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LTM9002

9002f

OPERATION

DYNAMIC PERFORMANCE DEFINITIONS

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 fundamen-

tal 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 +KVn2

(

)/V1

where V1 is the RMS amplitude of the fundamental

frequency 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 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. The 3rd order intermodulation products are

2fa + fb, 2fb + fa, 2fa – fb and 2fb – fa. The intermodula-

tion 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 spuri-

ous 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.

Aperture Delay Time

The time from when CLK reaches mid supply to the in-

stant 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

The amount of signal coupled from one channel into the

other. This is measured by applying a full-scale sinusoidal

input on channel A, shorting the inputs of channel B and

taking the ratio of the signal powers in an FFT.

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