Datasheet LTC2246H (Analog Devices) - 10

LTC2246H
APPLICATIONS INFORMATION DYNAMIC PERFORMANCE
is defi ned as the ratio of the RMS value of either input tone to the RMS value of the largest 3rd order intermodulation
Signal-to-Noise Plus Distortion Ratio
product. The signal-to-noise plus distortion ratio [S/(N + D)] is
Spurious Free Dynamic Range (SFDR)
the ratio between the RMS amplitude of the fundamen- tal input frequency and the RMS amplitude of all other Spurious free dynamic range is the peak harmonic or spuri- frequency components at the ADC output. The output is ous noise that is the largest spectral component excluding band limited to frequencies above DC to below half the the input signal and DC. This value is expressed in decibels sampling frequency. relative to the RMS value of a full scale input signal.
Signal-to-Noise Ratio Input Bandwidth
The signal-to-noise ratio (SNR) is the ratio between the The input bandwidth is that input frequency at which the RMS amplitude of the fundamental input frequency and amplitude of the reconstructed fundamental is reduced the RMS amplitude of all other frequency components by 3dB for a full scale input signal. except the fi rst fi ve harmonics and DC.
Aperture Delay Time Total Harmonic Distortion
The time from when CLK reaches mid-supply to the Total harmonic distortion is the ratio of the RMS sum instant that the input signal is held by the sample and of all harmonics of the input signal to the fundamental hold circuit. itself. The out-of-band harmonics alias into the frequency band between DC and half the sampling frequency. THD
Aperture Delay Jitter
is expressed as: The variation in the aperture delay time from conversion to conversion. This random variation will result in noise THD = 20Log V22 + V32 + V42 + ...Vn2 ( )/V1 ( ) when sampling an AC input. The signal to noise ratio due to the jitter alone will be: where V1 is the RMS amplitude of the fundamental fre- quency and V2 through Vn are the amplitudes of the second SNRJITTER = –20log (2π • fIN • tJITTER) through nth harmonics. The THD calculated in this data sheet uses all the harmonics up to the fi fth.
CONVERTER OPERATION Intermodulation Distortion
As shown in Figure 1, the LTC2246H is a CMOS pipelined multistep converter. The converter has six pipelined ADC If the ADC input signal consists of more than one spectral stages; a sampled analog input will result in a digitized component, the ADC transfer function nonlinearity can value fi ve cycles later (see the Timing Diagram section). produce intermodulation distortion (IMD) in addition to For optimal AC performance the analog inputs should be THD. IMD is the change in one sinusoidal input caused driven differentially. For cost sensitive applications, the by the presence of another sinusoidal input at a different analog inputs can be driven single-ended with slightly frequency. worse harmonic distortion. The CLK input is single-ended. If two pure sine waves of frequencies fa and fb are applied The LTC2246H has two phases of operation, determined to the ADC input, nonlinearities in the ADC transfer function by the state of the CLK input pin. can create distortion products at the sum and difference Each pipelined stage shown in Figure 1 contains an ADC, frequencies of mfa ± nfb, where m and n = 0, 1, 2, 3, etc. a reconstruction DAC and an interstage residue amplifi er. The 3rd order intermodulation products are 2fa + fb, 2fb In operation, the ADC quantizes the input to the stage and + fa, 2fa – fb and 2fb – fa. The intermodulation distortion the quantized value is subtracted from the input by the 2246hfb 10