Datasheet
AD7891
–19–
REV. D
Effective Number of Bits
The formula for signal-to-(noise + distortion) ratio (see Terminology
section) is related to the resolution or number of bits of the
converter. Rewriting the formula gives a measure of performance
expressed in effective number of bits (ENOB).
ENOB SNR=-
()
176 602./.
where SNR is the signal-to-(noise + distortion) ratio.
The effective number of bits for a device can be calculated from
its measured SNR. Figure 20 shows a typical plot of effective
number of bits versus frequency for the AD7891-1 and the
AD7891-2 from dc to 200 kHz. The sampling frequency is
500 kHz. The AD7891-1 inputs were configured for ± 10 V
operation. The AD7891-2 inputs were configured for 0 to 5 V
operation. The AD7891-1 plot only goes to 100 kHz as a
± 10 V sine wave of sufficient quality was unavailable at higher
frequencies.
Figure 20 shows that the AD7891-1 converts an input sine wave of
100 kHz to an effective number of bits of 11 which equates to a
signal-to-(noise + distortion) level of 68.02 dBs. The AD7891-2
converts an input sine wave of 200 kHz to an effective number
of bits of 11.07, which equates to a signal-to-(noise + distortion)
level of 68.4 dBs.
FREQUENCY – kHz
12.0
11.9
11.1
40 80 120
11.5
11.4
11.3
11.2
11.7
11.6
11.8
20060100 140 160 180 200
11.0
EFFECTIVE NUMBER OF BITS
AD7891-2 ENOB
AD7891-1 ENOB
Figure 20. Effective Number of Bits vs. Frequency
44-Lead Plastic Leaded Chip Carrier [PLCC]
(P-44A)
Dimensions shown in inches and (millimeters)
BOTTOM VIEW
(PINS UP)
6
PIN 1
IDENTIFIER
7
40
39
17
18
29
28
TOP VIEW
(PINS DOWN)
0.656 (16.66)
0.650 (16.51)
SQ
0.048 (1.22)
0.042 (1.07)
0.050
(1.27)
BSC
0.695 (17.65)
0.685 (17.40)
SQ
0.048 (1.22)
0.042 (1.07)
0.021 (0.53)
0.013 (0.33)
0.630 (16.00)
0.590 (14.99)
0.032 (0.81)
0.026 (0.66)
0.180 (4.57)
0.165 (4.19)
0.056 (1.42)
0.042 (1.07)
0.020 (0.51)
MIN
0.120 (3.05)
0.090 (2.29)
0.040 (1.01)
0.025 (0.64)
R
COMPLIANT TO JEDEC STANDARDS MO-047AC
CONTROLLING DIMENSIONS ARE IN INCHES; MILLIMETER DIMENSIONS
(IN PARENTHESES) ARE ROUNDED-OFF INCH EQUIVALENTS FOR
REFERENCE ONLY AND ARE NOT APPROPRIATE FOR USE IN DESIGN
OUTLINE DIMENSIONS