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LTC1749

1749f

TI I G DIAGRA

APPLICATIO S I FOR ATIO

DYNAMIC PERFORMANCE

Signal-to-Noise Plus Distortion Ratio

The signal-to-noise plus distortion ratio [S/(N + D)] is the

ratio between the RMS amplitude of the fundamental input

frequency and the RMS amplitude of all other frequency

components at the ADC output. The output is band limited

to frequencies above DC to below half the sampling

frequency.

Signal-to-Noise Ratio

The signal-to-noise ratio (SNR) is the ratio between the

RMS amplitude of the fundamental input frequency and

the RMS amplitude of all other frequency components

except the first five harmonics and DC.

Total Harmonic Distortion

Total harmonic distortion 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. The THD calculated in this

data sheet uses all the harmonics up to the fifth.

Intermodulation Distortion

If the ADC input signal consists of more than one spectral

component, the ADC transfer function nonlinearity can

produce intermodulation distortion (IMD) in addition to

THD. IMD is the change in one sinusoidal input caused by

the presence of another sinusoidal input at a different

frequency.

If two pure sine waves of frequencies fa and fb are applied

to the ADC input, nonlinearities in the ADC transfer func-

tion can create distortion products at the sum and differ-

ence frequencies of mfa

± nfb, where m and n = 0, 1, 2, 3,

etc. The 3rd order intermodulation products are 2fa + fb,

2fb + fa, 2fa – fb and 2fb – fa. The intermodulation

distortion is defined as the ratio of the RMS value of either

input tone to the RMS value of the largest 3rd order

intermodulation product.

1749 TD

t10

•

DATA (N – 5)

DB11 TO DB0

ANALOG

INPUT

ENC

DATA

CLKOUT

DATA (N – 4)

DB11 TO DB0

DATA (N – 3)

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