Information in the following applications sections is not part of the TI component specification, and TI does not warrant its accuracy or completeness. TI’s customers are responsible for determining suitability of components for their purposes. Customers should validate and test their design implementation to confirm system functionality.
Data acquisition systems often require stable voltage references to maintain accuracy. The REF50xx family features low noise, very low drift, and high initial accuracy for high-performance data converters. Figure 42 shows the REF5040 in a basic data acquisition system.
When using the REF50xx in the design, select a proper output capacitor that does not create gain peaking, thereby increasing total system noise. At the same time, the capacitor must be selected to provide required filtering performance for the system. In addition, input bypass capacitor and noise reduction capacitors must be added for optimum performances. During the design of the data acquisition system, equal consideration must be given to the buffering analog input signal as well as the reference voltage. Having a properly designed input buffer with an associated RC filter is a necessary requirement for good performance of the Data Acquisition System.
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 4 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 bandgap noise of the REF50xx.
SNR Measurements using different RC filters at the output of OPA365, different values of output capacitor for the REF50xx and different values of capacitors at the TRIM/NR pin are shown in Table 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|