AD7678ASTZ datasheet(21/29 Pages) AD

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AD7678

Rev. A | Page 20 of 28

DIGITAL INTERFACE

The AD7678 has a versatile digital interface; it can be interfaced

with the host system by using either a serial or parallel interface.

The serial interface is multiplexed on the parallel data bus. The

AD7678 digital interface also accommodates both 3 V and 5 V

logic by simply connecting the AD7678’s OVDD supply pin to

the host system interface digital supply. Finally, by using the

OB/2C input pin in any mode except 18-bit interface mode,

both twos complement and straight binary coding can be used.

The two signals, CS and RD, control the interface. When at least

one of these signals is high, the interface outputs are in high

impedance. Usually, CS allows the selection of each AD7678 in

multicircuit applications, and is held low in a single AD7678

design. RD is generally used to enable the conversion result on

the data bus.

CNVST

BUSY

DATA

BUS

CS = RD = 0

PREVIOUS CONVERSION DATA

NEW DATA

t10

t11

03084-0-035

Figure 29. Master Parallel Data Timing for Reading (Continuous Read)

PARALLEL INTERFACE

The AD7678 is configured to use the parallel interface with an

18-bit, a 16-bit, or an 8-bit bus width, according to Table 7. The

data can be read either after each conversion, which is during

the next acquisition phase, or during the following conversion,

as shown in Figure 30 and Figure 31, respectively. When the

data is read during the conversion, however, it is recommended

that it is read only during the first half of the conversion phase.

This avoids any potential feedthrough between voltage

transients on the digital interface and the most critical analog

conversion circuitry. Refer to Table 7 for a detailed description

of the different options available.

DATA

BUS

t12

t13

BUSY

CURRENT

CONVERSION

03084-0-036

Figure 30. Slave Parallel Data Timing for Reading (Read after Convert)

CS = 0

CNVST,

CONVERSION

DATA

BUS

t12

t13

BUSY

03084-0-037

Figure 31. Slave Parallel Data Timing for Reading (Read during Convert)

A0, A1

PINS D[15:8]

PINS D[7:0]

HI-Z

HIGH BYTE

LOW BYTE

HIGH BYTE

HI-Z

t12

t13

03084-0-038

Figure 32. 8-Bit and 16-Bit Parallel Interface

SERIAL INTERFACE

The AD7678 is configured to use the serial interface when

MODE0 and MODE1 are held high. The AD7678 outputs 18

bits of data, MSB first, on the SDOUT pin. This data is

synchronized with the 18 clock pulses provided on the SCLK

pin. The output data is valid on both the rising and falling edge

of the data clock.

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