LTC1286CS8 datasheet(19/24 Pages) LINER

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LTC1286/LTC1298

APPLICATION INFORMATION

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 3rd order IMD terms include (2fa + fb),

(2fa – fb), (fa + 2fb), and (fa – 2fb). If the two input sine

waves are equal in magnitudes, the value (in dB) of the 2nd

order IMD products can be expressed by the following

formula:

IMD f

mplitude f

()=

()













20log

amplitude at fa

For input frequencies of 5kHz and 6kHz, the IMD of the

LTC1286/LTC1298 is 73dB with a 5V supply.

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 dBs 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.

The full-linear bandwidth is the input frequency at which

the effective bits rating of the ADC falls to 11 bits. Beyond

this frequency, distortion of the sampled input signal

increases. The LTC1286/LTC1298 have been designed to

optimize input bandwidth, allowing the ADCs to

undersample input signals with frequencies above the

converters’ Nyquist 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 of the sampling frequency. THD

is defined as:

THD

+++ +

20log

VVV

...

where V1 is the RMS amplitude of the fundamental fre-

quency and V2 through VN are the amplitudes of the

second through the Nth harmonics. The typical THD speci-

fication in the Dynamic Accuracy table includes the 2nd

through 5th harmonics. With a 7kHz input signal, the

LTC1286/LTC1298 have typical THD of 80dB with VCC = 5V.

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.

Figure 12. Effective Bits and S/(N + D) vs Input Frequency

INPUT FREQUENCY (kHz)

100

1000

LTC 1286/98 G20

TA = 25°C

VCC = 5V

fCLK = 200kHz

fSMPL = 12.5kHz

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