LTC2360 datasheet(13/20 Pages) LINER

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LTC2360/LTC2361/LTC2362

236012f

APPLICATIONS INFORMATION

Intermodulation Distortion

If the ADC input signal consists of more than one spectral

component, the ADC transfer function nonlinearity can

produce intermoduation 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 function

can create distortion products at the sum and difference

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 magnitude, the

value (in decibels) of the 2nd order IMD products can be

expressed by the following formula:

IMD fa ± fb

()= 20log

Amplitude at fa ± fb

()

Amplitude at fa

Peak Harmonic or Spurious Noise

The peak harmonic or spurious noise is the largest spectral

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 reconstructed fundamental is reduced by

3dB for full-scale input signal.

The full-linear bandwidth is the input frequency at which the

SINAD has dropped to 68dB (11 effective bits). The LTC2362

has been designed to optimize input bandwidth, allowing the

ADC to undersample input signals with frequencies above

the converter’s Nyquist frequency. The noise ﬂoor stays

very low at high frequencies; SINAD becomes dominated

by distortion at frequencies far beyond Nyquist.

Figure 9. LTC2362 Intermodulation Distortion Plot

INPUT FREQUENCY (kHz)

–20

–60

–100

–40

–80

–120

100

200

236012 F09

250

150

VDD = 3.6V

fSMPL = 500ksps

fa = 99kHz

fb = 101kHz

IMD = –76.5dB

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