REV.

AD9058

–6–

THEORY OF OPERATION

The AD9058 contains two separate 8-bit analog-to-digital con-

verters (ADCs) on a single silicon die. The two devices can be

operated independently with separate analog inputs, voltage

references, and clocks.

In a traditional flash converter, 256 input comparators are required

to make the parallel conversion for 8-bit resolution. This is in

marked contrast to the scheme used in the AD9058, as shown

in Figure 1.

Unlike traditional “flash,” or parallel, converters, each of the two

ADCs in the AD9058 utilizes a patented interpolating archi-

tecture to reduce circuit complexity, die size, and input capacitance.

These advantages accrue because, compared to a conventional

flash design, only half the normal number of input comparator

cells is required to accomplish the conversion.

In this unit, each of the two independent ADCs uses only 128 (2

comparators to make the conversion. The conversion for the

seven most significant bits (MSBs) is performed by the 128

comparators. The value of the least significant bit (LSB) is

determined by interpolation between adjacent comparators in

the decoding register. A proprietary decoding scheme processes

the comparator outputs and provides an 8-bit code to the output

register of each ADC; the scheme also minimizes error codes.

8

256

ANALOG IN

8

128

127

2

1

Figure 1. Comparator Block Diagram

Analog input range is established by the voltages applied at the

operate from 0 V to 2 V using the internal voltage reference,

or anywhere between –1 V and +2 V using external references.

Input range is limited to 2 V p-p when using external references.

The internal resistor ladder divides the applied voltage reference

into 128 steps, with each step representing two 8-bit quanti-

zation levels.

COMP

ENCODE

A

ENCODE

AD9058

(J-LEAD)

ENCODE

B

CLOCK

ANALOG

IN A

0.5V

ANALOG

IN B

0.5V

0.1 F

0.1 F

+5V

–5V

0.1 F

–2V

+2V

AD9617

400

400

20k

200

200

800

800

20k

5

5

0.1 F

10pF

1k

50

(SEE TEXT)

CLOCK

8

8

1N4001

4, 19, 21,

25, 27, 42

40

1

43

3

2

6

38

8

7, 20,

26, 39

28

29

30

31

32

33

34

35

18

17

16

15

14

13

12

11

5, 9, 22,

24, 37, 41

10

36

AD9617

AD707

74HCT04

Figure 2. AD9058 Using Internal 2 V Voltage Reference

E