REV. D

AD8002

–12–

Driving A-to-D Converters

The AD8002 is well suited for driving high-speed analog-to-

digital converters such as the AD9058. The AD9058 is a dual

8-bit 50 MSPS ADC. In Figure 10, the AD8002 is shown driv-

ing the inputs of the AD9058 which are configured for 0 V to 2 V

ranges. Bipolar input signals are buffered, amplified (–2

×), and

offset (by 1.0 V) into the proper input range of the ADC. Using

0.1 F

20

50

1k

18

17

16

15

14

13

12

11

10pF

CLOCK

5, 9, 22,

24, 37, 41

4,19, 21

25, 27, 42

0.1 F

38

8

6

2

3

549

274

ANALOG

IN A

0.5V

1.1k

AD707

43

20k

0.1 F

–2V

1.1k

20k

549

ANALOG

IN B

0.5V

274

20

0.1 F

40

COMP

1

ENCODE A

ENCODE B

10

36

ENCODE

74ACT04

0.1 F

+5V

28

29

30

31

32

33

34

35

RZ1

RZ2

7, 20,

26, 39

–5V

1N4001

AD9058

(J-LEAD)

RZ1, RZ2 = 2,000

SIP (8-PKG)

8

8

1/2

AD8002

1/2

AD8002

50

50

Figure 10. AD8002 Driving a Dual A-to-D Converter

the AD9058’s internal 2 V reference connected to both ADCs

as shown in Figure 10 reduces the number of external compo-

nents required to create a complete data acquisition system. The

20

Ω resistors in series with ADC inputs are used to help the

AD8002s drive the 10 pF ADC input capacitance. The AD8002

adds only 100 mW to the power consumption, while not limit-

ing the performance of the circuit.