Total Harmonic Distortion Plus Noise
Total harmonic distortion is the evaluation standard for the accuracy of op amps. Usually, the smaller the distortion, the better. Usually a pure sinusoidal signal is sent to the operational amplifier (OPAMP). Since the integrated operational amplifier is actually composed of many transistors or field effect transistors, thermal noise and intrinsic noise are inevitably introduced.
The output terminal will not output a pure sinusoidal signal, except when the signal is at ffIn addition to the fundamental frequency signal at f , at integer multiples ffThere are still some higher-order harmonics at f , and the formula THD+N is expressed as
T H D + N = ∑ H a r m o n i c V o l t a g e s + N o i s e V o l t a g e s T o t a l O u t p u t V o l t a g e × 100 % THD+N=\frac{ {\sum{ {HarmonicVoltages+NoiseVoltages}}}}{ {TotalOutputVoltage}}\times 100\% THD+N=T o t a l O u t p u t V o l t a g e∑HarmonicVoltages+NoiseVoltages×100%
When testing THD, the test conditions must be indicated, that is, load, frequency, gain, amplitude value, etc. Usually the frequency is 1Khz. THD appears in the form of a percentage. As shown in the figure, the fundamental frequency signal occupies 99% of the energy of the entire spectrum, so THD+N is 1%.
Determine the number of bits driven by an OPAMP ADC
When the total harmonic distortion of OPAMP is too large and the number of ADC bits is high, the noise of the integrated operational amplifier will be collected into the signal by the ADC, thus directly affecting the signal-to-noise ratio (SNR). The price of low-noise OPAMP will be very high, so it is very important to choose the appropriate OPAMP while reducing costs. By parameter Signal-to-noise and distortion ratio (SINAD)
SINAD = 20 log Energy of fundamental frequency signal Energy of noise and harmonic signal SINAD=20\log \frac{ { Energy of fundamental frequency signal}}{ {Energy of noise and harmonic signal}}S I N A D=20logNoise and harmonic signal energy _The energy of the fundamental frequency signal
In fact, THD and SINAD have a reciprocal relationship, but one is in percentage and the other is in dB. Assume that OPA365 is taken as an example. The data sheet shows that the input frequency is 1Khz, the load is 600R, the output swing is 4V, and the THD+N when the gain is 1 is 0.0004%. First, calculate SINAD through the formula
S I N A D = 20 log 1 0.000004 = 108 d B SINAD=20\log \frac{1}{ {0.000004}}=108dB S I N A D=20log0.0000041=108dB
Through an empirical formula ENOB (Effective Number Of Bits)
E N O B = ( S I N A D − 1.76 ) 6.02 ENOB=\frac{ {(SINAD-1.76)}}{ {6.02}} E N O B=6.02( S I N A D − 1 . 7 6 )
It can be found that it is about 17.6 bits, so OPA365 can drive a 16-bit ADC.
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