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LTC2273/LTC2272

22732f

DEFINITIONS

DYNAMIC PERFORMANCE TERMS

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 lim-

ited to frequencies above DC to below half the sampling

frequency.

Signal-to-Noise Ratio

The signal-to-noise (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.

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 (

√(V

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.

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

(fa + 2fb), (2fa - fb) and (fa - 2fb). The 3rd order IMD is

deﬁned as the ratio of the RMS value of either input tone

to the RMS value of the largest 3rd order IMD product.

Spurious Free Dynamic Range (SFDR)

The ratio of the RMS input signal amplitude to the RMS

value of the peak spurious spectral component expressed

in dBc. SFDR may also be calculated relative to full scale

and expressed in dBFS.

Full Power Bandwidth

The full power 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)

SERIAL INTERFACE TERMS

8B/10B Encoding

A data encoding method designed to make an 8-bit data

word (octet) more suitable for serial transmission. The

resulting 10-bit word (code-group) has two fundamental

strengths: 1) The receiver does not require a high-speed

clock to capture the data. This is because the output

code-groups are run-length limited, ensuring that there

are enough transitions in the bit stream for the receiver

to lock onto the data and recover the high-speed clock.

2) AC coupling is permitted because the code-groups are

generated in a way that ensures the data stream is DC

balanced (see Running Disparity).

A table of the 256 possible input octets with the resulting

10-bit code-groups is documented in IEEE Std 802.3-2002

part3 Table 36-1. The name associated with each of the

256 data code-groups is formatted Dx.y, with x ranging

from 0 to 31 and y ranging from 0 to 7. Table 36-2 of

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