LTC1289BCSW datasheet(21/28 Pages) LINER

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LTC1289

1289fb

output spectrum of the LTC1289 is shown in Figures 17a

and 17b. The input (fIN) frequencies are 1kHz and 12kHz

with the sampling frequency (fS) at 25kHz. The SNR

obtained from the plot are 72.92dB and 72.23dB.

Rewriting the SNR expression it is possible to obtain the

equivalent resolution based on the SNR measurement.

This is the so-called effective number of bits (ENOB). For

the example shown in Figures 17a and 17b, N = 11.8 bits

and 11.7 bits, respectively. Figure 18 shows a plot of ENOB

as a function of input frequency. The curve shows the

A/D’s ENOB remain in the range of 11.8 to 11.7 for input

frequencies up to fS/2

APPLICATI

I FOR ATIO

7. LTC1289 AC Characteristics

Two commonly used figures of merit for specifying the

dynamic performance of the A/D’s in digital signal pro-

cessing applications are the Signal-to-Noise Ratio (SNR)

and the “effective number of bits (ENOB).” SNR is defined

as the ratio of the RMS magnitude of the fundamental to

the RMS magnitude of all the nonfundamental signals up

to the Nyquist frequency (half the sampling frequency).

The theoretical maximum SNR for a sine wave input is

given by:

SNR = (6.02N + 1.76dB)

where N is the number of bits. Thus the SNR is a function

of the resolution of the A/D. For an ideal 12-bit A/D the SNR

is equal to 74dB. A Fast Fourier Transform(FFT) plot of the

Figure 17a. fIN = 1kHz, fS = 25kHz, SNR = 72.92dB

Figure 19. fIN1 = 2.6kHz, fIN2 = 3.1kHz, fS = 25kHz

Figure 17b. fIN = 12kHz, fS = 25kHz, SNR = 72.23dB

Figure 18. LTC1289 ENOB vs Input Frequency

FREQUENCY (kHz)

LTC1289 • AIF18

fS = 25kHz

SNR – 1.76dB

6.02

N =

FREQUENCY (kHz)

–60

–40

610

LTC1289 F17a

–80

–100

812

–120

–140

–20

FREQUENCY (kHz)

–60

–40

610

LTC1289 F17b

–80

–100

812

–120

–140

–20

FREQUENCY (kHz)

–60

–40

610

LTC1289 F19

–80

–100

812

–120

–140

–20

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