HI1179 データシートの表示（PDF） - Intersil

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HI1179

8-Bit, 35 MSPS, Video A/D Converter

Intersil 

HI1179

INPUT SIGNAL

VOLTAGE

VRT

•

•

•

•

•

•

•

•

VRB

STEP

255

•

•

•

128

127

•

•

•

0

MSB

1

1

0

0

A/D OUTPUT CODE TABLE

DIGITAL OUTPUT CODE

1

1

1

1

1

•

•

•

0

0

0

0

0

1

1

1

1

1

•

•

•

0

0

0

0

0

LSB

1

1

0

0

1

1

0

0

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.

Bypass both the digital and analog VDD pins to their respec-

tive grounds with a ceramic 0.1µF 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

will float to about 1/2(VRT + VRB). The other option is to use

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

between VRT and VRB. The internal reference can be used

by shorting VRT to VRTS and VRB to VRBS. The internal

bias generator will set VRT to about 2.6V and VRB to about

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

ladder is connected directly to VDD and VSS. Any supply

variations can be minimized by good decoupling of VRT and

VR B .

An external reference can be used for increased accuracy,

by connecting the reference voltage to VRT and VRB. If an

external reference is used, VRT should be keep below 2.8V

and (VRT - VRB) should be less than 2.8V and greater than

1.8V. If a VRB below +0.6V is used the linearity of the part

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

outputs connect to VRT and VRB , makes a good, low cost

external reference.

Bypass VRT and VRB to analog ground with a 0.1µF

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

set by the VREF pin before the signal is digitized. The clamp

4-1119

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