LTC1410 데이터 시트보기 (PDF) - Linear Technology

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LTC1410

12-Bit, 1.25Msps Sampling A/D Converter with Shutdown

Linear Technology 

LTC1410

APPLICATIONS INFORMATION

where V1 is the RMS amplitude of the fundamental fre-

quency and V2 through Vn are the amplitudes of the

second through nth harmonics. THD vs Input Frequency is

shown in Figure 4. The LTC1410 has good distortion

performance up to the Nyquist frequency and beyond.

0

–10

–20

–30

–40

–50

–60

–70

–80

–90

–100

1k

3RD THD

2ND

10k

100k

1M

INPUT FREQUENCY (Hz)

10M

1410 G03

Figure 4. Distortion vs Input Frequency

Intermodulation Distortion (IMD)

If the ADC input signal consists of more than one spectral

component, the ADC transfer function nonlinearity can

produce Intermodulation Distortion 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 magnitude,

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

be expressed by the following formula:

( ) ( ) IMD

fa + fb

Amplitude at

= 20 log

fa ± fb

Amplitude at fa

0

(fa) (fb)

–20

–40

fSAMPLE = 1.25MHz

fIN1 = 88.19580078kHz

fIN2 = 111.9995117kHz

–60 (2fa–fb)

(fb–fa)

–80

(2fb–fa) (fa+fb) (2fa+fb)

(2fa) (2fb) (fa+2fb)

(3fa)

(3fb)

–100

–120

0

100

200

300

400

500

600

FREQUENCY (MHz)

1410 F05

Figure 5. Intermodulation Distortion Plot

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.

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

LTC1410 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)

does not become dominated by distortion until frequen-

cies far beyond Nyquist.

Driving the Analog Input

The differential analog inputs of the LTC1410 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

9

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