LTC1277CS datasheet(11/20 Pages) LINER

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LTC1274/LTC1277

APPLICATIONS INFORMATION

using an FFT algorithm, the ADCs’ spectral content can be

examined for frequencies outside the fundamental. Figures

2a and 2b show typical LTC1274 FFT plots.

Signal-to-Noise Ratio

The Signal-to-Noise plus Distortion Ratio [S/(N + D)] is

the ratio between the RMS amplitude of the fundamental

input frequency to the RMS amplitude of all other fre-

quency components at the A/D output. The output is band

limited to frequencies above DC and below half the sam-

pling frequency. Figure 2a shows a typical spectral content

with a 100kHz sampling rate and a 48.85kHz input. The

dynamic performance is excellent for input frequencies well

beyond Nyquist as shown in Figure 2b and Figure 3.

INPUT FREQUENCY (kHz)

–20

–40

–60

–80

–100

–120

LTC1274/77 • F02a

fSAMPLE = 100kHz

fIN = 48.85kHz

Figure 2a. LTC1274 Nonaveraged, 4096 Point

FFT Plot with 50kHz Input Frequency

INPUT FREQUENCY (kHz)

–20

–40

–60

–80

–100

–120

LTC1274/77 • F02b

fSAMPLE = 100kHz

fIN = 97.68kHz

Figure 2b. LTC1274 Nonaveraged, 4096 Point

FFT Plot with 100kHz Input Frequency

Effective Number of Bits

The Effective Number of Bits (ENOBs) is a measurement of

the resolution of an ADC and is directly related to the

S/(N + D) by the equation:

N = [S/(N + D) – 1.76]/6.02

where N is the Effective Number of Bits of resolution and

S/(N + D) is expressed in dB. At the maximum sampling

rate of 100kHz, the LTC1274/LTC1277 maintain very

good ENOBs over 300kHz. Refer to Figure 3.

Total Harmonic Distortion

Total Harmonic Distortion (THD) is the ratio of the RMS

sum of all harmonics of the input signal to the fundamen-

tal itself. The out-of-band harmonics alias into the

frequency band between DC and half the sampling

frequency. THD is expressed as:

THD = 20log

√V

2 + V32 + V42 ... + VN2

where V1 is the RMS amplitude of the fundamental fre-

quency and V2 through VN are the amplitudes of the

second through Nth harmonics. THD versus input fre-

INPUT FREQUENCY (Hz)

10k

100k

LTC1274/77 • F03

fSAMPLE = 100kHz

NYQUIST

FREQUENCY

Figure 3. ENOBs and S/(N + D) vs Input Frequency

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