4-1498

HI-5700

output word, plus an additional comparator to detect an

overflow condition.

The CMOS HI-5700 works by alternately switching between

a “Sample” mode and an “Auto Balance” mode. Splitting up

the comparison process in this CMOS technique offers a

number of significant advantages. The offset voltage of each

CMOS comparator is dynamically canceled with each

conversion cycle such that offset voltage drift is virtually

eliminated during operation. The block diagram and timing

diagram illustrate how the HI-5700 CMOS flash converter

operates.

The input clock which controls the operation of the HI-5700

is first split into a non-inverting

φ1 clock and an inverting φ2

clock. These two clocks, in turn, synchronize all internal

timing of analog switches and control logic within the

converter.

In the “Auto Balance” mode (

φ1), all φ1 switches close and

φ2 switches open. The output of each comparator is

momentarily tied to its own input, self-biasing the comparator

impedance to a small input capacitor. Each capacitor, in

turn, is connected to a reference voltage tap from the

resistor ladder. The Auto Balance mode quickly precharges

all 256 input capacitors between the self-bias voltage and

each respective tap voltage.

In the “Sample” mode (

φ2), all φ1 switches open and φ2

switches close. This places each comparator in a sensitive

high gain amplifier configuration. In this open loop state, the

input impedance is very high and any small voltage shift at

the input will drive the output either high or low. The

φ2 state

also switches each input capacitor from its reference tap to

the input signal. This instantly transfers any voltage

difference between the reference tap and input voltage to

the comparator input. All 256 comparators are thus driven

simultaneously to a defined logic state. For example, if the

input voltage is at mid-scale, capacitors precharged near

zero during

φ1 will push comparator inputs higher than the

self bias voltage at

φ2; capacitors precharged near the

reference voltage push the respective comparator inputs

lower than the bias point. In general, all capacitors

precharged by taps above the input voltage force a “low”

voltage at comparator inputs; those precharged below the

input voltage force “high” inputs at the comparators.

During the next

φ1 Auto-Balancing state, comparator output

data is latched into the encoder logic block and the first

stage of encoding takes place. The following

φ2 state

completes the encoding process. The 8 data bits (plus

overflow bit) are latched into the output flip-flops at the next

falling clock edge. The Overflow bit is set if the input voltage

enabled or disabled according to the state of CE1 and CE2

(See Table 2). When disabled, output bits assume a high

impedance state.

As shown in the timing diagram, the digital output word

becomes valid after the second

φ1 state. There is thus a one

and a half cycle pipeline delay between input sample and

digital output. “Data Output Delay” time indicates the slight

time delay for data to become valid at the end of the

φ1

Theory of Operation

The HI-5700 is an 8-bit analog-to-digital converter based on

a parallel CMOS “flash” architecture. This flash technique is

an extremely fast method of A/D conversion because all bit

decisions are made simultaneously. In all, 256 comparators

TABLE 1. PIN DESCRIPTION

PIN #

NAME

DESCRIPTION

1

CLK

Clock Input

2

D7

Bit 7, Output (MSB)

3

D6

Bit 6, Output

4

D5

Bit 5, Output

5

D4

Bit 4, Output

6

Digital Power Supply

8

GND

Digital Ground

9

10

D3

Bit 3, Output

11

D2

Bit 2, Output

12

D1

Bit 1, Output

13

D0

Bit 0, Output (LSB)

14

OVF

Overflow, Output

15

CE2

Three-State Output Enable Input, Active High.

(See Table 2)

16

CE1

Three-State Output Enable Input, Active Low.

(See Table 2)

17

Reference Voltage Positive Input

18

Analog Power Supply, +5V

19

AGND

Analog Ground

20

AGND

Analog Ground

21

Analog Power Supply, +5V

22

23

Analog Power Supply, +5V

24

AGND

Analog Ground

25

AGND

Analog Ground

26

Analog Power Supply, +5V

27

Reference Voltage Negative Input

28

Analog Input

TABLE 2. CHIP ENABLE TRUTH TABLE

CE1

CE2

D0

OVF

0

1

Valid

Valid

1

1

Three-State

Valid

X

0

Three-State

Three-State

X’s = Don’t Care.