PRELIMINARY TECHNICAL DATA

AD9430

-12-

4/01/2002 REV. PrG

APPLICATION NOTES

THEORY OF OPERATION

The AD9430 architecture is optimized for high speed and

ease of use. The analog inputs drive an integrated high

bandwidth track-and-hold circuit that samples the signal

prior to quantization by the 12-bit core. For ease of use the

part includes an onboard reference and input logic that

accepts TTL, CMOS, or LVPECL levels. The digital outputs

logic levels are user selectable as standard 3V CMOS or

LVDS (ANSI-644 compatible) via pin S2.

USING THE AD9430

ENCODE Input

Any high speed A/D converter is extremely sensitive to the

quality of the sampling clock provided by the user. A

track/hold circuit is essentially a mixer, and any noise,

distortion, or timing jitter on the clock will be combined

with the desired signal at the A/D output. For that reason,

considerable care has been taken in the design of the

ENCODE input of the AD9430, and the user is advised to

give commensurate thought to the clock source.

The AD9430 has an internal clock duty cycle stabilization

circuit that locks to the rising edge of ENCODE (falling

edge of ENCODE if driven differentially), and optimizes

timing internally. This allows for a wide range of input duty

cycles at the input without degrading performance. Jitter in

the rising edge of the input is still of paramount concern, and

is not reduced by the internal stabilization circuit. This

circuit is always on, and cannot be disabled by the user.

The ENCODE and ENCODE inputs are internally biased to

1.5V (nominal), and support either differential or single –

ended signals. For best dynamic performance, a differential

signal is recommended. Good performance is obtained

using an MC10EL16 in the circuit to drive the

encode inputs , as illustrated in figure below.

Driving Encode with EL16

Analog Input

The analog input to the AD9430 is a differential buffer. For

best dynamic performance, impedances at

____

AIN

should match. The analog input has been optimized to

provide superior wideband performance and requires that the

analog inputs be driven differentially. SNR and SINAD

performance will degrade significantly (~6dB) if the analog

input is driven with a single-ended signal. A wideband

transformer such as Minicircuits ADT1 -1WT can be used to

provide the

differential analog inputs for applications that require a

single-ended-to-differential conversion. Both analog inputs

are self-biased by an on-chip resistor divider to a nominal

2.8 V. (See Equivalent Circuits section TBD.)

Special care was taken in the design of the Analog Input

section of the AD9430 to prevent damage and corruption of

data when the input is overdriven. The nominal input range

is 1.5 V diff p-p. The nominal differential input range is 768

mV p-p × 2.

Differential Analog Input Range

Single Ended Analog Input Range