LTC1608 datasheet(18/20 Pages) LINER

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LTC1608

APPLICATIO S I FOR ATIO

The dynamic performance is excellent for input frequen-

cies up to and beyond the Nyquist limit of 250kHz.

Effective Number of Bits

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

the resolution of an ADC and is directly related to the

S/(N + D) by the equation:

ENOB = [S/(N + D) – 1.76]/6.02

where ENOB is the effective number of bits of resolution

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

sampling rate of 500kHz, the LTC1608 maintains above 14

bits up to the Nyquist input frequency of 250kHz (refer to

Figure 20).

FREQUENCY (kHz)

–40

–20

200

1608 F19b

–60

–80

100

150

250

–100

–120

–140

fSAMPLE = 500kHz

fIN = 98.754kHz

SINAD = 86.7dB

THD = –92.6dB

Figure 19b. Even with Inputs at 100kHz, the

LTC1608’s Dynamic Linearity Remains Robust

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

Log

VVV

...

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 21. The LTC1608 has good distortion

performance up to the Nyquist frequency and beyond.

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

FREQUENCY (kHz)

–40

–20

200

1608 F19a

–60

–80

100

150

250

–100

–120

–140

fSAMPLE = 500kHz

fIN = 2.807kHz

SINAD = 88.9dB

THD = –98dB

Figure 19a. This FFT of the LTC1608’s Conversion of a

Full-Scale 3kHz Sine Wave Shows Outstanding Response

with a Very Low Noise Floor When Sampling at 500ksps

FREQUENCY (Hz)

10k

100k

1608 F20

Figure 20. Effective Bits and Signal/(Noise + Distortion)

vs Input Frequency

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