LTC1406IGN datasheet(9/16 Pages) LINER

9 / 16 page

LTC1406

APPLICATIONS INFORMATION

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 magnitude, the

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

expressed by the following formula:

IMD f

Amplitude at

()=

()

20 log

Amplitude at f

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 decibel relative to the RMS value of

a full-scale input signal (see Figure 6).

Input Bandwidth

The input bandwidth is that input frequency at which the

amplitude of the reconstructed fundamental is reduced by

3dB for a full-scale input signal. The LTC1406 has been

designed for wide input bandwidth (250MHz), 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.

Analog Inputs

The LTC1406 has a unique differential sample-and-hold

circuit that allows rail-to-rail inputs. The AIN

+ and AIN–

inputs are sampled at the same time and the ADC will

always convert the difference of [(AIN

+) – (AIN–)] indepen-

dent of the common mode voltage. Any unwanted signal

that is common to both inputs will be rejected by the com-

mon mode rejection of the sample-and-hold circuit. The

common mode rejection holds up to extremely high fre-

quencies (see Figure 7).

The inputs can be driven differentially or single-ended. In

differential mode, both inputs are driven

±0.5V out of

phase with each other. In single-ended mode, the nega-

tive input is tied to a fixed voltage and AIN

+ is used as the

Figure 6. Spurious-Free Dynamic Range vs

Input Frequency

INPUT FREQUENCY (Hz)

100k

10M

100M

1406 G04

INPUT FREQUENCY (Hz)

100k

–10

–20

–30

–40

–50

–60

–70

–80

10M

100M

1406 G03

THD

3RD HARMONIC

2ND HARMONIC

Figure 4. Distortion vs Input Frequency

FREQUENCY (MHz)

–10

–20

–30

–40

–50

–60

–70

–80

–90

–100

1406 G05

0123

fSAMPLE = 20MHz

fIN1 = 3.500977MHz

fIN2 = 3.598633MHz

Figure 5. Intermodulation Distortion Plot

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