AD4685 datasheet(17/32 Pages) AD

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Data Sheet

AD4684/AD4685

APPLICATIONS INFORMATION

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Rev. 0 | 17 of 32

POWER SUPPLY

The typical application circuit in Figure 33 can be powered by a sin-

gle 5 V (V+) voltage source that supplies the whole signal chain.

The 5 V supply can come from a low noise, CMOS low dropout

(LDO) regulator (for example, the ADP7105). The driver amplifier

supply is provided by +5 V (V+) and −2.5 V (V−), which is derived

from the inverter (for example, the ADM660). The inverter then

converts +5 V to −5 V and supplies this voltage to the ADP7182 low

noise voltage regulator to output −2.5 V.

The two independent supplies of the AD4684/AD4685, VCC and

VLOGIC, that supply the analog circuitry and digital interface, respec-

tively, can be supplied by a low quiescent current LDO regulator like

the ADP166. The ADP166 is a suitable supply with a fixed output

voltage range from 1.2 V to 3.3 V for typical VCC and VLOGIC levels.

Decouple both the VCC supply and the VLOGIC supply separately

with a 1 µF capacitor, placed close to the AD4684/AD4685 and

connected using short and wide traces, to provide low impedance

paths and reduce the glitches in the power supply lines. Additional-

ly, there is an internal LDO regulator to supply the AD4684/AD4685.

The on-chip regulator provides a 1.9 V supply for internal use on

the device only. Decouple the REGCAP pin with a 1 µF capacitor

to GND with short and wide traces and place the capacitor close to

the AD4684/AD4685 REGCAP and GND pins.

Power-Up

The AD4684/AD4685 are not easily damaged in power supply

sequencing. VCC and VLOGIC can be applied in any sequence.

An external reference must be applied after VCC and VLOGIC are ap-

plied. Analog and digital signals must be applied after the external

reference is applied.

The AD4684/AD4685 require a tPOWERUP time from applying VCC

and VLOGIC until the ADC conversion results are stable. Figure 4

shows the recommended power-up timing and condition with CS

held high. It is recommended and a good practice to perform a

software reset after power-up. See the Software Reset section for

details.

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