ADS8699IPWR datasheet(58/74 Pages) TI2

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8.2.2 Detailed Design Procedure

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 its

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 by itself can be easily satisfied by using the ADS869x. This device

simplifies system design because the ADS869x 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 ADS869x because the device cannot load the system main

power supply

• A digital isolator is required to facilitate data transfer between the isolated ADS869x 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.

Main Power Supply

(> 4.3 V)

GND

Transformer

Driver

Isolation

Barrier

IGND

Full-Wave Rectifier and

Smoothing Capacitor

IGND

GND

VIN

GND

VIN

VOUT

Isolated 5 V

Isolated Switching Power Supply

1:NS (>1)

GND

IGND

GND

VIN

VOUT

Isolated 3.3 V

LDO

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 its 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 ADS869x 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 ADS869x at the full throughput

• Quad-channel device that facilitates excellent delay-matching between critical interface signals for reliable

operation at high speed

ADS8691, ADS8695, ADS8699

SBAS777B – DECEMBER 2016 – REVISED MARCH 2021

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