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ADS8671 Datasheet(PDF) 56 Page - Texas Instruments

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Part No. ADS8671
Description  14-Bit, High-Speed, Single-Supply, SAR ADC Data Acquisition System with Programmable, Bipolar Input Ranges
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Maker  TI1 [Texas Instruments]
Homepage  http://www.ti.com
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ADS8671 Datasheet(HTML) 56 Page - Texas Instruments

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Main Power Supply
(> 4.3 V)
GND
Transformer
Driver
xx
xx
xx
xx
Isolation
Barrier
IGND
IGND
Full-Wave Rectifier and
Smoothing Capacitor
IGND
GND
VIN
GND
VIN
VOUT
Isolated 5 V
Isolated Switching Power Supply
S2
S1
1:NS (>1)
GND
IGND
D1
D2
CS
M
IGND
GND
VIN
VOUT
Isolated 3.3 V
LDO
LDO
56
ADS8671, ADS8675
SBAS779 – DECEMBER 2016
www.ti.com
Product Folder Links: ADS8671 ADS8675
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Copyright © 2016, Texas Instruments Incorporated
Typical Application (continued)
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 90. 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 96, Figure 92, and Figure 93.
The system performance requirements by itself can be easily satisfied by using the ADS867x. This device
simplifies system design because the ADS867x 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 ADS867x because the device cannot load the system main
power supply
A digital isolator is required to facilitate data transfer between the isolated ADS867x 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 91.
Figure 91. Isolated Power-Supply Design
Recommended components for the circuit shown in Figure 91 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 ADS867x 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 ADS867x at the full throughput
Quad-channel device that facilitates excellent delay-matching between critical interface signals for reliable
operation at high speed


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