LTC1277CS datasheet(12/20 Pages) LINER

12 / 20 page

LTC1274/LTC1277

APPLICATIONS INFORMATION

Full-Power and Full-Linear Bandwidth

The full-power bandwidth is that input frequency at which

the amplitude of the reconstructed fundamental is re-

duced 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

LTC1274/LTC1277 have been designed to optimize input

bandwidth, allowing ADCs 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 analog input of the LTC1274/LTC1277 is easy to

drive. It draws only one small current spike while charg-

ing the sample-and-hold capacitor at the end of conver-

sion. During conversion the analog input draws only a

small leakage current. The only requirement is that the

amplifier driving the analog input must settle after the

small current spike before the next conversion starts.

Any op amp that settles in 2

µs to small current transients

will allow maximum speed operation. If slower op amps

are used, more settling time can be provided by increasing

the time between conversions. Suitable devices capable of

driving the ADC AIN input include the LT

®1006, LT1007,

LT1220, LT1223 and LT1224 op amps.

quency is shown in Figure 4. The ADCs have good distor-

tion performance up to the Nyquist frequency and beyond.

Intermodulation Distortion

If the ADC input signal consists of more than one spectral

component, the ADC transfer function nonlinearity can pro-

duce 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 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) and

(fa – fb) while the 3rd order IMD terms include (2fa + fb),

(2fa – fb), (fa + 2fb) and (fa – 2fb). 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

Figure 5 shows the IMD performance at a 97kHz input.

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.

Figure 4. Distortion vs Input Frequency

Figure 5. Intermodulation Distortion

FREQUENCY (Hz)

–20

–40

–60

–80

–100

–120

10k

20k

30k

40k

LTC1274/77 • F05

50k

2fb – fa

fb – fa

2fb

2fa – fb

2fa

3fb

2fb + fa

2fa + fb

fa + fb

3fa

fSAMPLE = 100kHz

fa = 96.948kHz

fb = 97.681kHz

THD

INPUT FREQUENCY (Hz)

10k

–20

–40

–60

–80

–100

–120

100k

LTC1274/77 • F04

3RD HARMONIC

2ND HARMONIC

fSAMPLE = 100kHz

LTC1277CS датащи(PDF) 12 Page - Linear Technology

LTC1277CS датащи(HTML) 12 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