LTC1404CS8 datasheet(11/24 Pages) LINER

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LTC1404

APPLICATIONS INFORMATION

Signal-to-Noise Ratio

The signal-to-noise plus distortion ratio [S/(N + D)] is the

ratio between the RMS amplitude of the fundamental input

frequency to the RMS amplitude of all other frequency

components at the A/D output. The output is band limited

to frequencies from DC to half the sampling frequency.

Figure 2a shows a typical spectral content with a 600kHz

sampling rate and a 100kHz input. The dynamic perfor-

mance is excellent for input frequencies up to the Nyquist

limit of 300kHz as shown in Figure 2b.

Effective Number of Bits

The effective number of bits (ENOBs) is a measurement of

the effective resolution of an ADC and is directly related to

the S/(N + D) by the equation:

SN D

()

/– .

176

602

where N is the effective number of bits of resolution and

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

rate of 600kHz, the LTC1404 maintains very good ENOBs

up to the Nyquist input frequency of 300kHz (refer to

Figure 3).

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:

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 LTC1404 has good distortion

performance up to the Nyquist frequency and beyond.

INPUT FREQUENCY (Hz)

10k

–10

–20

–30

–40

–50

–60

–70

–80

–90

–100

100k

1404 F04

fSAMPLE = 600kHz

3RD HARMONIC

THD

2ND HARMONIC

Figure 4. Distortion vs Input Frequency in

Bipolar Mode

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.

INPUT FREQUENCY (Hz)

10k

100k

1404 F03

NYQUIST

FREQUENCY

fSAMPLE = 600kHz

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

Distortion vs Input Frequency in Bipolar Mode

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