AD7885 AD [Analog Devices], AD7885 Datasheet - Page 7

• Manufacturer Part Number: AD7885
• Description: LC2MOS 16-Bit, High Speed Sampling ADCs
• Manufacturer: AD [Analog Devices]

REV. C
TERMINOLOGY
Integral Nonlinearity
This is the maximum deviation from a straight line passing
through the endpoints of the ADC transfer function.
Differential Nonlinearity
This is the difference between the measured and the ideal 1 LSB
change between any two adjacent codes in the ADC.
Bipolar Zero Error
This is the deviation of the midscale transition (all 0s to all 1s)
from the ideal (AGND).
Positive Gain Error
This is the deviation of the last code transition (01 . . . 110 to
01 . . . 111) from the ideal (+V
Zero Error has been adjusted out.
Negative Gain Error
This is the deviation of the first code transition (10 . . . 000 to
10 . . . 001) from the ideal (–V
Zero Error has been adjusted out.
Signal to (Noise + Distortion) Ratio
This is the measured ratio of signal to (noise + distortion) at the
output of the A/D converter. The signal is the rms amplitude of
the fundamental. Noise is the rms sum of all nonfundamental
signals up to half the sampling frequency (f
The ratio is dependent upon the number of quantization levels
in the digitization process; the more levels, the smaller the quan-
tization noise. The theoretical signal to (noise + distortion) ratio
for an ideal N-bit converter with a sine wave input is given by:
Thus for an ideal 16-bit converter, this is 98 dB.
Total Harmonic Distortion
Total harmonic distortion (THD) is the ratio of the rms sum of
harmonics to the fundamental. For the AD7884/AD7885, it is
defined as:
where V
V
sixth harmonics.
Peak Harmonic or Spurious Noise
Peak harmonic or spurious noise is defined as the ratio of the
rms value of the next largest component in the ADC output
spectrum (up to f
fundamental. Normally, the value of this specification is deter-
mined by the largest harmonic in the spectrum, but for parts
where the harmonics are buried in the noise floor, it will be a
noise peak.
Intermodulation Distortion
With inputs consisting of sine waves at two frequencies, fa and
fb, any active device with nonlinearities will create distortion
products at sum and difference frequencies of mfa
m, n = 0, 1, 2, 3, etc. Intermodulation terms are those for which
neither m or n are equal to zero. For example, the second order
terms include (fa + fb) and (fa – fb), while the third order terms
include (2fa + fb), (2fa – fb), (fa + 2fb) and (fa – 2fb).
4
, V
5
and V
Signal to (Noise + Distortion) = (6.02N + 1.76) dB
1
THD (dB) 20 log
is the rms amplitude of the fundamental and V
6
are the rms amplitudes of the second through the
S
/2 and excluding dc) to the rms value of the
REF+
V
REF+
2
2
S + 1 LSB), after Bipolar
S – 1 LSB), after Bipolar
V
3
2
V
V
1
S
4
2
/2), excluding dc.
V
5
2
V
nfb where
6
2
2
, V
3
,
–7–
The AD7884/AD7885 is tested using the CCIFF standard
where two input frequencies near the top end of the input band-
width are used. In this case, the second and third order terms
are of different significance. The second order terms are usually
distanced in frequency from the original sine waves while the
third order terms are usually at a frequency close to the input
frequencies. As a result, the second and third order terms are
specified separately. The calculation of the intermodulation dis-
tortion is as per the THD specification where it is the ratio of
the rms sum of the individual distortion products to the rms am-
plitude of the fundamental expressed in dBs.
Power Supply Rejection Ratio
This is the ratio, in dBs, of the change in positive gain error to
the change in V
OPERATIONAL DIAGRAM
An operational diagram for the AD7884/AD7885 is shown in
Figure 6. It is set up for an analog input range of 5 V. If a
The chosen input buffer amplifier (A1) should have low noise
and distortion and fast settling time for high bandwidth applica-
tions. Both the AD711 and the AD845 are suitable amplifiers.
A2 is the force, sense amplifier for AGND. The AGNDS pin
should be at zero potential. Therefore, the amplifier must have a
low input offset voltage and good noise performance. It must
also have the ability to deal with fast current transients on the
AGNDS pin. The AD817 has the required performance and is
the recommended amplifier.
If AGNDS and AGNDF are simply tied together to Star
Ground instead of buffering, the SNR and THD are not signifi-
cantly degraded. However, dc specifications like INL, Bipolar
Zero and Gain Error will be degraded.
3 V input range is required, A1 should drive 3 V
3 V
NOTE: POWER SUPPLY DECOUPLING NOT SHOWN
V
8
DD
AD780
IN
= +5V
Figure 6. AD7884/AD7885 Operational Diagram
F with 5 V
2
4
6
V
IN
10µF
DD
A2
A3
A4
A1
or V
AD845, AD817 OR
IN
AD845, AD817 OR
AD817
AD711
EQUIVALENT
S, 5 V
EQUIVALENT
AD817
SS
. It is a dc measurement.
IN
F being tied to system AGND.
±5V
±3V
AGNDS
V
V
V
±5V
±3V
AGNDF
V
AD7884/AD7885
AV
REF+
INV
REF+
REF–
IN
IN
DD
IN
IN
S
S
F
F
F
S
GND
+5V
V
DD
AD7884
AD7885
AV
DGND
SS
–5V
V
SS
IN
S and
OUTPUTS
CONTROL
INPUTS
DATA