Datasheet AD7880 (Analog Devices) - 9
| Manufacturer | Analog Devices |
| Description | CMOS, Single +5 V Supply, Low Power, 12-Bit Sampling ADC |
| Pages / Page | 17 / 9 — AD7880. 11.5. EFFECTIVE NUMBER OF BITS 10.5. SAMPLE FREQUENCY = 61kHz. T … |
| File Format / Size | PDF / 375 Kb |
| Document Language | English |
AD7880. 11.5. EFFECTIVE NUMBER OF BITS 10.5. SAMPLE FREQUENCY = 61kHz. T = 25 C. 30.5. INPUT FREQUENCY – kHz
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AD7880 12
Using the CCIF standard where two input frequencies near the top end of the input bandwidth are used, the second and third order terms are of different significance. The second order terms are usually distanced in frequency from the original sine waves,
11.5
while the third order terms are usually at a frequency close to the input frequencies. As a result, the second and third order terms are specified separately. The calculation of the inter- modulation distortion is as per the THD specification where it is
11
the ratio of the rms sum of the individual distortion products to the rms amplitude of the fundamental expressed in dBs. In this case, the input consists of two, equal amplitude, low distortion, sine waves. Figure 14 shows a typical IMD plot for the
EFFECTIVE NUMBER OF BITS 10.5 SAMPLE FREQUENCY = 61kHz
AD7880.
T = 25 C A 10 15 30.5 INPUT FREQUENCY – kHz
Figure 13. Effective Number of Bits vs. Frequency
Total Harmonic Distortion (THD)
THD is the ratio of the rms sum of harmonics to the rms value of the fundamental. For the AD7880, THD is defined as: 2 2 2 2 2 V +V +V +V +V THD = 20 log 2 3 4 5 6 (3) V1 where V1 is the rms amplitude of the fundamental and V2, V3, V4, V5 and V6 are the rms amplitudes of the second through the sixth harmonic. The THD is also derived from the FFT plot of the ADC output spectrum.
Intermodulation Distortion
Figure 14. IMD Plot With inputs consisting of sine waves at two frequencies, fa and fb, any active device with nonlinearities will create distortion
Peak Harmonic or Spurious Noise
products at sum and difference frequencies of mfa ± nfb where Peak harmonic or spurious noise is defined as the ratio of the m, n = 0, 1, 2, 3, etc. Intermodulation terms are those for which rms value of the next largest component in the ADC output neither m nor n are equal to zero. For example, the second or- spectrum (up to FS/2 and excluding dc) to the rms value of the der terms include (fa + fb) and (fa – fb), while the third order fundamental. Normally, the value of this specification will be terms include (2fa + fb), (2fa – fb), (fa + 2fb) and (fa – 2fb). determined by the largest harmonic in the spectrum, but for parts where the harmonics are buried in the noise floor the peak will be a noise peak. –8– REV. 0
Using the CCIF standard where two input frequencies near the top end of the input bandwidth are used, the second and third order terms are of different significance. The second order terms are usually distanced in frequency from the original sine waves,
11.5
while the third order terms are usually at a frequency close to the input frequencies. As a result, the second and third order terms are specified separately. The calculation of the inter- modulation distortion is as per the THD specification where it is
11
the ratio of the rms sum of the individual distortion products to the rms amplitude of the fundamental expressed in dBs. In this case, the input consists of two, equal amplitude, low distortion, sine waves. Figure 14 shows a typical IMD plot for the
EFFECTIVE NUMBER OF BITS 10.5 SAMPLE FREQUENCY = 61kHz
AD7880.
T = 25 C A 10 15 30.5 INPUT FREQUENCY – kHz
Figure 13. Effective Number of Bits vs. Frequency
Total Harmonic Distortion (THD)
THD is the ratio of the rms sum of harmonics to the rms value of the fundamental. For the AD7880, THD is defined as: 2 2 2 2 2 V +V +V +V +V THD = 20 log 2 3 4 5 6 (3) V1 where V1 is the rms amplitude of the fundamental and V2, V3, V4, V5 and V6 are the rms amplitudes of the second through the sixth harmonic. The THD is also derived from the FFT plot of the ADC output spectrum.
Intermodulation Distortion
Figure 14. IMD Plot With inputs consisting of sine waves at two frequencies, fa and fb, any active device with nonlinearities will create distortion
Peak Harmonic or Spurious Noise
products at sum and difference frequencies of mfa ± nfb where Peak harmonic or spurious noise is defined as the ratio of the m, n = 0, 1, 2, 3, etc. Intermodulation terms are those for which rms value of the next largest component in the ADC output neither m nor n are equal to zero. For example, the second or- spectrum (up to FS/2 and excluding dc) to the rms value of the der terms include (fa + fb) and (fa – fb), while the third order fundamental. Normally, the value of this specification will be terms include (2fa + fb), (2fa – fb), (fa + 2fb) and (fa – 2fb). determined by the largest harmonic in the spectrum, but for parts where the harmonics are buried in the noise floor the peak will be a noise peak. –8– REV. 0
