Datasheet LTC1745 (Analog Devices) - 10

LTC1745
U U W U APPLICATIO S I FOR ATIO DYNAMIC PERFORMANCE
If two pure sine waves of frequencies fa and fb are applied to the ADC input, nonlinearities in the ADC transfer func-
Signal-to-Noise Plus Distortion Ratio
tion can create distortion products at the sum and differ- The signal-to-noise plus distortion ratio [S / (N + D)] is the ence frequencies of mfa ± nfb, where m and n = 0, 1, 2, 3, ratio between the RMS amplitude of the fundamental input etc. The 3rd order intermodulation products are 2fa + fb, frequency and the RMS amplitude of all other frequency 2fb + fa, 2fa – fb and 2fb – fa. The intermodulation components at the ADC output. The output is band limited distortion is defined as the ratio of the RMS value of either to frequencies above DC to below half the sampling input tone to the RMS value of the largest 3rd order frequency. intermodulation product.
Signal-to-Noise Ratio Spurious Free Dynamic Range (SFDR)
The signal-to-noise ratio (SNR) is the ratio between the Spurious free dynamic range is the peak harmonic or RMS amplitude of the fundamental input frequency and spurious noise that is the largest spectral component the RMS amplitude of all other frequency components excluding the input signal and DC. This value is expressed except the first five harmonics and DC. in decibels relative to the RMS value of a full scale input signal.
Total Harmonic Distortion Input Bandwidth
Total harmonic distortion is the ratio of the RMS sum of all harmonics of the input signal to the fundamental itself. The The input bandwidth is that input frequency at which the out-of-band harmonics alias into the frequency band amplitude of the reconstructed fundamental is reduced by between DC and half the sampling frequency. THD is 3dB for a full scale input signal. expressed as:
Aperture Delay Time
V22 V32 V42 Vn2 ... The time from when a rising ENC equals the ENC voltage THD = + + + Log 20 V1 to the instant that the input signal is held by the sample and hold circuit. where V1 is the RMS amplitude of the fundamental fre- quency and V2 through Vn are the amplitudes of the
Aperture Delay Jitter
second through nth harmonics. The THD calculated in this The variation in the aperture delay time from conversion to data sheet uses all the harmonics up to the fifth. conversion. This random variation will result in noise
Intermodulation Distortion
when sampling an AC input. The signal to noise ratio due to the jitter alone will be: If the ADC input signal consists of more than one spectral component, the ADC transfer function nonlinearity can SNRJITTER = –20log (2π) • FIN • TJITTER 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. 1745f 10