AD7874

REV. C

–13–

MULTIPLE AD7874s

Figure 18 shows a system where a number of AD7874s can be

configured to handle multiple input channels. This type of con-

figuration is common in applications such as sonar, radar, etc.

The AD7874 is specified with maximum and minimum limits on

aperture delay. This means that the user knows the maximum

difference in the sampling instant between all channels. This al-

lows the user to maintain relative phase information between the

different channels.

A common read signal from the microprocessor drives the RD

input of all AD7874s. Each AD7874 is designated a unique ad-

dress selected by the address decoder. The reference output of

AD7874 number 1 is used to drive the reference input of all

other AD7874s in the circuit shown in Figure 18. One REF

OUT pin can drive several AD7874 REF IN pins. Alternatively,

an external or system reference can be used to drive all REF IN

inputs. A common reference ensures good full-scale tracking be-

tween all channels.

AD7874(1)

AD7874(2)

AD7874(n)

CS

RD

CS

RD

CS

RD

RD

ADDRESS

REF OUT

REF IN

REF IN

ADDRESS

DECODE

V

CH5

V

CH6

V

CH7

V

CH8

V

CH1

V

CH2

V

CH3

V

CH4

V

CHm

V

CHm+1

V

CHm+2

V

CHm+3

Figure 18. Multiple AD7874s in Multichannel System

DATA ACQUISITION BOARD

Figure 20 shows the AD7874 in a data acquisition circuit. The

corresponding printed circuit board (PCB) layout and silkscreen

are shown in Figures 21 to 23. A 26-contact IDC connector pro-

vides for a microprocessor connection to the board.

A component grid is provided near the analog inputs on the

PCB which may be used to provide antialiasing filters for the

analog input channels or to provide signal conditioning circuitry.

To facilitate this option, four shorting plugs (labeled LK1 to

LK4 on the PCB) are provided on the analog inputs, one plug

per input. If the shorting plug for a particular channel is used,

the input signal connects to the buffer amplifier driving the ana-

log input of the ADC. If the shorting plug is omitted, a wire link

can be used to connect the input signal to the PCB component

grid.

Microprocessor connections to the board are made via a 26-

contact IDC connector, SKT8, the pinout for which is shown in

Figure 19. This connector contains all data, control and status

signals of the AD7874 (with the exception of the CLK input

and the CONVST input which are provided via SKT5 and

SKT7, respectively). It also contains decoded R/W and STRB

inputs which are necessary for TMS32020 interfacing (and also

for 68000 interfacing although pin labels on the 68000 are dif-

ferent). Note that the AD7874 CS input must be decoded prior

to the AD7874 evaluation board.

place, these input signals are fed to four buffer amplifiers, IC1,

before being applied to the AD7874. The use of an external

clock source is optional; there is a shorting plug (LK5) on the

AD7874 CLK input which must be connected to either –5 V

(for the ADCs own internal clock) or to SKT5. SKT6 and

SKT7 provide the reference and CONVST inputs respectively.

Shorting plug LK6 provides the option of using the external ref-

erence or the ADCs own internal reference.

DB10

DB8

DB6

DB4

DB2

DB0

GND

R/W

RD

CS

1

3

5

7

9

13

15

17

19

25

23

21

11

N/C

N/C

2

4

6

8

10

14

16

18

20

26

24

22

12

DB11

DB9

DB7

DB5

DB3

DB1

GND

STRB

INT

N/C

N/C

N/C

5V

+

5V

+

Figure 19. SKT8, IDC Connector Pinout

POWER SUPPLY CONNECTIONS

The PCB requires two analog power supplies and one 5 V digi-

tal supply. The analog supplies are labeled V+ and V– and the

range for both supplies is 12 V to 15 V (see silkscreen in Figure

23). Connection to the 5 V digital supply is made via SKT8.

The +5 V supply and the –5 V supply required by the AD7874

are generated from voltage regulators (IC3 and IC4) on the V+

and V– supplies.