LTC1196 datasheet(22/28 Pages) LINER

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LTC1196/LTC1198

APPLICATI

I FOR ATIO

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 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 499kHz and 502kHz, the IMD of

the LTC1196/LTC1198 is 51dB 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 7 bits. Beyond

this frequency, distortion of the sampled input signal

increases. The LTC1196/LTC1198 have been designed to

optimize input bandwidth, allowing the ADCs to

undersample input signals with frequencies above the

converters’ Nyquist Frequency.

rate of 1.2MHz with a 5V supply the LTC1196 maintains

above 7.5 ENOBs at 400kHz input frequency. Above 500kHz

the ENOBs gradually decline, as shown in Figure 11, due

to increasing second harmonic distortion. The noise floor

remains low.

Figure 11. Effective Bits and S/(N + D) 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 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 100kHz input signal, the

LTC1196/LTC1198 have typical THD of 50dB and 49dB

with VCC = 5V and VCC = 3V, respectively.

Intermodulation Distortion

If the ADC input signal consists of more than one spectral

component, the ADC transfer function nonlinearity can

INPUT FREQUENCY (Hz)

10k

100k

1196/98 G24

VREF = VCC = 2.7V

fSMPL = 383kHz (LTC1196)

fSMPL = 287kHz (LTC1198)

VREF = VCC = 5V

fSMPL = 1MHz (LTC1196)

fSMPL = 750kHz (LTC1198)

TA = 25°C

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