adc1241cmj National Semiconductor Corporation, adc1241cmj Datasheet - Page 13

• Manufacturer Part Number: adc1241cmj
• Description: Self-calibrating 12-bit Plus Sign Mp-compatible A/d Converter With Sample-and-hold
• Manufacturer: National Semiconductor Corporation

4 0 Dynamic Performance
Many applications require the A D converter to digitize ac
signals but the standard dc integral and differential nonlin-
earity specifications will not accurately predict the A D 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
(S (N
time and aperture jitter are quantitative measures of the
A D converter’s capability
An A D converter’s ac performance can be measured using
Fast Fourier Transform (FFT) methods A sinusoidal wave-
form is applied to the A D converter’s input and the trans-
form is then performed on the digitized waveform S (N
is calculated from the resulting FFT data and a spectral plot
may also be obtained Typical values for S (N
shown in the table of Electrical Characteristics and spectral
plots are included in the typical performance curves
The A D 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
be seen in the S (N
curves will also give an indication of the full power band-
width (the frequency at which the S (N
Two sample hold 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 A D
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
a
D)) effective bits full power bandwidth aperture
a
D) versus frequency curves These
a
D) drops 3 dB)
a
distortion ratio
a
D) are
a
D)
13
Tantalum
Protecting the Analog Inputs
Power Supply Bypassing
TL H 10554 – 19
TL H 10554 – 20