LTC2292UP datasheet(15/28 Pages) LINER

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LTC2293/LTC2292/LTC2291

229321f

DYNAMIC PERFORMANCE

Signal-to-Noise Plus Distortion Ratio

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

ratio between the RMS amplitude of the fundamental input

frequency and the RMS amplitude of all other frequency

components at the ADC output. The output is band limited

to frequencies above DC to below half the sampling

frequency.

Signal-to-Noise Ratio

The signal-to-noise ratio (SNR) is the ratio between the

RMS amplitude of the fundamental input frequency and

the RMS amplitude of all other frequency components

except the first five harmonics and DC.

Total Harmonic Distortion

Total harmonic distortion 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 = 20Log √(V2

2 + V32 + V42 + . . . Vn2)/V1

where V1 is the RMS amplitude of the fundamental fre-

quency and V2 through Vn are the amplitudes of the

second through nth harmonics. The THD calculated in this

data sheet uses all the harmonics up to the fifth.

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 the sum and differ-

ence frequencies of mfa ± nfb, where m and n = 0, 1, 2, 3,

etc. The 3rd order intermodulation products are 2fa + fb,

APPLICATIO S I FOR ATIO

2fb + fa, 2fa – fb and 2fb – fa. The intermodulation

distortion is defined as the ratio of the RMS value of either

input tone to the RMS value of the largest 3rd order

intermodulation product.

Spurious Free Dynamic Range (SFDR)

Spurious free dynamic range is the peak harmonic or

spurious noise that is the largest spectral component

excluding the input signal and DC. This value is expressed

in decibels relative to the RMS value of a full scale input

signal.

Input Bandwidth

The input bandwidth is that input frequency at which the

amplitude of the reconstructed fundamental is reduced by

3dB for a full scale input signal.

Aperture Delay Time

The time from when CLK reaches midsupply to the instant

that the input signal is held by the sample and hold circuit.

Aperture Delay Jitter

The variation in the aperture delay time from conversion to

conversion. This random variation will result in noise

when sampling an AC input. The signal to noise ratio due

to the jitter alone will be:

SNRJITTER = –20log (2π) • fIN • tJITTER

Crosstalk

Crosstalk is the coupling from one channel (being driven

by a full-scale signal) onto the other channel (being driven

by a –1dBFS signal).

CONVERTER OPERATION

As shown in Figure 1, the LTC2293/LTC2292/LTC2291 are

dual CMOS pipelined multistep converters. The convert-

ers have six pipelined ADC stages; a sampled analog input

will result in a digitized value six cycles later (see the

Timing Diagram section). For optimal AC performance

the analog inputs should be driven differentially. For cost

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