AD7776 Просмотр технического описания (PDF) - Analog Devices
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AD7776 Datasheet PDF : 12 Pages
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AD7776/AD7777/AD7778
Short Circuit Current
This is defined as the maximum current which flows either
into or out of the REFOUT pin if this pin is shorted to any
potential between 0 V and VCC. This condition can be allowed
for up to 10 seconds provided that the power dissipation of the
package is not exceeded.
Signal-to-Noise and Distortion Ratio, S/(N+D)
Signal-to-noise and distortion ratio, S/(N+D), is the ratio of the
rms value of the measured input signal to the rms sum of all
other spectral components below the Nyquist frequency, includ-
ing harmonics, but excluding dc. The value for S/(N+D) is
given in decibels.
Total Harmonic Distortion, THD
Total harmonic distortion is the ratio of the rms sum of the first
five harmonic components to the rms value of a full-scale input
signal and is expressed in decibels. For the AD7776/AD7777/
AD7778, total harmonic distortion (THD) is defined as:
( ) V22 + V32 + V4 2 + V52 + V62 1 2
20 log
V1
where V1 is the rms amplitude of the fundamental and V2,
V3, V4, V5, and V6 are the rms amplitudes of the individual
harmonics.
Intermodulation Distortion, IMD
With inputs consisting of sine waves at two frequencies, fa and fb,
any active device with nonlinearities creates distortion products,
of order (m + n), at sum and difference frequencies of mfa + nfb,
where m, n = 0, 1, 2, 3. Intermodulation terms are those for which
m or n is not equal to zero. For example, the second order terms
include (fa + fb) and (fa – fb) and the third order terms include
(2 fa + fb), (2 fa – fb), (fa + 2 fb), and (fa – 2 fb).
Channel-to-Channel Isolation
Channel-to-channel isolation is a measure of the level of cross-talk
between channels. It is measured by applying a full-scale 100 kHz
sine wave signal to any one of the input channels and monitoring
the remaining channels. The figure given is the worst case across
all channels.
DIGITAL SIGNAL PROCESSING APPLICATIONS
In digital signal processing (DSP) application areas like voice
recognition, echo cancellation, and adaptive filtering, the dynamic
characteristics S/(N+D), THD, and IMD of the ADC are critical.
The AD7776/AD7777/AD7778 are specified dynamically as well
as with standard dc specifications. Because the track/hold
amplifier has a wide bandwidth, an antialiasing filter should be
placed on the analog inputs to avoid aliasing high frequency noise
back into the bands of interest.
The dynamic performance of the ADC is evaluated by applying a
sine wave signal of very low distortion to a single analog input
which is sampled at 380.95 kHz. A fast Fourier transform (FFT)
plot or histogram plot is then generated from which the signal to
noise and distortion, harmonic distortion, and dynamic differential
nonlinearity data can be obtained. Similarly, for intermodulation
distortion, an input signal consisting of two pure sine waves at
different frequencies is applied to the AD7776/AD7777/AD7778.
Figure 11 shows a 2048-point FFT plot for a single channel of
the AD7778 with an input signal of 99.88 kHz. The SNR is
58.71 dB. It can be seen that most of the harmonics are buried
in the noise floor. It should be noted that the harmonics are
taken into account when calculating the S/(N+D).
0
INPUT FREQUENCY =
99.88kHz
SAMPLE FREQUENCY =
–20
380.95kHz
SNR = 58.7dB
TA = 25؇C
–40
–60
–80
–90
0
99.88
FREQUENCY – kHz
Figure 11. ADC FFT Plot
The relationship between S/(N+D) and resolution (n) is ex-
pressed by the following equation:
S (N + D ) = (6.02n + 1.76) dB
This is for an ideal part with no differential or integral linearity
errors. These errors cause a degradation in S/(N+D). By work-
ing backwards from the above equation, it is possible to get a
measure of ADC performance expressed in effective number
of bits (n).
n(effective) = S
(N
+ D) (dB ) − 1.76
6.02
The effective number of bits plotted versus frequency for a
single channel of the AD7778 is shown in Figure 12. The effec-
tive number of bits is typically 9.5.
10.0
9.5
SAMPLE FREQUENCY = 378.4kHz
TA = 24؇C
9.0
8.5
8.0
7.5
0
INPUT FREQUENCY – kHz
189.2
Figure 12. Effective Number of Bits vs. Frequency
–10–
REV. A
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