AD7763 datasheet(15/32 Pages) AD

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AD7763

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AD7763 INTERFACE

READING DATA USING THE SPI INTERFACE

The timing diagram in Figure 2 shows how the AD7763 transmits

its conversion results using the SPI-compatible serial interface.

The data being read from the AD7763 is clocked out using the

serial clock output, SCO. The SCO frequency is dependent on

the state of the serial clock output rate pin, SCR, and the clock

divider mode chosen by the state of the clock divider pin, CDIV

(see the Clocking the AD7763 section). Table 7 shows both the

SCO frequency and the ICLK frequency for the AD7763, resulting

from the states of both the CDIV and SCR pins.

Table 7. SCO Frequency

Clock Divide

Mode

CDIV

SCR

SCO

Frequency

ICLK

Frequency

Divide by 1

MCLK

Divide by 2

MCLK/2

MCLK1

MCLK/2

1 In decimate × 32 mode, when CDIV = 0 and SCR = 1, FSO pulses low for

32 SCO clock cycles, as shown in Figure 2. For all other combinations of CDIV

and SCR in decimate × 32 mode, FSO is continuously low.

An active low pulse of one SCO period on the data-ready output,

DRDY, indicates a new conversion result is available at the

AD7763 serial data output, SDO.

Each bit of the new conversion result is clocked onto the SDO

line on the rising SCO edge and is valid on the falling SCO edge

(for SCP = 0). The conversion result spans 32 SCO clock cycles

and consists of 24 data bits in twos complement form, followed

by 7 status bits.

ADR2

ADR1

ADR0

DVALID

OVR

LPWR

FILTER_OK

The conversion result output on the SDO line is framed by the

frame synchronization output, FSO, which is sent logic low for

32 SCO cycles following the rising edge of the DRDY signal.

Note that the SDO line is in three-state for one clock cycle

before the FSO signal returns to logic high, which means that

only 31 actual data bits are output in each conversion.

The first three status bits, ADR[2:0], are the device address bits.

The DVALID bit is asserted when the data being clocked out on

the SDO line is valid. Table 19 contains descriptions of the other

status bits: OVR, LPWR, and FILTER_OK.

There is an exception to the behavior of FSO when the AD7763

operates in decimate × 32 mode (see Endnote 1 of Table 7). If SCR

and CDIV are chosen so that the SCO frequency output has the

capability to clock through only 32 SCO cycles before the MSB

of the next conversion result is output, then FSO stays logic low

continuously.

The AD7763 also features a serial data latch output, SDL, which

outputs a pulse every 16 data bits. The SDL output offers an

alternative framing signal for serial transfers, which require

a framing signal more frequent than every 32 bits.

SYNCHRONIZATION

The SYNC input to the AD7763 provides a synchronization

function that allows the user to begin gathering samples of the

analog front-end input from a known point in time.

The SYNC function allows multiple AD7763s, operated from

the same master clock and using the same SYNC signal, to be

synchronized so that each ADC simultaneously updates its

output register.

Using a common SYNC signal to all AD7763 devices in a

system allows synchronization to occur. On the falling edge of

the SYNC signal, the digital filter sequencer is reset to 0. The

filter is held in reset state until a rising edge of the SCO senses

SYNC high. Thus, to perform a synchronization of devices, a

SYNC pulse of a minimum of 2.5 ICLK cycles in length can be

applied, synchronous to the falling edge of SCO. On the first

rising edge of SCO after SYNC goes logic high, the filter is taken

out of reset, and the multiple parts gather input samples

synchronously.

Following a SYNC, the digital filter needs time to settle before

valid data can be read from the AD7763. The user knows there

is valid data on the SDO line by checking the DVALID status bit

(see D3 in the status bits listing) that is output with each conversion

result. The time from the rising edge of SYNC until the DVALID

bit is asserted is dependent on the filter configuration used. See the

Theory of Operation section and the figures listed in Table 6 for

details on calculating the time until DVALID is asserted.

SHARING THE SERIAL BUS

The AD7763 functionality allows up to eight devices to share

the same serial bus, SDO, depending on the decimation rate

that is chosen.

Table 8 details the maximum number of devices that can share

the same SDO line for each decimation rate (×32, ×64, ×128,

×256).

Table 8. Maximum Number of Devices Sharing SDO

Decimation Rate

SCO

(MHz)

×32

×64

×128

×256

Maximum Number of

Devices Sharing SDO

N/A

The Share Pins SH[2:0] of all the devices sharing the serial bus

must be programmed with the number of devices that are

sharing the serial bus.

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