AD7863 Analog Devices, AD7863 Datasheet - Page 15

• Manufacturer Part Number: AD7863
• Description: Simultaneous Sampling Dual 175 kSPS 14-Bit A/D Converter
• Manufacturer: Analog Devices

Specifications of AD7863

• Resolution (bits): 14bit
• # Chan: 4
• Sample Rate: 175kSPS
• Interface: Par
• Analog Input Type: SE-Bip,SE-Uni
• Ain Range: Bip 10V,Bip 2.5V,Uni 2.5V
• Adc Architecture: SAR
• Pkg Type: SOIC,SOP

TOTAL HARMONIC DISTORTION (THD)
Total harmonic distortion (THD) is the ratio of the rms sum of
harmonics to the rms value of the fundamental. For the
AD7863, THD is defined as
where:
V
V
the fifth harmonic.
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 creates distortion
products at sum and difference frequencies of mfa ± nfb where
m, n = 0, 1, 2, 3 . . . Intermodulation terms are those for which
neither m nor n is equal to zero. For example, the second order
terms include (fa + fb) and (fa − fb) and the third order terms
include (2fa + fb), (2fa − fb), (fa + 2fb), and (fa − 2fb).
In this case, 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 intermodulation distortion is
as per the THD specification where it is 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 15 shows a typical IMD plot for the AD7863.
1
2
, V
is the rms amplitude of the fundamental.
THD
3
, V
–100
–110
–120
–130
–140
–150
–10
–20
–30
–40
–50
–60
–70
–80
–90
4
, and V
0
( )
0
dB
=
10
20
5
are the rms amplitudes of the second through
log
20
Figure 15. IMD Plot
V
2
30
2
FREQUENCY (kHz)
+
V
3
40
2
V
+
1
V
4
50
2
+
INPUT FREQUENCIES
IMD
V
F1 = 50.13kHz
F2 = 49.13kHz
f
2ND ORDER TERM
3RD ORDER TERM
5
SAMPLE
–93.91dB
60
–98.21dB
2
70
= 175kHz
80
90
Rev. B | Page 15 of 24
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PEAK HARMONIC OR SPURIOUS NOISE
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 f
fundamental. Normally, the value of this specification is
determined by the largest harmonic in the spectrum, but for
parts where the harmonics are buried in the noise floor, the
peak is a noise peak.
DC LINEARITY PLOT
Figure 16 and Figure 17 show typical DNL and INL plots for
the AD7863.
–0.5
–1.0
–0.5
–1.0
1.0
0.5
1.0
0.5
0
0
0
0
2048
2048
S
/2 and excluding dc) to the rms value of the
4096
4096
Figure 16. DC DNL Plot
Figure 17. DC INL Plot
6144
6144
ADC CODE
ADC CODE
8192
8192
10240
10240
12288
12288
14336
14336
AD7863
16383
16383