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

20

060

100

140

160

180

200

11.0

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