AD7643BSTZ datasheet(21/29 Pages) AD

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AD7643

Rev. 0 | Page 20 of 28

POWER SUPPLY

The AD7643 uses three sets of power supply pins: an analog

2.5 V supply AVDD, a digital 2.5 V core supply DVDD, and a

digital input/output interface supply OVDD. The OVDD supply

allows direct interface with any logic working between 2.3 V

and 5.25 V. To reduce the number of supplies needed, the digital

core (DVDD) can be supplied through a simple RC filter from

the analog supply, as shown in Figure 23.

Power Sequencing

The AD7643 is independent of power supply sequencing and

thus free from supply induced voltage latch-up. In addition, it is

insensitive to power supply variations over a wide frequency

range, as shown in Figure 29.

65.0

45.0

10000

FREQUENCY (kHz)

100

1000

62.5

60.0

57.5

55.0

52.5

50.0

47.5

INT REF

EXT REF

Figure 29. PSRR vs. Frequency

Power-Up

At power-up, or when returning to operational mode from the

power-down mode (PD = high), the AD7643 engages an

initialization process. During this time, the first 128 conversions

should be ignored or the RESET input could be pulsed to

engage a faster initialization process. Refer to the Digital

Interface section for RESET and timing details.

A simple power-on reset circuit, as shown in Figure 23, can be

used to minimize the digital interface. As OVDD powers up, the

capacitor is shorted and brings RESET high; it is then charged

returning RESET to low. However, this circuit only works when

powering up the AD7643 because the power-down mode

(PD = high) does not power down any of the supplies and as a

result, RESET is low.

It should be noted that the digital interface remains active even

during the acquisition phase. To reduce the operating digital

supply currents even further, drive the digital inputs close to

the power rails (that is, OVDD and OGND).

CONVERSION CONTROL

The AD7643 is controlled by the CNVST input. A falling edge

on CNVST is all that is necessary to initiate a conversion.

Detailed timing diagrams of the conversion process are shown

in Figure 30. Once initiated, it cannot be restarted or aborted,

even by the power-down input, PD, until the conversion is

complete. The CNVST signal operates independently of CS and

RD signals.

BUSY

MODE

CONVERT

ACQUIRE

CONVERT

CNVST

Figure 30. Basic Conversion Timing

For optimal performance, the rising edge of CNVST should not

occur after the maximum CNVST low time, t1, or until the end

of conversion.

Although CNVST is a digital signal, it should be designed with

special care with fast, clean edges and levels with minimum

overshoot and undershoot or ringing.

The CNVST trace should be shielded with ground and a low

value serial resistor (for example, 50 Ω) termination should be

added close to the output of the component that drives this line.

In addition, a 50 pF capacitor is recommended to further reduce

the effects of overshoot and undershoot as shown in Figure 23.

For applications where SNR is critical, the CNVST signal should

have very low jitter. This can be achieved by using a dedicated

oscillator for CNVST generation, or by clocking CNVST with a

high frequency, low jitter clock, as shown in Figure 23.

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