AD7938 Analog Devices, AD7938 Datasheet - Page 14

• Manufacturer Part Number: AD7938
• Description: 8-Channel, 1.5 MSPS, 12-Bit Parallel ADCs with a Sequencer
• Manufacturer: Analog Devices

Specifications of AD7938

• Resolution (bits): 12bit
• # Chan: 8
• Sample Rate: 1.5MSPS
• Interface: Byte,Par
• Analog Input Type: Diff-Uni,SE-Uni
• Ain Range: (2Vref) p-p,5V p-p,Uni (Vref),Uni (Vref) x 2,Uni 2.5V,Uni 5.0V
• Adc Architecture: SAR
• Pkg Type: CSP,QFP

AD7938/AD7939
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 ±½ 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
ratio is dependent on the number of quantization levels in the
digitization process; the more levels, the smaller the
quantization noise.
The theoretical signal-to-noise and distortion ratio for an ideal
N-bit converter with a sine wave input is given by
Thus, for a 12-bit converter, SINAD is 74 dB, and for a 10-bit
converter, it is 62 dB.
Total Harmonic Distortion (THD)
THD is the ratio of the rms sum of harmonics to the
fundamental. For the AD7938/AD7939, it is defined as
where:
V
V
through the sixth harmonics.
1
2
, V
is the rms amplitude of the fundamental.
SINAD = (6.02 N + 1.76) dB
THD
3
, V
4
, V
( )
dB
5
, and V
=
−
20
6
are the rms amplitudes of the second
log




V
2
2
SAMPLE
+
V
3
2
/2), excluding dc. The
+
V
V
1
4
2
+
V
5
2
+
V
6
2




Rev. C | Page 14 of 36
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
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), while
the third-order terms include (2fa + fb), (2fa − fb), (fa + 2fb),
and (fa − 2fb).
The AD7938/AD7939 are 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- 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.
SAMPLE
/2 and excluding dc) to the rms value of
Data Sheet