AD7608 Analog Devices, AD7608 Datasheet - Page 18

• Manufacturer Part Number: AD7608
• Description: 8-Channel DAS with 18-Bit, Bipolar, Simultaneous Sampling ADC
• Manufacturer: Analog Devices

Specifications of AD7608

• Resolution (bits): 18bit
• # Chan: 8
• Sample Rate: 200kSPS
• Interface: Par,Ser,SPI
• Analog Input Type: Diff-Bip
• Ain Range: Bip 10V,Bip 5.0V
• Adc Architecture: SAR
• Pkg Type: QFP

AD7608
TERMINOLOGY
Integral Nonlinearity
The maximum deviation from a straight line passing through
the endpoints of the ADC transfer function. The endpoints of
the transfer function are zero scale, at ½ LSB below the first
code transition; and full scale, at ½ LSB above the last code
transition.
Differential Nonlinearity
The difference between the measured and the ideal 1 LSB
change between any two adjacent codes in the ADC.
Bipolar Zero Code Error
The deviation of the midscale transition (all 1s to all 0s) from
the ideal, which is 0 V − ½ LSB.
Bipolar Zero Code Error Match
The absolute difference in bipolar zero code error between any
two input channels.
Positive Full-Scale Error
The deviation of the actual last code transition from the ideal
last code transition (10 V − 1½ LSB (9.99988) and 5 V − 1½ LSB
(4.99994)) after bipolar zero code error is adjusted out. The
positive full-scale error includes the contribution from the
internal reference buffer.
Positive Full-Scale Error Match
The absolute difference in positive full-scale error between any
two input channels.
Negative Full-Scale Error
The deviation of the first code transition from the ideal first
code transition (−10 V + ½ LSB (−9.99996) and −5 V + ½ LSB
(−4.99998)) after the bipolar zero code error is adjusted out.
The negative full-scale error includes the contribution from
the internal reference buffer.
Negative Full-Scale Error Match
The absolute difference in negative full-scale error between any
two input channels.
Signal-to-(Noise + Distortion) Ratio
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 sum of all nonfundamental signals
up to half the sampling frequency (f
The ratio 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 an 18-bit converter, the signal-to-(noise + distortion)
is 110.12 dB.
Signal-to-(Noise + Distortion) = (6.02 N + 1.76) dB
S
/2, excluding dc).
Rev. A | Page 18 of 32
Total Harmonic Distortion (THD)
The ratio of the rms sum of the harmonics to the fundamental.
For the AD7608, it is defined as
where:
V
V
harmonics.
Peak Harmonic or Spurious Noise
The ratio of the rms value of the next largest component in the
ADC output spectrum (up to f
of the fundamental. Normally, the value of this specification is
determined by the largest harmonic in the spectrum, but for
ADCs where the harmonics are buried in the noise floor, it is
determined by a noise peak.
Intermodulation Distortion (IMD)
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. Intermodulation distortion terms are those for which
neither m nor n is equal to 0. For example, the second-order
terms include (fa + fb) and (fa − fb), and the third-order terms
include (2fa + fb), (2fa − fb), (fa + 2fb), and (fa − 2fb).
The calculation of the intermodulation distortion is per the
THD specification, where it is the ratio of the rms sum of the
individual distortion products to the rms amplitude of the
sum of the fundamentals expressed in decibels (dB).
Power Supply Rejection Ratio (PSRR)
Variations in power supply affect the full-scale transition but not
the converter’s linearity. PSR is the maximum change in full-
scale transition point due to a change in power supply voltage
from the nominal value. The PSR ratio (PSRR) is defined as the
ratio of the power in the ADC output at full-scale frequency, f,
to the power of a 100 mV p-p sine wave applied to the ADC’s
V
where:
Pf is equal to the power at Frequency f in the ADC output.
Pf
supply.
Channel-to-Channel Isolation
Channel-to-channel isolation is a measure of the level of crosstalk
between all input channels. It is measured by applying a full-scale
sine wave signal, up to 160 kHz, to all unselected input channels
and then determining the degree to which the signal attenuates
in the selected channel with a 1 kHz sine wave signal applied (see
Figure 25).
1
2
DD
S
is the rms amplitude of the fundamental.
to V
is equal to the power at Frequency f
and V
THD (dB) =
20log
PSRR (dB) = 10 log (Pf/Pf
9
are the rms amplitudes of the second through ninth
SS
supplies of Frequency f
V
2
2
+
V
3
2
+
V
4
2
+
S
/2, excluding dc) to the rms value
S
V
)
5
2
V
+
1
S
.
V
6
S
2
coupled onto the AV
+
V
7
2
Data Sheet
+
V
8
2
+
V
9
2
CC