LTC1400CS8 datasheet(8/20 Pages) LINER

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LTC1400

APPLICATIONS INFORMATION

THD

+++…

222

log

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 LTC1400 has good distortion

performance up to the Nyquist frequency and beyond.

Figure 2b. LTC1400 Nonaveraged, 4096 Point FFT

Plot with 200kHz Input Frequency in Bipolar Mode

FREQUENCY (kHz)

80 100

140

180

120

160

200

LTC1400 • F02b

–10

–20

–30

–40

–50

– 60

–70

–80

–90

–100

–110

–120

fSAMPLE = 400kHz

fIN = 199.121kHz

SINAD = 72.1dB

THD = – 80dB

Figure 4. Distortion vs Input Frequency in

Bipolar Mode

INPUT FREQUENCY (Hz)

10k

–10

–20

–30

–40

–50

–60

–70

–80

–90

–100

100k

LTC1400 • F04

2ND HARMONIC

THD

3RD HARMONIC

fSAMPLE = 400kHz

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 includes (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

() =

20log

Amplitude at (

)

Amplitude at fa

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 of the sampling frequency. THD

is expressed as:

Figure 3. Effective Bits and Signal-to-Noise +

Distortion vs Input Frequency in Bipolar Mode

INPUT FREQUENCY (Hz)

10k

100k

LTC1400 • F03

NYQUIST

FREQUENCY

fSAMPLE = 400kHz

where N is the effective number of bits of resolution and

S/(N + D) is expressed in dB. At the maximum sampling

rate of 400kHz, the LTC1400 maintains very good ENOBs

up to the Nyquist input frequency of 200kHz (refer to

Figure 3).

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