Datasheet
ADS802
13
SBAS039B
www.ti.com
FIGURE 8. Optional External Reference to Set the Full-Scale Range Utilizing a Dual, Single-Supply Op Amp.
2kΩ
+2.5V to +3.25V
+5V
+5V
R
P
220Ω
R
P
220Ω
10kΩ
6.2kΩ
0.1µF+2.5V
10kΩ
1/2
OPA2234
1/2
OPA2234
A
1
A
2
Bottom
Reference
Top
Reference
REF1004
+1.25V
10kΩ
10kΩ
(1)
10kΩ
(1)
NOTE: (1) Use parts alternatively for adjustment capability.
ground planes can give excellent results. It is recommended
that the analog and digital ground pins of the ADS802 be
connected directly to the analog ground plane. In our experi-
ence, this gives the most consistent results. The A/D converter
power-supply commons should be tied together 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 ADS802 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 ADS802. 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 SignalPower
Noise Harmonic Power first harmonics+
(
)
2. Signal-to-Noise Ratio (SNR):
10 log
Sinewave SignalPower
Noise Power
3. Intermodulation Distortion (IMD):
10
5
log
PrHighest IMD oduct Power to th order
Sinewave SignalPower
−
(
)
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.