AD1837AASZ datasheet(22/24 Pages) AD

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REV. A

AD1837A

–22–

CASCADE MODE

Dual AD1837A Cascade

The AD1837A can be cascaded to an additional AD1837A,

which, in addition to six external stereo ADCs, can be used to

create a 32-channel audio system with 16 inputs and 16 outputs.

The cascade is designed to connect to a SHARC DSP and operates

in a time division multiplexing (TDM) format. Figure 14 shows

the connection diagram for cascade operation. The digital inter-

face for both parts must be set to operate in Auxiliary 512 mode

by programming ADC Control Register 2. AD1837A No. 1 is

set as a master device by connecting the

M/S pin to DGND and

AD1837A No.2 is set as a slave device by connecting the

M/S to

ODVDD. Both devices should be run from the same MCLK

and

PD/RST signals to ensure that they are synchronized.

With Device 1 set as a master, it will generate the frame-sync

and bit clock signals. These signals are sent to the SHARC and

Device 2 ensuring that both know when to send and receive data.

The cascade can be thought of as two 256-bit shift registers, one

for each device. At the beginning of a sample interval, the shift

registers contain the ADC results from the previous sample

interval. The first shift register (Device 1) clocks data into the

SHARC and also clocks in data from the second shift register

(Device 2). While this is happening, the SHARC is sending

DAC data to the second shift register. By the end of the sample

interval, all 512 bits of ADC data in the shift registers will have

been clocked into the SHARC and been replaced by DAC data,

which is subsequently written to the DACs. Figure 15 shows the

timing diagram for the cascade operation.

AUX ADC

(SLAVE)

ALRCLK

ABCLK

ASDATA

DSDATA

ALRCLK

ABCLK

ASDATA

DSDATA

AD1837A NO. 1

(MASTER)

AD1837A NO. 2

(SLAVE)

SHARC

(SLAVE)

AUX ADC

(SLAVE)

AUX ADC

(SLAVE)

AUX ADC

(SLAVE)

AUX ADC

(SLAVE)

AUX ADC

(SLAVE)

DRx

RFSx

RCLKx

TCLKx

DTx

TFSx

Figure 14. AD1837A Cascade

AD1837A NO. 1 DACs

AD1837A NO. 2 DACs

TFSx/

RFSx

DTx

AD1837A NO. 1 ADCs

AD1837A NO. 2 ADCs

DRx

256 ABCLKs

MSB

MSB – 1

LSB

32 ABCLKs

ABCLK

DTx

MSB

LSB

DRx

DON’T CARE

MSB – 1

Figure 15. AD1837A Cascade Timing

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