AD7864 Analog Devices, AD7864 Datasheet - Page 9

• Manufacturer Part Number: AD7864
• Description: High Speed, Low Power, 4-channel Simultaneous Sampling, 12-Bit ADC
• Manufacturer: Analog Devices

Specifications of AD7864

• Resolution (bits): 12bit
• # Chan: 4
• Sample Rate: 520kSPS
• Interface: Par
• Analog Input Type: SE-Bip,SE-Uni
• Ain Range: Bip 10V,Bip 2.5V,Bip 5.0V,Uni 2.5V,Uni 5.0V
• Adc Architecture: SAR
• Pkg Type: QFP

TERMINOLOGY
Signal-to-(Noise + Distortion) Ratio
This is the measured ratio of signal-to-(noise + distortion) at
the output of the ADC. 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
depends on the number of quantization levels in the digitization
process; the more levels, the smaller the quantization noise. The
theoretical signal-to-(noise + distortion) ratio for an ideal N-bit
converter with a sine wave input is given by
Thus, for a 12-bit converter, this is 74 dB.
Total Harmonic Distortion (THD)
THD is the ratio of the rms sum of harmonics to the
fundamental. For the AD7864, it is defined as
where V
V
fifth 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 is a noise peak.
Intermodulation Distortion
With inputs consisting of sine waves at two frequencies, fa and
fb, any active device with nonlinearities creates distortion products
at sum and difference frequencies of mfa ± nfb, where m, n = 0,
1, 2, 3, and so on. Intermodulation terms are those for which
neither m nor n are equal to zero. For example, second-order
terms include (fa + fb) and (fa − fb), whereas third-order terms
include (2 fa + fb), (2 fa − fb), (fa + 2 fb), and (fa − 2 fb).
The AD7864 is tested using the CCIF standard, where two input
frequencies near the top end of the input bandwidth 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, whereas 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 distortion is as per the
THD specification where it is the ratio of the rms sum of the
individual distortion products to the rms amplitude of the funda-
mental expressed in decibels.
4
, V
Signal-to-(Noise + Distortion) = (6.02 N + 1.76) dB
THD
5
, and V
1
is the rms amplitude of the fundamental, and V
(
dB
6
)
are the rms amplitudes of the second through the
=
20
S
/2 and excluding dc) to the rms value of the
log
V
2
2
+
V
3
2
+
S
/2), excluding dc. The ratio
V
V
4
1
2
+
V
5
2
+
V
6
2
2
, V
3
Rev. D | Page 9 of 28
,
Channel-to-Channel Isolation
Channel-to-channel isolation is a measure of the level of
crosstalk between channels. It is measured by applying a full-
scale 50 kHz sine wave signal to all nonselected input channels
and determining how much that signal is attenuated in the
selected channel. The figure given is the worst case across all
four channels.
Relative Accuracy
Relative accuracy, or endpoint nonlinearity, 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.
Positive Full-Scale Error
This is the deviation of the last code transition (01...110 to 01...111)
from the ideal, 4 × V
V
(AD7864-3, ±2.5 V range), after the bipolar offset error has
been adjusted out.
Positive Full-Scale Error (AD7864-2, 0 V to 2.5 V and 0 V to 5 V)
This is the deviation of the last code transition (11...110 to 11...111)
from the ideal 2 × V
or V
unipolar offset error has been adjusted out.
Bipolar Zero Error (AD7864-1, ±10 V/±5 V, AD7864-3, ±2.5 V)
This is the deviation of the midscale transition (all 0s to all 1s)
from the ideal, AGND − 1/2 LSB.
Unipolar Offset Error (AD7864-2, 0 V to 2.5 V and 0 V to 5 V)
This is the deviation of the first code transition (00...000 to
00...001) from the ideal, AGND + 1/2 LSB.
Negative Full-Scale Error (AD7864-1, ±10 V/±5 V, and
AD7864-3, ±2.5 V)
This is the deviation of the first code transition (10...000 to
10...001) from the ideal, −4 × V
−2 × V
(AD7864-3, ±2.5 V range), after bipolar zero error has been
adjusted out.
Track-and-Hold Acquisition Time
Track-and-hold acquisition time is the time required for the
output of the track-and-hold amplifier to reach its final value,
within ±1/2 LSB, after the end of a conversion (the point at
which the track-and-hold returns to track mode). It also applies
to situations where there is a step input change on the input
voltage applied to the selected V
REF
REF
− 3/2 LSB (AD7864-1, ±5 V range), or V
REF
− 3/2 LSB (AD7864-2, 0 V to 2.5 V range), after the
+ 1/2 LSB (AD7864-1, ±5 V range) or −V
REF
REF
− 3/2 LSB (AD7864-2, 0 V to 5 V range)
− 3/2 LSB (AD7864-1, ±10 V), or 2 ×
REF
INxA
+ 1/2 LSB (AD7864-1, ±10 V),
/V
INxB
input of the AD7864.
REF
− 3/2 LSB
REF
+ 1/2 LSB
AD7864