40 Dynamic Performance

Many applications require the AD converter to digitize ac

signals but the standard dc integral and differential nonlin-

earity specifications will not accurately predict the AD con-

verter’s performance with ac input signals The important

specifications for ac applications reflect the converter’s abil-

ity to digitize ac signals without significant spectral errors

and without adding noise to the digitized signal Dynamic

time and aperture jitter are quantitative measures of the

AD converter’s capability

An AD converter’s ac performance can be measured using

Fast Fourier Transform (FFT) methods A sinusoidal wave-

form is applied to the AD converter’s input and the trans-

is calculated from the resulting FFT data and a spectral plot

shown in the table of Electrical Characteristics and spectral

plots are included in the typical performance curves

The AD converter’s noise and distortion levels will change

with the frequency of the input signal with more distortion

and noise occurring at higher signal frequencies This can

curves will also give an indication of the full power band-

Two samplehold specifications aperture time and aperture

jitter are included in the Dynamic Characteristics table

since the ADC1241 has the ability to track and hold the

analog input voltage Aperture time is the delay for the AD

to respond to the hold command In the case of the

ADC1241 the hold command is internally generated When

the Auto-Zero function is not being used the hold command

occurs at the end of the acquisition window or seven clock

periods after the rising edge of the WR The delay between

the internally generated hold command and the time that

the ADC1241 actually holds the input signal is the aperture

time For the ADC1241 this time is typically 100 ns Aper-

ture jitter is the change in the aperture time from sample to

sample Aperture jitter is useful in determining the maximum

slew rate of the input signal for a given accuracy For exam-

ple an ADC1241 with 100 ps of aperture jitter operating with

a 5V reference can have an effective gain variation of about

1 LSB with an input signal whose slew rate is 12 V ms

Power Supply Bypassing

TLH10554 – 19

Tantalum

Protecting the Analog Inputs

TLH10554 – 20

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