AD7658BSTZ datasheet(27/32 Pages) AD

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Data Sheet

AD7656/AD7657/AD7658

Rev. D | Page 27 of 32

Daisy-Chain Mode (DCEN = 1, SER/PAR = 1)

When reading conversion data back from the AD7656/AD7657/

AD7658 using their three/two/one DOUT pins, it is possible

to configure the parts to operate in daisy-chain mode, using

the DCEN pin. This daisy-chain feature allows multiple AD7656/

AD7657/AD7658 devices to be cascaded together and is useful

for reducing component count and wiring connections. An

example connection of two devices is shown in Figure 33. This

configuration shows two DOUT lines being used. Simultaneous

sampling of the 12 analog inputs is possible by using a common

CONVSTx signal. The DB5, DB4, and DB3 data pins are used

as data input pins DCIN [A:C] for the daisy-chain mode.

The rising edge of CONVST is used to initiate a conversion on

the AD7656/AD7657/AD7658. After the BUSY signal has gone

low to indicate that the conversion is complete, the user can

begin to read the data from the two devices. Figure 34 shows the

serial timing diagram when operating two AD7656/AD7657/

D7658 devices in daisy-chain mode.

The CS falling edge is used to frame the serial transfer from the

AD7656/AD7657/AD7658 devices, to take the bus out of three-

state, and to clock out the MSB of the first conversion result. In

the example shown in Figure 34, all 12 ADC channels are

simultaneously sampled. Two DOUT lines are used to read the

conversion results in this example. CS frames a 96 SCLK transfer.

During the first 48 SCLKs, the conversion data is transferred

from Device 2 to Device 1. DOUT A on Device 2 transfers

conversion data from V1, V2, and V5 into DCIN A in Device 1.

DOUT B on Device 2 transfers conversion results from V3, V4,

and V6 to DCIN B in Device 1. During the first 48 SCLKs,

Device 1 transfers data into the digital host. DOUT A on

Device 1 transfers conversion data from V1, V2, and V5.

DOUT B on Device 1 transfers conversion data from V3, V4,

and V6. During the last 48 SCLKs, Device 2 clocks out zeros

and Device 1 shifts the data clocked in from Device 2 during

the first 48 SCLKs into the digital host. This example can

also be implemented using six 16 SCLK individually framed

transfers if DCEN remains high during the transfers.

Figure 35 shows the timing if two AD7656/AD7657/AD7658

devices are configured in daisy-chain mode and are operating

with three DOUT lines. Assuming a simultaneous sampling of

all 12 inputs occurs, the CS frames a 64 SCLK transfer during

the read operation. During the first 32 SCLKs of this transfer,

the conversion results from Device 1 are clocked into the digital

host and the conversion results from Device 2 are clocked into

Device 1. During the last 32 SCLKs of the transfer, the conver-

sion results from Device 2 are clocked out of Device 1 and into

the digital host. Device 2 clocks out zeros.

Standby/Partial Power-Down Modes of Operation

Each ADC pair can be individually placed into partial power-

down mode by bringing the CONVST x signal low before the

falling edge of BUSY. To power the ADC pair back up, the

CONVST x signal should be brought high to tell the ADC pair

to power up and place the track-and-hold into track mode.

After the power-up time from partial power-down has elapsed,

the CONVST x signal should receive a rising edge to initiate a

valid conversion. In partial power-down mode, the reference

buffers remain powered up. While an ADC pair is in partial power-

down mode, conversions can still occur on the other ADCs.

The AD7656/AD7657/AD7658 have a standby mode whereby

the devices can be placed into a low power consumption mode

(100 µW maximum). The AD7656/AD7657/AD7658 are placed

into standby mode by bringing the logic input STBY low and

can be powered up again for normal operation by bringing

STBY logic high. The output data buffers are still operational

when the AD7656/AD7657/AD7658 are in standby mode,

meaning the user can continue to access the conversion results

of the parts. This standby feature can be used to reduce the

average power consumed by the AD7656/AD7657/AD7658

when operating at lower throughput rates. The parts can be

placed into standby at the end of each conversion when

BUSY goes low and taken out of standby again prior to the

next conversion. The time for the AD7656/AD7657/AD7658

to come out of standby is called the wake-up time. The wake-

up time limits the maximum throughput rate at which the

AD7656/AD7657/AD7658 can operate when powering down

between conversions. See the Specifications section.

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