LTC1278 Просмотр технического описания (PDF) - Linear Technology

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LTC1278

12-Bit, 500ksps Sampling A/D Converter with Shutdown

Linear Technology 

LTC1278

APPLICATIONS INFORMATION

frequency is shown in Figure 4. The LTC1278 has good

distortion performance up to the Nyquist frequency and

beyond.

0

fSAMPLE = 500kHz

–10

–20

–30

–40

–50

–60

–70

2ND HARMONIC

–80

THD

–90

3RD HARMONIC

–100

10k

100k

INPUT FREQUENCY (Hz)

1M 2M

LT1278 G6

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 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

Amplitude at

±

fa

fb)

Figure 5 shows the IMD performance at a 100kHz input.

0

fSAMPLE = 500kHz

–20

fIN1 = 96.80kHz

fIN2 = 101.68kHz

–40

–60

–80

–100

–120

0

50k 100k 150k 200k 250k

FREQUENCY (Hz)

LTC1278 G8

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 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 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

LTC1278 has been designed to optimize input bandwidth,

allowing ADC to undersample input signals with frequen-

cies above the converter’s Nyquist Frequency. The noise

floor stays very low at high frequencies; S/(N + D) be-

comes dominated by distortion at frequencies far beyond

Nyquist.

Driving the Analog Input

The analog input of the LTC1278 is easy to drive. It draws

only one small current spike while charging the sample-

and-hold capacitor at the end of conversion. During con-

version the analog input draws no current. The only

requirement is that the amplifier driving the analog input

must settle after the small current spike before the next

10

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