AD7366/AD7367
TERMINOLOGY
Total Harmonic Distortion (THD)
THD is the ratio of the rms sum of harmonics to the
fundamental. For the AD7366/AD7367, it is defined as:
Differential Nonlinearity (DNL)
DNL is the difference between the measured and the ideal
1 LSB change between any two adjacent codes in the ADC.
2
2
2
2
2
Integral Nonlinearity (INL)
V2 +V3 +V4 +V5 +V6
THD(dB) = 20log
INL is 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 single (1)
LSB point below the first code transition and full scale, a point
1 LSB above the last code transition.
V1
where:
V1 is the rms amplitude of the fundamental.
V2, V3, V4, V5, and V6 are the rms amplitudes of the second
through the sixth harmonics.
Zero Code Error
Peak Harmonic or Spurious Noise
It is the deviation of the midscale transition (all 1s to all ꢀs)
from the ideal ꢁIN voltage, that is, AGND – ½ LSB for bipolar
ranges and 2 × ꢁREF − 1 LSB for the unipolar range.
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 fS/2, excluding dc) to the rms value of the
fundamental. Normally, the value of this specification is deter-
mined by the largest harmonic in the spectrum. However, for
ADCs where the harmonics are buried in the noise floor, it is
a noise peak.
Positive Full-Scale Error
It is the deviation of the last code transition (ꢀ11…11ꢀ) to
(ꢀ11…111) from the ideal (that is, +4 × ꢁREF − 1 LSB or +2 ×
ꢁREF – 1 LSB) after the zero code error has been adjusted out.
Negative Full-Scale Error
Channel-to-Channel Isolation
This is the deviation of the first code transition (1ꢀ…ꢀꢀꢀ) to
(1ꢀ…ꢀꢀ1) from the ideal (that is, −4 × ꢁREF + 1 LSB, −2 × ꢁREF
+ 1 LSB, or AGND + 1 LSB) after the zero code error has been
adjusted out.
Channel-to-channel isolation is a measure of the level of cross-
talk between any two channels when operating in any of the
input ranges. It is measured by applying a full-scale, 1±ꢀ kHz
sine wave signal to all unselected input channels and determin-
ing how much that signal is attenuated in the selected channel
with a ±ꢀ kHz signal. The figure given is the typical across all
four channels for the AD7366/AD7367 (see the Typical
Performance Characteristics section for more information).
Zero Code Error Match
This is the difference in zero code error across all 12 channels.
Positive Full-Scale Error Match
The difference in positive full-scale error across all channels.
Intermodulation Distortion
Negative Full-Scale Error Match
The difference in negative full-scale error across all channels.
With inputs consisting of sine waves at two frequencies, fa and
fb, any active device with nonlinearities creates distortion prod-
ucts at the sum, and different frequencies of mfa ± nfb where m,
n = ꢀ, 1, 2, 3, and so on. Intermodulation distortion terms are
those for which neither m nor n is 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).
Track-and-Hold Acquisition Time
The track-and-hold amplifier returns to track mode at the end
of a conversion. Track-and-hold acquisition time is the time
required for the output of the track-and-hold amplifier to reach
its final value, within ±½ LSB, after the end of conversion.
Signal-to-Noise (+ Distortion) Ratio (SINAD)
This ratio 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 sum of all nonfundamental signals up
to half the sampling frequency (fS/2), excluding dc. The ratio is
dependent on the number of quantization levels in the digitiza-
tion 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:
The AD7366/AD7367 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-order terms are usually dis-
tanced 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
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 sum of the fundamentals expressed in decibels.
Signal-to-Noise (+ Distortion) = (6.ꢀ2N + 1.76) dB
Thus, for a 12-bit converter, this is 74 dB.
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