AD7653ACPZ datasheet(20/27 Pages) AD

20 / 27 page

Data Sheet

AD7653

Rev. C | Page 19 of 26

POWER DISSIPATION VS. THROUGHPUT

Operating currents are very low during the acquisition phase,

allowing significant power savings when the conversion rate is

reduced (see Figure 25). This power savings depends on the

mode used. In Impulse mode, the AD7653 automatically

reduces power consumption at the end of each conversion

phase. This makes the part ideal for very low power battery

applications. The digital interface and the reference remain

active even during the acquisition phase. To reduce operating

digital supply currents even further, digital inputs need to be

driven close to the power supply rails (i.e., DVDD or DGND),

and OVDD should not exceed DVDD by more than 0.3 V.

SAMPLE RATE (SPS)

02966-0-041

1000000

100

10000

100000

1000

10000

100

1000

1000000

WARP MODE POWER

IMPULSE MODE POWER

100000

PDREF = PDBUF = HIGH

Figure 25. Power Dissipation vs. Sampling Rate

CONVERSION CONTROL

Figure 26 shows the detailed timing diagrams of the conversion

process. The AD7653 is controlled by the CNVST signal, which

initiates conversion. Once initiated, it cannot be restarted or

aborted, even by the power-down input PD, until the

conversion is complete. CNVST operates independently of CS

and RD.

In Impulse mode, conversions can be automatically initiated. If

CNVST is held LOW when BUSY is LOW, the AD7653 controls

the acquisition phase and automatically initiates a new con-

version. By keeping CNVST LOW, the AD7653 keeps the

conversion process running by itself. It should be noted that the

analog input must be settled when BUSY goes low. Also, at

power-up, CNVST should be brought LOW once to initiate the

conversion process. In this mode, the AD7653 can run slightly

faster than the guaranteed 666 kSPS limits in Impulse mode.

This feature does not exist in Warp and Normal modes.

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 (i.e., 50 Ω) termination should be added

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

For applications where SNR is critical, the CNVST signal should

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

oscillator for CNVST generation, or to clock CNVST with a

high frequency, low jitter clock, as shown in Figure 22.

BUSY

MODE

ACQUIRE

CONVERT

ACQUIRE

CONVERT

02966-0-011

CNVST

Figure 26. Basic Conversion Timing

RESET

DATA

BUSY

02966-0-011

CNVST

Figure 27. RESET Timing

t11

BUSY

DATA

BUS

CS = RD = 0

t10

PREVIOUS CONVERSION DATA

NEW DATA

02966-0-012

CNVST

Figure 28. Master Parallel Data Timing for Reading (Continuous Read)

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