Datasheet LTC1278 (Analog Devices) - 10
| Fabricante | Analog Devices |
| Descripción | 12-Bit, 500ksps Sampling A/D Converter with Shutdown |
| Páginas / Página | 16 / 10 — APPLICATIONS INFORMATION. Figure 5. Intermodulation Distortion Plot. Peak … |
| Formato / tamaño de archivo | PDF / 290 Kb |
| Idioma del documento | Inglés |
APPLICATIONS INFORMATION. Figure 5. Intermodulation Distortion Plot. Peak Harmonic or Spurious Noise
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LTC1278
U U W U APPLICATIONS INFORMATION
frequency is shown in Figure 4. The LTC1278 has good 0 distortion performance up to the Nyquist frequency and fSAMPLE = 500kHz fIN1 = 96.80kHz beyond. –20 fIN2 = 101.68kHz –40 0 fSAMPLE = 500kHz –10 –60 –20 –30 AMPLITUDE (dB) –80 –40 –100 –50 –60 –120 2ND HARMONIC –70 0 50k 100k 150k 200k 250k FREQUENCY (Hz) –80 THD LTC1278 G8 3RD HARMONIC –90 AMPLITUDE (dB BELOW THE FUNDAMENTAL)
Figure 5. Intermodulation Distortion Plot
–100 10k 100k 1M 2M INPUT FREQUENCY (Hz) LT1278 G6
Peak Harmonic or Spurious Noise Figure 4. Distortion vs Input Frequency
The peak harmonic or spurious noise is the largest spec- tral component excluding the input signal and DC. This value is expressed in decibels relative to the RMS value of
Intermodulation Distortion
a full-scale input signal. If the ADC input signal consists of more than one spectral component, the ADC transfer function nonlinearity can
Full Power and Full Linear Bandwidth
produce intermodulation distortion (IMD) in addition to The full power bandwidth is that input frequency at which THD. IMD is the change in one sinusoidal input caused by the amplitude of the reconstructed fundamental is re- the presence of another sinusoidal input at a different duced by 3dB for a full-scale input signal. frequency. The full linear bandwidth is the input frequency at which If two pure sine waves of frequencies fa and fb are applied the S/(N + D) has dropped to 68dB (11 effective bits). The to the ADC input, nonlinearities in the ADC transfer func- LTC1278 has been designed to optimize input bandwidth, tion can create distortion products at sum and difference allowing ADC to undersample input signals with frequen- frequencies of mfa ± nfb, where m and n = 0, 1, 2, 3, etc. cies above the converter’s Nyquist Frequency. The noise For example, the 2nd order IMD terms include (fa + fb) and floor stays very low at high frequencies; S/(N + D) be- (fa – fb) while the 3rd order IMD terms include (2fa + fb), comes dominated by distortion at frequencies far beyond (2fa – fb), (fa + 2fb), and (fa – 2fb). If the two input sine Nyquist. waves are equal in magnitude, the value (in decibels) of the 2nd order IMD products can be expressed by the
Driving the Analog Input
following formula: The analog input of the LTC1278 is easy to drive. It draws only one small current spike while charging the sample- Amplitude at (fa ± fb) IMD (fa ± fb) = 20log and-hold capacitor at the end of conversion. During con- Amplitude at fa version the analog input draws no current. The only Figure 5 shows the IMD performance at a 100kHz input. requirement is that the amplifier driving the analog input must settle after the small current spike before the next 10
U U W U APPLICATIONS INFORMATION
frequency is shown in Figure 4. The LTC1278 has good 0 distortion performance up to the Nyquist frequency and fSAMPLE = 500kHz fIN1 = 96.80kHz beyond. –20 fIN2 = 101.68kHz –40 0 fSAMPLE = 500kHz –10 –60 –20 –30 AMPLITUDE (dB) –80 –40 –100 –50 –60 –120 2ND HARMONIC –70 0 50k 100k 150k 200k 250k FREQUENCY (Hz) –80 THD LTC1278 G8 3RD HARMONIC –90 AMPLITUDE (dB BELOW THE FUNDAMENTAL)
Figure 5. Intermodulation Distortion Plot
–100 10k 100k 1M 2M INPUT FREQUENCY (Hz) LT1278 G6
Peak Harmonic or Spurious Noise Figure 4. Distortion vs Input Frequency
The peak harmonic or spurious noise is the largest spec- tral component excluding the input signal and DC. This value is expressed in decibels relative to the RMS value of
Intermodulation Distortion
a full-scale input signal. If the ADC input signal consists of more than one spectral component, the ADC transfer function nonlinearity can
Full Power and Full Linear Bandwidth
produce intermodulation distortion (IMD) in addition to The full power bandwidth is that input frequency at which THD. IMD is the change in one sinusoidal input caused by the amplitude of the reconstructed fundamental is re- the presence of another sinusoidal input at a different duced by 3dB for a full-scale input signal. frequency. The full linear bandwidth is the input frequency at which If two pure sine waves of frequencies fa and fb are applied the S/(N + D) has dropped to 68dB (11 effective bits). The to the ADC input, nonlinearities in the ADC transfer func- LTC1278 has been designed to optimize input bandwidth, tion can create distortion products at sum and difference allowing ADC to undersample input signals with frequen- frequencies of mfa ± nfb, where m and n = 0, 1, 2, 3, etc. cies above the converter’s Nyquist Frequency. The noise For example, the 2nd order IMD terms include (fa + fb) and floor stays very low at high frequencies; S/(N + D) be- (fa – fb) while the 3rd order IMD terms include (2fa + fb), comes dominated by distortion at frequencies far beyond (2fa – fb), (fa + 2fb), and (fa – 2fb). If the two input sine Nyquist. waves are equal in magnitude, the value (in decibels) of the 2nd order IMD products can be expressed by the
Driving the Analog Input
following formula: The analog input of the LTC1278 is easy to drive. It draws only one small current spike while charging the sample- Amplitude at (fa ± fb) IMD (fa ± fb) = 20log and-hold capacitor at the end of conversion. During con- Amplitude at fa version the analog input draws no current. The only Figure 5 shows the IMD performance at a 100kHz input. requirement is that the amplifier driving the analog input must settle after the small current spike before the next 10