Datasheet LTC1400 (Analog Devices) - 8

LTC1400
U U W U APPLICATIO S I FOR ATIO
0 –10 fSAMPLE = 400kHz 2 2 2 2 f V2 V3 V4 Vn IN = 199.121kHz –20 SINAD = 72.1dB THD= + + +… 20log –30 THD = –80dB V1 –40 –50 where V1 is the RMS amplitude of the fundamental fre- –60 quency and V2 through Vn are the amplitudes of the second –70 through nth harmonics. THD vs input frequency is shown AMPLITUDE (dB) –80 in Figure 4. The LTC1400 has good distortion performance –90 –100 up to the Nyquist frequency and beyond. –110 –120 0 20 40 60 80 100 120 140 160 180 200 0 FREQUENCY (kHz) f –10 SAMPLE = 400kHz 1400 F02b –20
Figure 2b. LTC1400 Nonaveraged, 4096 Point FFT
–30
Plot with 200kHz Input Frequency in Bipolar Mode
–40 where N is the effective number of bits of resolution and –50 –60 S/(N + D) is expressed in dB. At the maximum sampling –70 3RD HARMONIC rate of 400kHz, the LTC1400 maintains very good ENOBs THD –80 up to the Nyquist input frequency of 200kHz (refer to –90 Figure 3). 2ND HARMONIC AMPLITUDE (dB BELOW THE FUNDAMENTAL) –10010k 100k 1M INPUT FREQUENCY (Hz) 12 74 1400 F04 11 68 SIGNAL/(NOISE + DISTORTION) (dB)
Figure 4. Distortion vs Input Frequency in Bipolar Mode
10 NYQUIST 62 FREQUENCY 9 56 8 50
Intermodulation Distortion
7 6 If the ADC input signal consists of more than one spectral 5 component, the ADC transfer function nonlinearity can 4 produce intermodulation distortion (IMD) in addition to 3 EFFECTIVE NUMBER OF BITS 2 THD. IMD is the change in one sinusoidal input caused 1 f by the presence of another sinusoidal input at a different SAMPLE = 400kHz 0 frequency. 10k 100k 1M INPUT FREQUENCY (Hz) If two pure sine waves of frequencies fa and fb are applied 1400 F03 to the ADC input, nonlinearities in the ADC transfer func-
Figure 3. Effective Bits and Signal-to-Noise + Distortion vs Input Frequency in Bipolar Mode
tion can create distortion products at sum and difference frequencies of mfa ± nfb, where m and n = 0, 1, 2, 3, etc.
Total Harmonic Distortion
For example, the 2nd order IMD terms include (fa + fb) and (fa – fb) while the 3rd order IMD terms includes (2fa Total harmonic distortion (THD) is the ratio of the RMS + fb), (2fa – fb), (fa + 2fb) and (fa – 2fb). If the two input sum of all harmonics of the input signal to the fundamental sine waves are equal in magnitude, the value (in decibels) itself. The out-of-band harmonics alias into the frequency of the 2nd order IMD products can be expressed by the band between DC and half of the sampling frequency. THD following formula. is expressed as: fa Amplitude at ( fb) IMD(fa ± fb) = ± 20log Amplitude at fa 1400fa 8