9

LTC1419

Figure 4. Distortion vs Input Frequency

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

fb

Log

+

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 77dB (12.5 effective bits).

The LTC1419 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.

Driving the Analog Input

The differential analog inputs of the LTC1419 are easy to

drive. The inputs may be driven differentially or as a single-

unwanted signal that is common mode to both inputs will

be reduced by the common mode rejection of the sample-

and-hold circuit. The inputs draw only one small current

spike while charging the sample-and-hold capacitors at

the end of conversion. During conversion the analog

inputs draw only a small leakage current. If the source

impedance of the driving circuit is low, then the LTC1419

inputs can be driven directly. As source impedance in-

creases so will acquisition time (see Figure 6). For mini-

mum acquisition time with high source impedance, a

buffer amplifier should be used. The only requirement is

Figure 5. Intermodulation Distortion Plot

FREQUENCY (kHz)

0

–120

–100

–80

–60

–40

100

200

300

400

1419 G05

–20

0

50

150

250

350

INPUT FREQUENCY (Hz)

0

–10

–20

–30

–40

–50

–60

–70

–80

–90

–100

– 110

1419 G03

THD

2ND

3RD

1k

100k

1M

2M

10k