Datasheet

AD7895
–11–
REV. 0
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
–120
0 9.9k10k 30k 50k 70k 90k
–20
–40
–60
–80
–100
–10
–30
–50
–70
–90
–110
F
SAMPLE
= 198656
F
IN
= 10kHz
SNR = –73.04dB
THD = –84.91dB
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 f
SAMPLING
/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.
0 1000200 400 600 800
10.0
11.4
11.2
11.0
10.8
11.8
11.6
12.0
10.6
10.4
FREQUENCY – kHz
ENOB
10.2
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.
0.1 9010 20 30 40
0
5
4
3
2
7
6
8
1
FREQUENCY – kHz
POWER – mW
50 60 70 80
10
9
11
Figure 13. Power vs. Sample Rate in Auto Power-Down
Mode