Data Sheet

AD7902

Rev. B | Page 23 of 28

Chain Mode with Busy Indicator

Chain mode with a busy indicator can also be used to daisy-

chain both ADCs within an AD7902 on a 3-wire serial interface

while providing a busy indicator. This feature is useful for reducing

component count and wiring connections, for example, in isolated

multiconverter applications or for systems with limited interfacing

capacity. Data readback is analogous to clocking a shift register.

See Figure 48 for a connection diagram example using three

AD7902 ADCs. The corresponding timing is shown in Figure 49.

When SDIx and CNVx are low, SDOx is driven low. With SCKx

high, a rising edge on CNVx initiates a conversion, selects chain

mode, and enables the busy indicator feature. In this mode, CNVx

is held high during the conversion phase and the subsequent data

readback. When all ADCs in the chain have completed their

conversions, the SDOx pin of the ADC closest to the digital host

(see the ADC labeled ADCx in the AD7902 B box in Figure 48)

is driven high. This transition on SDOx can be used as a busy

indicator to trigger the data readback controlled by the digital host.

The AD7902 then enters the acquisition phase and powers down.

The data bits stored in the internal shift register are clocked out,

MSB first, by subsequent SCKx falling edges. For each ADC,

SDIx feeds the input of the internal shift register and is clocked

by the SCKx falling edge. Each ADC in the chain outputs its

data MSB first, and 16 × N + 1 clocks are required to read back

the N ADCs. Although the rising edge can be used to capture the

data, a digital host using the SCKx falling edge allows a faster

reading rate and, consequently, more ADCs in the chain,

provided that the digital host has an acceptable hold time.

Figure 48. Chain Mode with a Busy Indicator Connection Diagram

Figure 49. Chain Mode with a Busy Indicator Serial Interface Timing

CONVERT

DATA IN

CLK

DIGITAL HOST

AD7902

SDIxB

SDOxB

CNVx

SCKx

AD7902

AD7902 A

AD7902 B

SDI1A

SDO1A

CNVx

SCKx

IRQ

AD7902

SDI2A

SDO2A

CNVx

ADCx

ADC1

ADC2

SCKx

NOTES

1. DASHED LINE DENOTED ADCs ARE WITHIN A GIVEN PACKAGE.

2. SDI1A AND SDO1A REFER TO THE SDI1 AND SDO1 PINS IN ADC1 IN THE FIRST AD7902 OF THE CHAIN (AD7902 A).

SDI2A AND SDO2A REFER TO THE SDI2 AND SDO2 PINS IN ADC2 OF AD7902 A. LIKEWISE, SDIxB AND SDOxB REFER

TO THE SDIx AND SDOx PINS IN BOTH ADC1 AND ADC2 OF THE SECOND AD7902 IN THE CHAIN (AD7902 B).

ACQUISITION

ACQUISITION

CONVERSION

SCKx

1

2

3

15

16

17

4

18

19

31

32

33

34

35

47

48

49