LTC1418AIN datasheet(10/28 Pages) LINER

10 / 28 page

LTC1418

APPLICATIONS INFORMATION

Figure 3. Effective Bits and Signal/(Noise + Distortion)

vs Input Frequency

shown in Figure 4. The LTC1418 has good distortion

performance up to the Nyquist frequency and beyond.

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. For example, the 2nd order IMD terms include

(fa + fb). If the two input sine waves are equal in magni-

tude, the value (in decibels) of the 2nd order IMD products

can be expressed by the following formula:

IMD fa

Log

Amplitude

()=

()

at fa + fb

Amplitude at fa

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

Figure 4. Distortion vs Input Frequency

INPUT FREQUENCY (Hz)

–20

–40

–60

–80

–100

–120

1418 G03

100k

10k

THD

2ND

3RD

Peak Harmonic or Spurious Noise

The peak harmonic or spurious noise is the largest spec-

tral component excluding the input signal and DC. This

value is expressed in decibels relative to the RMS value of

a full-scale input signal.

FREQUENCY (kHz)

–120

–100

–80

–60

–40

100

1418 G05

–20

30 40

fSAMPLE = 200kHz

fIN1 = 97.65625kHz

fIN2 = 104.248046kHz

Figure 5. Intermodulation Distortion Plot

INPUT FREQUENCY (Hz)

10k

100k

1418 F03

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