The design uses galvanic isolation between the DAQ system inputs and main power supply to achieve extremely high CMRR, as indicated by Figure 8-1. The system not only tolerates large common-mode voltages beyond the absolute maximum ratings but also delivers excellent performance largely independent of common-mode amplitude and frequency (within the specified operating limits). The relevant performance characteristics are illustrated in Figure 8-9, Figure 8-3, and Figure 8-4.

The system performance requirements can be easily satisfied by using the ADS868x. This device simplifies system design because the ADS868x eliminates the need for designing a discrete high-performance signal chain needed with most other SAR ADCs. In addition, the use of galvanic isolation has the following system design implications:

• A local floating supply is needed to power the ADS868x because the device cannot load the system main power supply
• A digital isolator is required to facilitate data transfer between the isolated ADS868x serial interface and the digital host controller

The floating power supply can be realized as an isolated transformer-based, push-pull converter followed by a rectifier and low-dropout (LDO) regulator to largely eliminate the ADC power-supply ripple by taking advantage of the high PSRR provided by most LDOs. A schematic of this design is shown in Figure 8-2.

Figure 8-2 Isolated Power-Supply Design

Recommended components for the circuit shown in Figure 8-2 are given below:

• The SN6501 transformer driver is selected for the low input voltage requirement, small form-factor, and the flexibility offered for easily adjusting the system isolation voltage rating by substituting the transformer
• A miniature printed circuit board (PCB)-mount, center-tapped transformer with a gain > 1 maintains line regulation at the LDO outputs
• Schottky rectifiers for minimal forward voltage drop
• Smoothing capacitor for sufficiently low ripple at the LDO input
• The TPS7A4901 LDOs for an ultra-low noise contribution relative to the ADS868x and high PSRR over a wide frequency range to attenuate output ripple to levels below the LDO output noise level

With regard to the digital isolator, the ISO7640FM is recommended for the following reasons:

• Supports > a 50-MHz SCLK and the required logic levels for operating the ADS868x at the full throughput
• Quad-channel device that facilitates excellent delay-matching between critical interface signals for reliable operation at high speed