1418CG Просмотр технического описания (PDF) - Linear Technology

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1418CG

Low Power, 14-Bit, 200ksps ADC with Serial and Parallel I/O

Linear Technology 

LTC1418

APPLICATIONS INFORMATION

14

13

12

11

10

9

8

7

6

5

4

3

2

1k

10k

100k

1M

INPUT FREQUENCY (Hz)

1418 F03

Figure 3. Effective Bits and Signal/(Noise + Distortion)

vs Input Frequency

Total Harmonic Distortion

Total harmonic distortion (THD) is the ratio of the RMS

sum of all harmonics of the input signal to the fundamental

itself. The out-of-band harmonics alias into the frequency

band between DC and half the sampling frequency. THD

is expressed as:

THD = 20Log V22 + V32 + V42 +...VN2

V1

where V1 is the RMS amplitude of the fundamental

frequency and V2 through VN are the amplitudes of the

second through nth harmonics. THD vs Input Frequency

is shown in Figure 4. The LTC1418 has good distortion

performance up to the Nyquist frequency and beyond.

0

–20

–40

–60

–80

–100

3RD

THD

2ND

–120

1k

10k

100k

1M

INPUT FREQUENCY (Hz)

1418 F04

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 function

can create distortion products at the 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).

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

0

fSAMPLE = 200kHz

– 20

fIN1 = 97.65625kHz

fIN2 = 104.248046kHz

– 40

– 60

– 80

– 100

– 120

0 10 20 30 40 50 60 70 80 90 100

FREQUENCY (kHz)

1418 F05

Figure 5. Intermodulation Distortion Plot

Peak Harmonic or Spurious Noise

The peak harmonic or spurious noise is the largest spectral

component excluding the input signal and DC. This value

is expressed in decibels relative to the RMS value of a

full-scale input signal.

For more information www.linear.com/LTC1418

1418fa

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