Datasheet

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ADS820
14
SBAS037B
PC-BOARD LAYOUT AND BYPASSING
A well-designed, clean PC-board layout will assure proper
operation and clean spectral response. Proper grounding
and bypassing, short lead lengths, and the use of ground
planes are particularly important for high frequency circuits.
Multilayer PC boards are recommended for best perfor-
mance but if carefully designed, a two-sided pc board with
large, heavy ground planes can give excellent results. It is
recommended that the analog and digital ground pins of the
ADS820 be connected directly to the analog ground plane. In
our experience, this gives the most consistent results. The
A/D converter power-supply commons should be tied to-
gether at the analog ground plane. Power supplies should be
bypassed with 0.1µF ceramic capacitors as close to the pin
as possible.
DYNAMIC PERFORMANCE TESTING
The ADS820 is a high-performance converter and careful
attention to test techniques is necessary to achieve accurate
results. Highly accurate phase-locked signal sources allow
high-resolution FFT measurements to be made without using
data windowing functions. A low jitter signal generator such
as the HP8644A for the test signal, phase-locked with a low
jitter HP8022A pulse generator for the A/D converter clock,
gives excellent results. Low-pass filtering (or bandpass filter-
ing) of test signals is absolutely necessary to test the low
distortion of the ADS820. Using a signal amplitude slightly
lower than full scale will allow a small amount of “headroom”
so that noise or DC-offset voltage will not overrange the
A/D converter and cause clipping on signal peaks.
DYNAMIC PERFORMANCE DEFINITIONS
1. Signal-to-Noise-and-Distortion Ratio (SINAD):
10
15
log
Sinewave Signal Power
Noise Harmonic Power first harmonics+
( )
2. Signal-to-Noise Ratio (SNR):
10 log
Sinewave Signal Power
Noise Power
3. Intermodulation Distortion (IMD):
10
5
log
PrHighest IMD oduct Power to th order
Sinewave Signal Power
( )
IMD is referenced to the larger of the test signals f
1
or f
2
. Five
“bins” either side of peak are used for calculation of funda-
mental and harmonic power. The “0” frequency bin (DC) is
not included in these calculations, as it is of little importance
in dynamic signal processing applications.