11

Detailed Description

The HI1179 is a 2-step A/D converter featuring a 4-bit upper

comparator group and two lower comparator groups of 4 bits

each. The reference voltage can be obtained from the

onboard bias generator or be supplied externally. This IC

uses an offset canceling type comparator that operates syn-

chronously with an external clock. The operating modes of

the part are input sampling/autozero (S), hold (H), and

compare (C).

The operation of the part is illustrated in Figure 2. A refer-

ence voltage that is between VRT-VRB is constantly applied

to the upper 4-bit comparator group. VI(1) is sampled with

the falling edge of the first clock by the upper comparator

block. The lower block A also samples VI(1) on the same

edge. The upper comparator block finalizes comparison data

MD(1) with the rising edge of the first clock. Simultaneously

the reference supply generates a reference voltage RV(1)

that corresponds to the upper results and applies it to the

lower comparator block A. The lower comparator block final-

izes comparison data LD(1) with the rising edge of the sec-

ond clock. MD(1) and LD(1) are combined and output as

OUT(1) with the rising edge of the third clock. There is a 2.5

clock cycle delay from the analog input sampling point to the

corresponding digital output data. Notice how the lower com-

parator blocks A and B alternate generating the lower data in

order to increase the overall A/D sampling rate.

Power, Grounding, and Decoupling

Separate analog and digital grounds to reduce noise effects,

connecting them at a single point near the HI1179. Analog

and digital power should also be separated for optimum per-

formance. If a single 5V supply is used, isolate the analog

and digital power with an inductor or ferrite bead to minimize

the digital noise on the analog supply.

tive grounds with a ceramic 0.1

µF capacitor close to the pin.

Analog Input

The analog input capacitance is small when compared with

other flash type A/D converters. However, it is necessary to

drive the input with a low impedance source with sufficient

bandwidth and drive capability.

Op amps such as the HA-2544, the HA5020 and the

HFA1100 family should make excellent input amplifiers

depending on the applications requirements. In order to pre-

vent parasitic oscillation, it may be necessary to insert a

resistor between the output of the amplifier and the A/D

input.

The input can be AC or DC coupled. If AC coupled the input

the internal clamp, which will be discussed later. When DC

coupling the input be sure to disable the clamp function

(CLE, pin 29).

Reference Input

The HI1179 has an internal reference with the option to use

an external reference if more accuracy is desired.

The analog input range of the A/D is set by the voltage

0.6V. The analog input range of the A/D will now be from

0.6V to 2.6V. The internal reference may be subjected to

power supply variations since the internal reference resistor

An external reference can be used for increased accuracy,

may degrade. An ICL8069 reference and a dual op amp,

cost external reference.

capacitor when using either internal or external references.

Clamp Operation

The HI1179 provides a clamp (DC restore) option that allows

the user to clamp a portion of the analog input to a voltage

A/D OUTPUT CODE TABLE

INPUT SIGNAL

VOLTAGE

STEP

DIGITAL OUTPUT CODE

MSB

LSB

255

1111

1111

•

•

•

•

•

•

•

•

•

•

•

•

•

•

128

1000

0000

127

0111

1111

•

•

•

•

•

•

0

0000

0000

HI1179