AD7891 datasheet(19/20 Pages) AD

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AD7891

–19–

REV. A

Effective Number of Bits

The formula for signal to (noise + distortion) Ratio (see termi-

nology) is related to the resolution, or number of bits, of the

converter. Rewriting the formula, below, gives a measure of

performance expressed in effective number of bits (ENOB):

ENOB = (SNR - 1.76)/6.02

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 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 effec-

tive 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

120

11.5

11.4

11.3

11.2

11.7

11.6

11.8

100

140

160

180

200

11.0

AD7891-2 ENOB

AD7891-1 ENOB

Figure 20. Effective Number of Bits vs. Frequency

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