AD7657 Analog Devices, AD7657 Datasheet - Page 18

• Manufacturer Part Number: AD7657
• Description: 250 kSPS, 6-Channel, Simultaneous Sampling, Bipolar, 14-Bit A/D Converter
• Manufacturer: Analog Devices

Specifications of AD7657

• Resolution (bits): 14bit
• # Chan: 6
• Sample Rate: 250kSPS
• Interface: Par,Ser,SPI
• Analog Input Type: SE-Bip
• Ain Range: Bip (Vref) x 2,Bip (Vref) x 4,Bip 10V,Bip 5.0V
• Adc Architecture: SAR
• Pkg Type: QFP

AD7656/AD7657/AD7658
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, a 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 V
Bipolar Zero Code Error Matching
The difference in bipolar zero code error between any two input
channels.
Positive Full-Scale Error
The deviation of the last code transition (011…110) to (011…111)
from the ideal (+4 × V
adjusting for the bipolar zero code error.
Positive Full-Scale Error Matching
The difference in positive full-scale error between any two input
channels.
Negative Full-Scale Error
The deviation of the first code transition (10…000) to (10…001)
from the ideal (−4 × V
adjusting for the bipolar zero code error.
Negative Full-Scale Error Matching
The difference in negative full-scale error between any two
input channels.
Track-and-Hold Acquisition Time
The track-and-hold amplifier returns to track mode at the end
of the conversion. The 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 LSB, after the end of the conversion.
See the Track-and-Hold Section for more details.
Signal-to-Noise Ratio (SNR)
SNR is the ratio of the rms value of the measured output signal
to the rms sum of all other spectral components below the
Nyquist frequency. The value for SNR is expressed in decibels.
Signal-to-(Noise + Distortion) Ratio (SINAD)
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
IN
voltage, that is, AGND − 1 LSB.
REF
REF
− 1 LSB, +2 × V
+ 1 LSB, −2 × V
S
/2, excluding dc).
REF
REF
− 1 LSB) after
+ 1 LSB) after
Rev. C | Page 18 of 32
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, this is 98 dB for a 16-bit converter, 86.04 dB for a 14-bit
converter, and 74 dB for a 12-bit converter.
Total Harmonic Distortion (THD)
The ratio of the rms sum of the harmonics to the fundamental.
For the AD7656/AD7657/AD7658, it is defined as
where:
V
V
through sixth harmonics.
Peak Harmonic or Spurious Noise (SFDR)
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 create 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 are 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 AD7656/AD7657/AD7658 are tested using the CCIF standard
in which two input frequencies near the top end of the input
bandwidth are used. In this case, the second-order terms are
usually distanced in frequency from the original sine waves,
and 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 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.
Power Supply Rejection (PSR)
Variations in power supply affect the full-scale transition but
not the converter’s linearity. Power supply rejection is the
maximum change in full-scale transition point due to a change
in power supply voltage from the nominal value. See the Typical
Performance Characteristics section.
1
2
, V
is the rms amplitude of the fundamental.
Signal-to-(Noise + Distortion) = (6.02 N + 1.76) dB
THD
3
, V
4
, V
(
dB
5
, and V
)
=
20
log
6
are the rms amplitudes of the second
V
2
2
S
+
/2, excluding dc) to the rms value
V
3
2
+
V
V
1
4
2
+
V
5
2
+
V
6
2