Datasheet

AD7894
–11–REV. 0
Dynamic Performance (Mode 1 Only)
With a conversion time of 5 µs, the AD7894 is ideal for wide
bandwidth signal processing applications. These applications
require information on the ADC’s effect on the spectral con-
tent of the input signal. Signal to (Noise + Distortion), Total
Harmonic Distortion, Peak Harmonic or Spurious Noise and
Intermodulation Distortion are all specified. Figure 11 shows a
typical FFT plot of a 10 kHz, ±10␣ V input after being digitized
by the AD7894-10 operating at a 160 kHz sampling rate. The
signal to (noise + distortion) ratio is 80.24 dB and the total
harmonic distortion is –96.35 dB.
The formula for signal to (noise + distortion) ratio (see Ter-
minology section) is related to the resolution or number of bits
in the converter. Rewriting the formula, below, gives a mea-
sure of performance expressed in effective number of bits (N):
N =
(SNR –1.76)
6.02
where SNR is Signal to (Noise + Distortion) Ratio.
FREQUENCY – kHz
0
–140
08010
dBs
20 30 40 50 60 70
–20
–60
–80
–100
–120
–40
f
S
= 160kHz
F
IN
= 10kHz
SNR = 80.24dB
THD = –96.35dB
Figure 11. AD7894 FFT Plot
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 AD7894 from dc to f
SAMPLING
/2. The sampling fre-
quency is 160 kHz. The plot shows that the AD7894 converts
an input sine wave of 10␣ kHz to an effective numbers of bits of
13.00, which equates to a signal to (noise + distortion) level of
80.02 dB.
FREQUENCY – kHz
14
13
9
10 1000100
ENOBs
12
11
10
Figure 12. Effective Number of Bits vs. Frequency
Power Considerations
In the automatic power-down mode the part may be operated at
a sample rate that is considerably less than 160 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 con-
sumption versus sampling rates from 1 Hz to 100 kHz in the
automatic power-down mode. The conditions are 5 V supply
+25°C. The SCLK pin was held low and no data was read from
the part.
SAMPLING FREQUENCY – Hz
100
0.1
1 10000010
POWER – mW
100 1000 10000
10
1
Figure 13. Power vs. Sampling Rate in Automatic Power-
Down Mode
82
81
78
–40 80–20
0
20 40 60
80
79
f
S
= 160kHz
F
IN
= 10kHz
TEMPERATURE – 8C
SNR+D – dB
Figure 14. SNR + D vs. Temperature
FREQUENCY – kHz
100
80
10 1000100
THD – dB
90
70
60
50
40
30
20
10
0
Figure 15. THD vs. Frequency