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

characteristics such as signal-to-noise ratio (SN) signal-to-

bandwidth aperture 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-

and SN are calculated from the resulting FFT data and a

spectral plot may also be obtained Typical values for SN

are shown in the table of Electrical Characteristics and

formance 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-

3 dB)

Effective number of bits can also be useful in describing the

AD’s noise performance An ideal AD converter will have

some amount of quantization noise determined by its reso-

lution which will yield an optimum SN ratio given by the

following equation

where n is the AD’s resolution in bits

The effective bits of a real AD converter therefore can be

found by

602

wave input signal will typically have a SN of 78 dB which is

equivalent to 126 effective bits

Two samplehold specifications aperture time and aperture

jitter are included in the Dynamic Characteristics table

since the ADC12441 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

ADC12441

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 ADC12441 actually holds the input signal is

the aperture time For the ADC12441 this time is typically

100 ns Aperture jitter is the change in the aperture time

from sample to sample Aperture jitter is useful in determin-

ing the maximum slew rate of the input signal for a given

accuracy For example an ADC12441 with 100 ps of aper-

ture jitter operating with a 5V reference can have an effec-

tive gain variation of about 1 LSB with an input signal whose

slew rate is 12 V ms

Power Supply Bypassing

TLH11017 – 22

Tantalum

Ceramic

Protecting the Analog Inputs

TLH11017 – 23

Note

External protection diodes should be able to withstand the op amp

current limit

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