LTC1417IGN datasheet(11/32 Pages) LINER

11 / 32 page

LTC1417

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, 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

Log

Amplitude

()=

()

at fa

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 the input frequency at which

the amplitude of the reconstructed fundamental is

reduced by 3dB from 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 LTC1417 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 LTC1417 are easy to

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

ended input (i.e., the AIN– input is grounded). The AIN+ and

AIN– inputs are sampled at the same instant. Any

unwanted signal that is common 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 LTC1417

inputs can be driven directly. As source impedance

increases, so will acquisition time (see Figure 7). For

minimum acquisition time, with high source impedance, a

buffer amplifier must be used. The only requirement is that

the amplifier driving the analog input(s) must settle after

the small current spike before the next conversion starts —

500ns for full throughput rate.

APPLICATIONS INFORMATION

Figure 5. Distortion vs Input Frequency

INPUT FREQUENCY (kHz)

–120

–100

–80

–60

–40

100

THD

2ND

3RD

1417 G05

1000

–20

Figure 6. Intermodulation Distortion Plot

FREQUENCY (kHz)

–120

–100

–80

–60

–40

100

140

200

1417 G09

–20

60 80

120

160 180

fSAMPLE = 400kHz

fIN1 = 97.303466kHz

fIN2 = 104.632568kHz

VIN = 4.096VP-P

LTC1417IGN Datasheet(PDF) 11 Page - Linear Technology

LTC1417IGN Datasheet(HTML) 11 Page - Linear Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32

Datasheet Download

Link URL