### 摘要

In this application note a circuit with two switched-capacitor filters reconstructs the output of a digital-to-analog converter (DAC) while providing anti-aliasing and sinc-compensation functions. Filter IC prevents alias frequencies by excluding spectral energy above fs/2. The MAX265 filter is featured.

The dual-biquad filter chips and some external components (Figure 1) form a multipurpose filter for the reconstruction of D/A converter signals. Connected to a converter's output (Figure 2), the filter aids in generating the analog signal represented by digital-data samples at the converter's input. In addition, the filter provides anti-aliasing, (sinπx)/πx (sinc) compensation, and reduction of the D/A converter's quantization noise.

At, DC, a D/A converter's output is easily predicted from it's data sheet specs. Time-varying signals, however, produce staircase-output waveforms whose reconstruction errors are best discussed in the frequency domain. The converter's output spectrum, for example, consists of spectra (±f_{1}, where f_{1} is the spectrum represented by the digital input samples) that repeat at integral multiples of the sample rate f_{S} (Figure 3).

The filter's first job is to prevent alias frequencies by excluding spectral energy above f_{S}/2. In practice, f_{1} < f_{S}/2. The filter should pass f_{1} with an acceptably low error while sufficiently attenuating all frequencies above f_{S}/2.

A second filter requirement stems from the presence of sinc attenuation, introduced by the effect of rectangular-pulse components in the staircase waveform (Figure 4). These pulses have the same 1/f_{S} width, but differ in amplitude according to the digital-sample magnitudes. The spectrum of each pulse is the Fourier transform (the sinc function of f/f_{S}). These spectra combine with the f_{1} spectrum to form an overall frequency response for the converter output. Note the sinc expression's variation in amplitude for various values of f:

f | [(sin)(πf/f_{S}]/(πf/f_{S}) |

0 | 1.0 |

f_{S}/4 |
0.9003 (-0.9dB) |

f_{S}/3 |
0.8270 (-1.65dB) |

f_{S}/2 |
0.6366 (-3.92dB) |

Clearly, the staircase approximation causes an increase amplitude error as f approaches the Nyquist frequency f_{S}/2. To compensate for this attenuation, the Figure 1 circuit incorporates the inverse expression (πf/f_{S})/sin(πf/f_{S}) in it's passband-magnitude response.

Ideally, the resulting filter response would provide sinc compensation to f_{S}/2, drop abruptly to zero, and maintain that infinite attenuation for all frequencies above f_{S}/2. But actual filters cannot provide abrupt transitions or infinite attenuation. As a practical compromise, the circuit makes its transition over a finite bandwidth (transition ratio), and then provides an out-of-band rejection comparable to the D/A converter's signal-to-noise ratio SNR.

SNR for an ideal D/A converter is about 6dB/bit, or 72dB for a 12-bit device. Quantization error further degrades this number, yielding about 68dB for a typical 12-bit converter. Thus a reasonable goal in Figure 1 is 70dB rejection above f_{S}/2.

To prevent aliasing, the stopband edge must be no greater than the Nyquist frequency (f_{S}/2). The passband edge must threrfore be less than f_{S}/2. To achieve 70dB stopband rejection in the 8th-order circuit of Figure 1, the required transition ratio (f_{Stopband}/f_{Passband)}) is 1.5, which sets the passband edge at f_{S}/3. A rising amplitude response within this passband compensates for the converter's sinc attenuation.

Perfect sinc compensation would provide 1.65dB of gain at the Nyquist frequency, but tolerance uncertainties in the ±1% resistors and within the filter ICs limits the actual correction to about 1dB. The circuit does, however, achieve the 70dB stopband rejection and the 1.5 transition ratio. Figure 5 compares the Figure 1 response with that of an ideal filter.

To assure maximum dynamic range, the four biquad-filter sections (two in each IC) exhibit increasing Q from input to output, The pole-zero pairs of each section also axhibit increasing frequency, which minimizes the spread in component values. The following pole and zero values produce a 1-rad/sec filter passband:

Section | f_{pole} (Hz) |
Q_{pole} |
f_{Zero}(Hz) |

1 | 0.1005 | 0.5603 | 0.2397 |

2 | 0.1310 | 1.0540 | 0.2777 |

3 | 0.1564 | 2.3876 | 0.4273 |

4 | 0.1685 | 8.5145 | 1.4016 |

Note the feedback capacitors C1-C4 across each output op amp. These capacitors have two purposes; they improve the quality of transmission zeroes, and the form 1-pole lowpass filters that help to smooth out the discrete-level steps introduced by the filter's switched-capacitor action. The 1-pole filters have little effect on the passband shape because their high corner frequencies introduce only 0.1dB of loss at 1kHz.

Note also, that the applied clock frequency in Figure 2 (192kHz) allows use of a convenient binary-64 divider for setting the necessary 3X ratio betweeen the converter's sample rate and the filter's 1kHz corner frequency, f_{0}. Each chip is programmed for an f_{CLK}/f_{0} ratio of 191.64 by V+ and V- connections to the filter inputs, F0-F5.