AD8021 datasheet(17/28 Pages) AD

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AD7986

Rev. B | Page 17 of 28

DIGITAL INTERFACE

Although the AD7986 has a reduced number of pins, it offers

flexibility in its serial interface modes.

When in CS mode, the AD7986 is compatible with SPI,

MICROWIRE™, QSPI™, and digital hosts. In this mode, the

AD7986 can use either a 3-wire or a 4-wire interface. A 3-wire

interface using the CNV, SCK, and SDO signals minimizes

wiring connections, which is useful, for instance, in isolated

applications. A 4-wire interface using the SDI, CNV, SCK, and

SDO signals allows CNV, which initiates conversions, to be

independent of the readback timing (SDI). This is useful in low

jitter sampling or simultaneous sampling applications.

When in chain mode, the AD7986 provides a daisy-chain feature

using the SDI input for cascading multiple ADCs on a single

data line similar to a shift register. Chain mode is only available

in normal mode (TURBO = low).

The mode in which the part operates depends on the SDI level

when the CNV rising edge occurs. The CS mode is selected if

SDI is high, and the chain mode is selected if SDI is low. The

SDI hold time is such that when SDI and CNV are connected

together, the chain mode is always selected.

In normal mode operation, the AD7986 offers the option of

forcing a start bit in front of the data bits. This start bit can be

used as a busy signal indicator to interrupt the digital host and

trigger the data reading. Otherwise, without a busy indicator,

the user must time out the maximum conversion time prior to

readback.

The busy indicator feature is enabled in CS mode if CNV or

SDI is low when the ADC conversion ends (see Figure 28 and

Figure 32), and TURBO must be kept low for both digital

interfaces.

When CNV is low, reading can occur during conversion and

acquisition, and when split across acquisition and conversion,

as detailed in the following sections.

A discontinuous SCK is recommended because the part is

selected with CNV low, and SCK activity begins to clock

out data.

Note that in the following sections, the timing diagrams

indicate digital activity (SCK, CNV, SDI, and SDO) during

the conversion. However, due to the possibility of performance

degradation, digital activity should occur only prior to the safe

data reading time, tDATA, because the AD7986 provides error

correction circuitry that can correct for an incorrect bit decision

during this time. From tDATA to tCONV, there is no error correction,

and conversion results may be corrupted. Similarly, tQUIET, the

time from the last falling edge of SCK to the rising edge of CNV,

must remain free of digital activity. The user should configure

the AD7986 and initiate the busy indicator (if desired in normal

mode) prior to tDATA. It is also possible to corrupt the sample by

having SCK near the sampling instant. Therefore, it is recom-

mended to keep the digital pins quiet for approximately 20 ns

before and 10 ns after the rising edge of CNV, using a

discontinuous SCK whenever possible to avoid any potential

performance degradation.

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