LTC1409CSW datasheet(10/20 Pages) LINER

10 / 20 page

LTC1409

APPLICATIONS INFORMATION

FREQUENCY (Hz)

–20

–40

–60

–80

–100

–120

100k

200k

300k

400k

LTC1409 • F05

50k

150k

250k

350k

fb – fa

2fb – fa

2fa – fb

2fa

2fb

3fb

fa + 2fb

3fa

2fa + fb

fa + fb

fSAMPLE = 800kHz

fIN1 = 88.19580078kHz

fIN2 = 111.9995117kHz

Figure 5. Intermodulation Distortion Plot

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 68dB (11 effective bits). The

LTC1409 has been designed to optimize input bandwidth,

allowing the ADC to undersample input signals with fre-

quencies 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 LTC1409 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 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

LTC1409 inputs can be driven directly. As source imped-

ance increases so will acquisition time (see Figure 6). For

minimum acquisition time, with high source impedance,

a buffer amplifier should be used. The only requirement

is that the amplifier driving the analog input(s) must

settle after the small current spike before the next conver-

sion starts (settling time must be 150ns for full through-

put rate).

Choosing an Input Amplifier

Choosing an input amplifier is easy if a few requirements

are taken into consideration. First, to limit the magnitude

of the voltage spike seen by the amplifier from charging

the sampling capacitor, choose an amplifier that has a

low output impedance (< 100

Ω) at the closed-loop band-

width frequency. For example, if an amplifier is used in a

gain of 1 and has a unity-gain bandwidth of 50MHz, then

the output impedance at 50MHz should be less than

100

Ω. The second requirement is that the closed-loop

SOURCE RESISTANCE (k

Ω)

0.01

LTC1409 • F06

0.1

0.01

0.1

110

100

Figure 6. Acquisition Time vs Source Resistance

LTC1409CSW датащи(PDF) 10 Page - Linear Technology

LTC1409CSW датащи(HTML) 10 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

датащи скачать

ссылки URL