AD7890 datasheet(19/20 Pages) AD

Part #	AD7890

Description	LC2MOS 8-Channel, 12-Bit Serial, Data Acquisition System

PDF	20 Pages

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Manufacturer	AD [Analog Devices]

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AD7890

–19–

REV. A

Effective Number of Bits

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 measure

of performance expressed in effective number of bits (N):

N = (SNR — 1.76)/6.02

where SNR is Signal to (Noise + Distortion) Ratio

The effective number of bits for a device can be calculated

from its measured Signal to (Noise + Distortion) Ratio. Fig-

ure 21 shows a typical plot of effective number of bits versus

frequency for the AD7890-2 from dc to 40 kHz. The sampling

frequency is 102.4 kHz. The plot shows that the AD7890 con-

verts an input sine wave of 40 kHz to an effective numbers of

bits of 11 which equates to a Signal to (Noise + Distortion)

level of 68 dB.

040

11.5

10.5

INPUT FREQUENCY – kHz

Figure 21. Effective Number of Bits vs. Frequency

AD7890 Datasheet(PDF) 19 Page - Analog Devices