ADS8694 Datasheet, PDF (66/77 Pages) Texas Instruments – 4- and 8-Channel, Single-Supply, SAR ADCs

ADS8694, ADS8698
SBAS686 – JULY 2015
www.ti.com
A low-cost, constant, on-time, synchronous buck regulator in fly-buck configuration with an external transformer
(LM5017) generates the isolated power supply. The LM5017 has a wide input supply range, making this device
ideal for accepting a 24-V industrial supply. This transformer can accept up to 100 V, thereby making reliable
transient protection of the input supply more easily achievable. The fly-buck power supply isolates and steps the
input voltage down to 6 V. The supply then provides that voltage to the TPS70950 (the low dropout regulator) to
generate 5 V to power the ADS8698 and other circuitry. The LM5017 also features a number of other safety and
reliability functions, such as undervoltage lockout (UVLO), thermal shutdown, and peak current limit protection.
Input analog signals are protected against high-voltage, fast-transient events often expected in an industrial
environment. The protection circuitry makes use of the transient voltage suppressor (TVS) and ESD diodes. The
RC low-pass mode filters are used on each analog input before the input reaches the ADS8698, thus eliminating
any high-frequency noise pickups and minimizing aliasing.
For a step-by-step design procedure, circuit schematics, bill of materials, PCB files, simulation results, and test
results, see 16-Bit, 8-Channel, Integrated Analog Input Module for Programmable Logic Controllers (PLCs)
(TIDU365).
10 Power-Supply Recommendations
The device uses two separate power supplies: AVDD and DVDD. The internal circuits of the device operate on
AVDD; DVDD is used for the digital interface. AVDD and DVDD can be independently set to any value within the
permissible range.
The AVDD supply pins must be decoupled with AGND by using a minimum 10-µF and 1-µF capacitor on each
supply. Place the 1-µF capacitor as close to the supply pins as possible. Place a minimum 10-µF decoupling
capacitor very close to the DVDD supply to provide the high-frequency digital switching current. The effect of
using the decoupling capacitor is illustrated in the difference between the power-supply rejection ratio (PSRR)
performance of the device. Figure 110 shows the PSRR of the device without using a decoupling capacitor. The
PSRR improves when the decoupling capacitors are used, as shown in Figure 111.
140
140
120
120
100
100
80
60
40
100
----- ± 2.5*V_REF
----- ± 1.25*V_REF
----- ± 0.625*V_REF
------ +2.5*V_REF
------+1.25*V_REF
1000
10000 100000 1000000
Input Frequency (Hz)
Code output near 131072
10000000
C046
Figure 110. PSRR Without a Decoupling Capacitor
80
60
40
50
----- ± 2.5*V_REF
----- ± 1.25*V_REF
----- ± 0.625*V_REF
------ +2.5*V_REF
------+1.25*V_REF
500
5000 50000 500000
Input Frequency (Hz)
Code output near 131072
5000000
C045
Figure 111. PSRR With a Decoupling Capacitor
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