AD7776AN データシートの表示（PDF） - Analog Devices
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AD7776AN Datasheet PDF : 12 Pages
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AD7776/AD7777/AD7778
Short Circuit Current
This is defined as the maximum current which will flow 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:
20 log =
(V22 + V32 + V42 + V52 + V62)1/2
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 will create 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 dy-
namic characteristics S/(N+D), THD & IMD of the ADC are
critical. The AD7776/AD7777/AD7778 are specified dynami-
cally 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 of 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 a 380.95 kHz sampling rate. 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 con-
sisting 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 =
99.88 kHz
SAMPLE FREQUENCY =
–20
380.95 kHz
SNR = 58.7 dB
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 will cause a degradation in S/(N+D). By
working backwards from the above equation, it is possible to get
a measure of ADC performance expressed in effective number
of bits (n).
S/(N+D) (dB) – 1.76
n(effective) =
6.02
The effective number of bits plotted vs. frequency for a single
channel of the AD7778 is shown in Figure 12. The effective
number of bits is typically 9.5.
10
9.5
9
SAMPLE FREQUENCY = 378.4 kHz
8.5
TA = +24°C
8
7.5
0
INPUT FREQUENCY – kHz
189.2
Figure 12. Effective Number of Bits vs. Frequency
–10–
REV. 0
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