Datasheet

AD7938-6 Data Sheet
Rev. C | Page 12 of 32
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Track-and-Hold Acquisition Time
The track-and-hold amplifier returns to track mode at the end
of 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 ±1/2 LSB, after the end of conversion.
Signal to Noise and Distortion Ratio (SINAD)
This is the measured ratio of signal-to-noise and 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
SAMPLE
/2), excluding dc. The
ratio is dependent on the number of quantization levels in the
digitization process; the more levels, the smaller the
quantization noise.
The theoretical SINAD ratio for an ideal N-bit converter with a
sine wave input is given by
SINAD = (6.02 N + 1.76) dB
Thus, for a 12-bit converter, SINAD is 74 dB.
Total Harmonic Distortion (THD)
THD is the ratio of the rms sum of harmonics to the
fundamental. For the AD7938-6, it is defined as:
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
++++
−=
1
65432
V
VVVVV
THD
22222
log20dB
where:
V
1
is the rms amplitude of the fundamental.
V
2
, V
3
, V
4
, V
5
, and V
6
are the rms amplitudes of the second
through the sixth 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
SAMPLE
/2 and 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 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 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 AD7938-6 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 distanced
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-order and third-order terms are
specified separately. The intermodulation distortion is
calculated per the THD specification, as the ratio of the rms
sum of the individual distortion products to the rms amplitude
of the sum of the fundamentals, expressed in dB.