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LTC2201

2201f

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 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

(√(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.

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 ap-

plied 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 CLK reaches 0.45 of VDD 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 (2

π • fIN • tJITTER)

APPLICATIONS INFORMATION

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