1 Introduction
Based on the 0.35 micron N-well process, the full-time division of the power supply is designed. At the same time, in order to reduce the disordering of the residual voltage, T / H (Tracking-Maintenance) demodulation technology is used. When the circuit is operating at 150 kHz in the chopping frequency, the input equivalent noise reaches 31.12 nV / Hz.
In the op amp design of the D audio amplifier, the signal's low harmonic distortion and the noise design format the design. For audio signals in the range of 20 to 20 kHz, the distortion of op amp is mainly caused by voltage disorders and low frequency 1 / F noise. The CMOS process relatively high 1 / f noise and voltage disorders make this problem are particularly serious. When the offset voltage of the circuit is required to be lower than 1mV and the input equivalent noise is less than 100 nV / Hz. Ordinary CMOS is difficult to meet demand. Common static disorders zero technology, such as Trimming, although it can eliminate the impact of voltage disorders, but cannot reduce 1 / F noise. The best way to solve this problem is to use dynamic offset-cancellation techniques such as automatic steady and chopping techniques. Auto Zero Tiechnique is sampled by samples low frequency noise and offset, and then subtracts them from the instantaneous value of the operational amplifier, achieving reduction in offset and noise. Since automatic steady zero technology is used as the principle of capacitive sampling, it is easily folded into the baseband frequency in the circuit operation, and the wider bandwidth of the op amp, the more noise on the sampling capacitor, usually 70 nV / HZ. Chopper Technique is the principle of modulation and demodulation, movement of low frequency noise and offset to the high frequency portion, using low-pass filter filtration, due to no thermal noise, the noise voltage is automatically stable The zero technology is lower. However, the influence of chopper switch charge injection and charge feedthrough effects can still produce a residual voltage disorder of 100UV. Moreover, the use of chopper switches, the thermal noise level of the device will increase.
2 Working principle of chopping
The principle of chopping is shown in Figure 1, wherein Vin is an input audio signal, a chopper switch modulation of a frequency of fch, an amplitude of 1, according to the Nyquist sampling principle, in order to avoid the aliasing of the letter input signal, The FCH must be much greater than 2 times the signal bandwidth.
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Figure 1 Principle of chopping
After modulation, the signal is moved to the wonderful harmonic frequency of the chopping square wave. This signal is enlarged by an operation amplifier that is gain AV. At the same time, the input noise and input offset voltage are also amplified. The output signal is 1, the frequency is 1, the frequency is the chopper switch modulation of the FCH, the output signal is:
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As can be seen from the formula (1), after the second chopping, the input audio signal is demodulated to the low frequency segment, and the voltage offset and low frequency noise are only moved to the chopped square wave after only one modulation. On the frequency harmonics, after low-pass filtering, the high frequency component in the output signal is filtered out, and the low frequency component is reduced to an audio signal, thereby achieving precise amplification of the audio signal.
Fourier analysis is performed on the output signal to obtain the final input noise spectrum density (PSD) of the op amp:
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The factor k is related to the noise parameters of the process. , Reading the full text, the technology area
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