ad7883br Analog Devices, Inc., ad7883br Datasheet - Page 8

• Manufacturer Part Number: ad7883br
• Description: Lc2mos 12-bit, 3.3 V Sampling Adc
• Manufacturer: Analog Devices, Inc.

AD7883
Total Harmonic Distortion (THD)
THD is the ratio of the rms sum of harmonics to the rms value
of the fundamental. For the AD7883, THD is defined as:
where V
V
sixth harmonic. The THD is also derived from the FFT plot of
the ADC output spectrum.
Intermodulation Distortion
With inputs consisting of sine waves at two frequencies, fa and
fb, any active device with nonlinearities will create distortion
products at sum and difference frequencies of mfa
m, n = 0, 1, 2, 3, etc. Intermodulation terms are those for which
neither m nor n are equal to zero. For example, the second or-
der terms include (fa + fb) and (fa – fb), while the third order
terms include (2fa + fb), (2fa – fb), (fa + 2fb) and (fa – 2fb).
4
, V
Figure 11. Effective Number of Bits vs. Frequency
5
and V
11.5
10.5
THD 20 log
1
12
10
11
is the rms amplitude of the fundamental and V
6
are the rms amplitudes of the second through the
5
V
INPUT FREQUENCY – kHz
2
2
10
V
3
2
SAMPLE FREQUENCY = 50kHz
T A = +25 C
V
V
1
4
2
15
V
5
2
V
6
2
20
nfb where
2
, V
25
3
(3)
,
–8–
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, 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 in-
termodulation distortion is as per the THD specification where
it is the ratio of the rms sum of the individual distortion prod-
ucts to the rms amplitude of the fundamental expressed in dBs.
In this case, the input consists of two, equal amplitude, low dis-
tortion, sine waves. Figure 12 shows a typical IMD plot for the
AD7883.
Peak Harmonic or Spurious Noise
Peak harmonic or spurious noise is defined as the ratio of the
rms value of the next largest component in the ADC output
spectrum (up to FS/2 and excluding dc) to the rms value of the
fundamental. Normally, the value of this specification will be
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.
–120
–30
–60
–90
0
0
2.5
Figure 12. IMD Plot
INPUT FREQUENCY
SAMPLE FREQUENCY = 50kHz
TA = +25
IMD
F1 = 0.983kHz
F2 = 1.05kHz
ALL TERMS = 81.5dB
2ND ORDER TERMS = 83.6dB
3RD ORDER TERMS = 85.4dB
FREQUENCY – kHz
°
C
25
REV. 0