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LTC1603

1603f

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,

APPLICATIONS INFORMATION

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

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.

Full-Power and Full-Linear Bandwidth

The full-power bandwidth is that input frequency at which

the amplitude of the reconstructed fundamental is

reduced by 3dB for a full-scale input signal.

The full-linear bandwidth is the input frequency at which

the S/(N + D) has dropped to 84dB (13.66 effective bits).

The LTC1603 has been designed to optimize input band-

width, allowing the ADC to undersample input signals with

frequencies above the converter’s Nyquist Frequency. The

noise floor stays very low at high frequencies; S/(N + D)

becomes dominated by distortion at frequencies far

beyond Nyquist.

Figure 21. Intermodulation Distortion Plot

FREQUENCY (kHz)

020

100

–20

–40

–60

–80

–100

–120

–140

1603 F21

120

fSAMPLE = 250kHz

fIN1 = 29.3kHz

fIN2 = 32.4kHz

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