For applications that require a reference voltage with lower temperature drift or a common reference voltage for multiple devices, the ADS8588H offers a provision to use an external reference, using the internal buffer to drive the ADC reference pin. In order to select the external reference mode, either tie the REFSEL pin low or connect this pin to AGND. In this mode, an external 2.5-V reference must be applied at REFIN/REFOUT (pin 42), which becomes a high-impedance input pin. Any low-drift, small-size external reference can be used in this mode because the internal buffer is optimally designed to handle the dynamic loading on the ADC reference input. The output of the external reference must be filtered to minimize the resulting effect of the reference noise on system performance. Figure 7-10 shows a typical connection diagram for this mode.

Figure 7-10 Device Connections for Using an External 2.5-V Reference

For closed-loop operation of the internal reference buffer, the REFCAPA and REFCAPB pins must be externally shorted together. The output of the internal reference buffer appears at the REFCAP pin. A minimum capacitance of 10 µF must be placed between the REFCAPA, REFCAPB pins and REFGND (pin 43). Do not use this internal reference buffer to drive external ac or dc loads because of the limited current output capability of the buffer.

The performance of the internal buffer output (as shown in Figure 7-11) is very stable across the entire operating temperature range of –40°C to +125°C. The typical specified value of the reference buffer drift over temperature is 5 ppm/°C (Figure 7-12).

Figure 7-11 Variation of Reference Buffer Output (REFCAPA, REFCAPB) vs Free-Air Temperature and Supply

Number of samples = 30

Figure 7-12 Reference Buffer Temperature Drift Histogram