LTC2230UP datasheet(18/32 Pages) LINER

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LTC2230/LTC2231

22301fb

APPLICATIO S I FOR ATIO

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 (

√(V22 + 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,

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 a rising ENC+ equals the ENC– voltage

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)

CONVERTER OPERATION

As shown in Figure 1, the LTC2230/LTC2231 is a CMOS

pipelined multistep converter. The converter has five

pipelined ADC stages; a sampled analog input will result in

a digitized value five cycles later (see the Timing Diagram

section). For optimal AC performance the analog inputs

should be driven differentially. For cost sensitive applica-

tions, the analog inputs can be driven single-ended with

slightly worse harmonic distortion. The encode input is

differential for improved common mode noise immunity.

The LTC2230/LTC2231 has two phases of operation,

determined by the state of the differential ENC+/ENC– input

pins. For brevity, the text will refer to ENC+ greater than

ENC– as ENC high and ENC+ less than ENC– as ENC low.

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