LTC2365 datasheet(13/24 Pages) LINER

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LTC2365/LTC2366

23656f

APPLICATIONS INFORMATION

Figure 9b. LTC2366 Intermodulation Distortion Plot

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

The LTC2365/LTC2366 have good IMD as shown in Figure

9a and Figure 9b respectively.

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

LTC2365/LTC2366 have been designed to optimize input

bandwidth, allowing the ADC to undersample input sig-

nals with frequencies above the converter’s Nyquist Fre-

quency. The noise ﬂoor stays very low at high frequencies;

SINAD becomes dominated by distortion at frequencies

far beyond Nyquist.

Figure 9a. LTC2365 Intermodulation Distortion Plot

INPUT FREQUENCY (kHz)

–140

–120

–80

–60

–40

250

350

23656 F09a

–100

–20

200

450 500

100 150

300

400

VDD = 3V

fSMPL = 1Msps

fb = 396kHz

fb = 424kHz

IMD = –73.5dB

INPUT FREQUENCY (kHz)

–20

–40

–60

–80

–100

–120

–140

750

1250

23656 F09b

250

500

1000

1500

VDD = 3V

fSMPL = 3Msps

fa = 935kHz

fb = 1.045kHz

IMD = –71.5dB

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