AD7606/AD7606-6/AD7606-4
TERMINOLOGY
Integral Nonlinearity
Total Harmonic Distortion (THD)
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.
The ratio of the rms sum of the harmonics to the fundamental.
For the AD7606/AD7606-6/AD7606-4, it is defined as
THD (dB) =
2
2
V22 +V32 +V42 +V52 +V6 2 +V7 +V8 +V9
2
20log
V1
Differential Nonlinearity
The difference between the measured and the ideal 1 LSB
change between any two adjacent codes in the ADC.
where:
V1 is the rms amplitude of the fundamental.
V2 to V9 are the rms amplitudes of the second through ninth
harmonics.
Bipolar Zero Code Error
The deviation of the midscale transition (all 1s to all 0s) from
the ideal, which is 0 V − ½ LSB.
Peak Harmonic or Spurious Noise
The ratio of the rms value of the next largest component in the
ADC output spectrum (up to fS/2, excluding dc) to the rms value
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.
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 (9.99954) and 5 V − 1½ LSB
(4.99977)) after bipolar zero code error is adjusted out. The
positive full-scale error includes the contribution from the
internal reference buffer.
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. 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).
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 (−9.99984) and −5 V + ½ LSB
(−4.99992)) after the bipolar zero code error is adjusted out.
The negative full-scale error includes the contribution from the
internal reference buffer.
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)
Negative Full-Scale Error Match
The absolute difference in negative full-scale error between any
two input channels.
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
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 (fS/2, excluding dc).
V
DD and VSS supplies of Frequency fS.
PSRR (dB) = 10 log (Pf/PfS)
The ratio depends on the number of quantization levels in
the digitization process: the more levels, the smaller the
quantization noise.
where:
Pf is equal to the power at Frequency f in the ADC output.
PfS is equal to the power at Frequency fS coupled onto the AVCC
supply.
The theoretical signal-to-(noise + distortion) ratio for an ideal
N-bit converter with a sine wave input is given by
Channel-to-Channel Isolation
Signal-to-(Noise + Distortion) = (6.02 N + 1.76) dB
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 28).
Thus, for a 16-bit converter, the signal-to-(noise + distortion)
is 98 dB.
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