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LTC1417

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 frequency

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

to frequencies from above DC and below half the sampling

frequency. Figure 3b shows a typical spectral content with

a 400kHz sampling rate and a 200kHz input. The dynamic

performance is excellent for input frequencies up to and

beyond the Nyquist limit of 200kHz.

Figure 4. Effective Bits and Signal/(Noise + Distortion)

vs Input Frequency

APPLICATIONS INFORMATION

Figure 3a. LTC1417 Nonaveraged, 4096 Point FFT,

Input Frequency = 10kHz

Figure 3b. LTC1417 Nonaveraged, 4096 Point FFT,

Input Frequency = 200kHz

FREQUENCY (kHz)

100

–20

–40

–60

–80

–100

–120

150

1417 G07

200

fSAMPLE = 400kHz

fIN = 10.05859375kHz

SFDR = –97.44dB

SINAD = 81.71dB

FREQUENCY (kHz)

100

–20

–40

–60

–80

–100

–120

150

1417 G08

200

fSAMPLE = 400kHz

fIN = 197.949188kHz

SFDR = –98dB

SINAD = 81.1dB

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:

ENOB (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 400kHz, the LTC1417 maintains near ideal ENOBs

up to the Nyquist input frequency of 200kHz (refer to

Figure 4).

INPUT FREQUENCY (Hz)

10k

100k

1417 TA02

Total Harmonic Distortion

Total harmonic distortion (THD) is the ratio of the RMS

sum of all harmonics of the input signal to the fundamental

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

band between DC and half the sampling frequency. THD is

expressed as:

THD

Log

VVV

...

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 vs Input Frequency is

shown in Figure 5. The LTC1417 has good distortion

performance up to the Nyquist frequency and beyond.

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