ADC. The bandwidth of the driving amplifier must at least be four times the bandwidth of the RC filter.
The REF5040 is used to drive the REF pin of the ADS8326. Proper selection of voltage reference output capacitor is very important for this design. Very low equivalent series resistance (ESR) creates gain-peaking, which degrades SNR of the total system. If the ESR of the capacitor is not enough, then an additional resistor must be added in series with the output capacitor. A capacitance of 1 μF can be connected to the NR pin to reduce band-gap noise of the REF50xx.
SNR measurements using different RC filters at the output of the OPA365, different values of output capacitor for the REF50xx and different values of capacitors at the TRIM/NR pin are shown in Table 10-1.
|TEST CONDITION 1||TEST CONDITION 2|
|OPA365 RC filter||124 Ω, 1 nF||124 Ω, 1 nF|
|REF5040 output capacitor||10 μF||10 μF + 47 μF|
|TRIM/NR pin capacitor||0 μF||1 μF|
|SNR||86.7 dB||92.8 dB|