Low-noise, low residual offset, chopped amplifiers for high-end applications

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6.2. Low pass filtering in a digital audio system. Application specific constraints

Fig.6.1 illustrates the output filters of a bitstream D/A converter used in a digital audio interface [6]. System level aspects related to this will be presented in Chapter 7. The 1 bit input of the shift registers is delivered by a digital noise shaper. A

Fig.6.1: The output filters of a bitstream D/A converter

Fig.6.2: The output spectrum of the FIR filter (LPD)

sampled data low-pass FIR filter (LPD) attenuates the out of band quantization noise to -55dB. The filter has been realized with weighted current sources controlled by switches. The outputs of the shift register decide if a current source is switched to ground or to the input of the continuous-time analog low-pass filter (LPA). The output of the FIR filter has to be again filtered in order to eliminate the unwanted repetitions of the spectra due to the sampling process. The analog filter has to do the reconstruction of the analog signal without increasing the in-band noise and with high linearity. The extra current source I0 biases the output of the opamp to have maximum possible swing at the load and the right common mode level at the input. The load of the opamp is a headphone.

The input signal of the opamp used for low-pass filtering consists of a baseband spectrum and the attenuated aliases centered around multiples of the sampling frequency as illustrated in fig.6.2. The spectra around multiples of sampling frequency fs are caused by the filtering effect of the hold function performed at the output of the FIR filter. The attenuation of the quantization noise Nq in the first band is close to -55dB, attenuation ensured by the FIR filter. In this particular case, 64 times oversampling is used such that the effective sample frequency is 2.8MHz (see Chapter 7). The continuous time low-pass filter attenuates further the out of band noise delivering to the headphone the baseband audio signal and some extra out of band noise sufficiently attenuated. There are strong requirements in terms of linearity, noise and offset for the operational amplifier. For example, 16 bit D/A interface requires a signal to noise ratio of 98dB. The headphone can withstand offsets up to 1-2mV without damages but the acoustic efficiency will decrease.

A chopper stabilized opamp can be an useful solution for the reduction of noise and offset. In this case, chopping at multiples of sampling frequency is an advantage because of sin(x)/x dips in the spectrum (see fig.6.2) and advantageous for clock generation circuits. Fig.6.3 shows the chopped amplifier with a gain stage and an output stage for low-pass filtering. The two chopper modulators are shown explicitly. Chopping can be considered an internal operation inside the feedback loop. The input of the class AB stage provides the filtering needed to recover the baseband spectrum after chopper modulation whereas the RC combination outside amplifier provides the filtering needed to attenuate the unwanted repetitions of the sampling spectra.

Fig.6.3: Chopped amplifer for low-pass filtering

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