Total harmonic distortion – Rainbow Electronics MAX194 User Manual
Page 23
MAX194
14-Bit, 85ksps ADC with 10µA Shutdown
______________________________________________________________________________________
23
measure of the ADC’s usefulness. Figure 26 shows the
effective number of bits as a function of the MAX194’s
input frequency calculated from the SINAD.
Total Harmonic Distortion
If a pure sine wave is input to an ADC, AC integral non-
linearity (INL) of an ADC’s transfer function results in
harmonics of the input frequency being present in the
sampled output data.
Total Harmonic Distortion (THD) is the ratio of the RMS
sum of all the harmonics (in the frequency band above
DC and below one-half the sample rate, but not includ-
ing the DC component) to the RMS amplitude of the
fundamental frequency. This is expressed as follows:
√
(V
2
2
+ V
3
2
+ V
4
2
+ ... + V
N
2
)
THD = 20log——————————————
V1
where V
1
is the fundamental RMS amplitude, and V
2
through V
N
are the amplitudes of the 2nd through Nth
harmonics. The THD specification in the
Electrical
Characteristics
includes the 2nd through 5th harmon-
ics. In the MAX194, this distortion is caused primarily
by the changes in on-resistance of the AIN sampling
switches with changing input voltage. These resistance
changes, together with the DAC’s capacitance (which
can also vary with input voltage), cause a varying time
delay for AC signals, which causes significant distortion
at moderately high frequencies (Figure 27).
CK
(2 x CLK)
J
Q
+5V
K
CLK
BP/UP/SHDN
CK
2 x CLK
1/2 74HC73
CLOCK SHUTDOWN
Q
(CLK)
J
(CLOCK SHUTDOWN)
MAX194
Figure 24. Circuit to Stop Free-Running Asynchronous CLK
-150
-130
-110
-90
0
5
10
20
25
40
-30
-50
-70
-10
FREQUENCY (kHz)
SIGNAL AMPLITUDE (dB)
15
30
35
f
IN
= 1kHz
f
S
= 85kHz
T
A
= +25°C
Figure 25. MAX194 FFT Plot