Datasheet LTC1401 (Analog Devices) - 8

LTC1401
U U W U APPLICATIONS INFORMATION
0
Total Harmonic Distortion
f –10 SAMPLE = 200kHz fIN = 99.072266kHz –20 SINAD = 65dB Total harmonic distortion (THD) is the ratio of the RMS –30 THD = –66dB V sum of all harmonics of the input signal to the fundamental –40 CC = 3V TA = 25°C –50 itself. The out-of-band harmonics alias into the frequency – 60 band between DC and half of the sampling frequency. THD –70 is expressed as: AMPLITUDE (dB) –80 –90 –100 √V22 + V32 + ...Vn2 THD = 20log –110 V1 –120 0 10 20 30 40 50 60 70 80 90 100 Where V1 is the RMS amplitude of the fundamental FREQUENCY (kHz) LTC1400 • F02b frequency and V2 through Vn are the amplitudes of the second through nth harmonics. THD vs input frequency is
Figure 2b. LTC1401 Nonaveraged, 4096 Point FFT
shown in Figure 4. The LTC1401 has good distortion
Plot with 100kHz Input Frequency
performance up to the Nyquist frequency and beyond.
Effective Number of Bits
0 TA = 25°C –10 The effective number of bits (ENOBs) is a measurement of fSAMPLE = 200kHz –20 the effective resolution of an ADC and is directly related to –30 the S/(N + D) by the equation: –40 S (/N + D) – 1.76 –50 N = 2ND HARMONIC THD –60 . 6 02 –70 where N is the effective number of bits of resolution and 3RD HARMONIC –80 S/(N + D) is expressed in dB. Figure 3 shows ENOBs vs –90 AMPLITUDE (dB BELOW THE FUNDAMENTAL) Input Frequency. –100 10k 100k 1M INPUT FREQUENCY (Hz) 12 74 LTC1401 • F04 11 68 SIGNAL/(NOISE + DISTORTION) (dB) 10 62
Figure 4. Distortion vs Input Frequency
9 56 8 50 7
Intermodulation Distortion
6 5 If the ADC input signal consists of more than one spectral 4 component, the ADC transfer function nonlinearity can 3 EFFECTIVE NUMBER OF BITS produce intermodulation distortion (IMD) in addition to 2 TA = 25°C THD. IMD is the change in one sinusoidal input caused by 1 fSAMPLE = 200kHz 0 the presence of another sinusoidal input at a different 10k 100k 1M frequency. INPUT FREQUENCY (Hz) LTC1401 • F03 If two pure sine waves of frequencies fa and fb are applied
Figure 3. Effective Bits and Signal-to-Noise +
to the ADC input, nonlinearities in the ADC transfer
Distortion vs Input Frequency
function can create distortion products at sum and differ- 1401fa 8