Datasheet LTC1409 (Analog Devices) - 9

LTC1409
U U W U APPLICATIONS INFORMATION Signal-to-Noise Ratio
V22 V32 V42 Vn2 THD = + + +… 20 Log The signal-to-noise plus distortion ratio [S/(N + D)] is the V1 ratio between the RMS amplitude of the fundamental input where V1 is the RMS amplitude of the fundamental fre- frequency to the RMS amplitude of all other frequency quency and V2 through Vn are the amplitudes of the components at the A/D output. The output is band limited second through Nth harmonics. THD vs input frequency is to frequencies from above DC and below half the sampling shown in Figure 4. The LTC1409 has good distortion frequency. Figure 2 shows a typical spectral content with performance up to the Nyquist frequency and beyond. an 800kHz sampling rate and a 100kHz input. The dynamic 0 performance is excellent for input frequencies up to and –10 beyond the Nyquist limit of 400kHz. –20
Effective Number of Bits
–30 –40 The Effective Number of Bits (ENOBs) is a measurement of –50 the resolution of an ADC and is directly related to the –60 S/(N + D) by the equation: –70 –80 3RD N = [S/(N + D) – 1.76]/6.02 THD –90 2ND AMPLITUDE (dB BELOW THE FUNDAMENTAL) where N is the effective number of bits of resolution and –100 1k 10k 100k 1M 10M S/(N + D) is expressed in dB. At the maximum sampling INPUT FREQUENCY (Hz) rate of 800kHz the LTC1409 maintains near ideal ENOBs LTC1409 • F04 up to the Nyquist input frequency of 400kHz. Refer to
Figure 4. Distortion vs Input Frequency
Figure 3.
Intermodulation Distortion
12 If the ADC input signal consists of more than one spectral 11 10 component, the ADC transfer function nonlinearity can 9 produce intermodulation distortion (IMD) in addition to 8 THD. IMD is the change in one sinusoidal input caused by 7 the presence of another sinusoidal input at a different 6 5 frequency. EFFECTIVE BITS 4 If two pure sine waves of frequencies fa and fb are applied 3 2 to the ADC input, nonlinearities in the DC transfer function 1 fSAMPLE = 800kHz can create distortion products at the sum and difference 0 1k 10k 100k 1M 10M frequencies of mfa + –nfb, where m and n = 0, 1, 2, 3, etc. INPUT FREQUENCY (Hz) For example, the 2nd order IMD terms include (fa + fb). If LTC1409 • F03 the two input sine waves are equal in magnitude, the value
Figure 3. Effective Bits and Signal/(Noise +
(in decibels) of the 2nd order IMD products can be
Distortion) vs Input Frequency
expressed by the following formula:
Total Harmonic Distortion
Amplitude at (fa + fb) IMD(fa + fb) = 20 Log Total Harmonic Distortion (THD) is the ratio of the RMS sum Amplitude at fa of all harmonics of the input signal to the fundamental itself.
Peak Harmonic or Spurious Noise
The out-of-band harmonics alias into the frequency band between DC and half the sampling frequency. THD is The peak harmonic or spurious noise is the largest spec- expressed as: tral component excluding the input signal and DC. This 9