AD7895AN-10 Ver la hoja de datos (PDF) - Analog Devices
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AD7895AN-10 Datasheet PDF : 12 Pages
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Dynamic Performance (Mode 1 Only)
With a combined conversion and acquisition time of 4.1 µs, the
AD7895 is ideal for wide bandwidth signal processing applica-
tions. These applications require information on the ADC’s
effect on the spectral content of the input signal. Signal to
(Noise + Distortion), Total Harmonic Distortion, Peak Har-
monic or Spurious Noise, and Intermodulation Distortion are
all specified. Figure 11 shows a typical FFT plot of a 10 kHz,
0 V to +5 V input after being digitized by the AD7895 operating
at a 198.656 kHz sampling rate. The Signal to (Noise + Distor-
tion) Ratio is 73.04 dB, and the Total Harmonic Distortion is
–84.91 dB.
The formula for Signal to (Noise + Distortion) Ratio (see
Terminology section) is related to the resolution or number of
bits in the converter. Rewriting the formula, below, gives a
measure of performance expressed in effective number of bits (N):
N = (SNR 1.76)/6.02
where SNR is Signal to (Noise + Distortion) Ratio.
–0
–10
FSAMPLE = 198656
FIN = 10kHz
–20
SNR = –73.04dB
THD = –84.91dB
–30
–40
–50
–60
–70
–80
–90
–100
–110
–120
0 10k
30k
50k
70k
90k 9.9k
Figure 11. AD7896 FFT Plot Effective Number of Bits
The effective number of bits for a device can be calculated from
its measured Signal to (Noise + Distortion) Ratio. Figure 12
shows a typical plot of effective number of bits versus frequency
for the AD7895 from dc to fSAMPLING/2. The sampling frequency
is 198.656 kHz. The plot shows that the AD7895 converts an
input sine wave of 10 kHz to an effective numbers of bits of
11.84, which equates to a Signal to (Noise + Distortion) level of
73.04 dB.
AD7895
12.0
11.8
11.6
11.4
11.2
11.0
10.8
10.6
10.4
10.2
10.0
0
200
400
600
800
FREQUENCY – kHz
1000
Figure 12. Effective Number of Bits vs. Frequency
Power Considerations
In the automatic power-down mode, then, the part may be
operated at a sample rate that is considerably less than
100 kHz. In this case, the power consumption will be reduced
and will depend on the sample rate. Figure 13 shows a graph
of the power consumption versus sampling rates from 100 Hz to
90 kHz in the automatic power-down mode. The conditions
are 5 V supply 25°C, serial clock frequency of 8.33 MHz, and
the data was read after conversion.
11
10
9
8
7
6
5
4
3
2
1
0
0.1 10 20 30 40 50 60 70 80 90
FREQUENCY – kHz
Figure 13. Power vs. Sample Rate in Auto Power-Down
Mode
REV. 0
–11–
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