Inside a 1980s filter chip that uses switched capacitors

本文为原文前 6,000 字符的节选翻译，完整内容请查看原文。

Inside a 1980s filter chip that uses switched capacitors. Sometimes it’s easier to identify an IC with a microscope. While sorting a box of old ICs, CuriousMarc came across some Harris ICs labeled “F1-10-5”, a mysterious part number that didn’t show up in any databooks. Since unidentifiable ICs are useless, he gave me one to analyze. Conveniently, it was in a ceramic package, so I could open it up with a quick tap from a chisel. Under the microscope, the chip’s most striking feature was a grid of square capacitors. With all those capacitors, I guessed that it was a switched-capacitor filter. The die provided another clue: the part number HF-10. With this information, we quickly found that the chip was Harris’s version of the standard MF10 switched-capacitor filter chip.

深入了解 1980 年代的开关电容滤波器芯片。有时，通过显微镜识别集成电路（IC）会更容易。在整理一盒旧芯片时，CuriousMarc 发现了一些标有“F1-10-5”的 Harris 芯片，这是一个在任何数据手册中都查不到的神秘零件编号。由于无法识别的芯片毫无用处，他给了我一个进行分析。幸运的是，它采用的是陶瓷封装，所以我可以用凿子轻轻一敲就将其打开。在显微镜下，该芯片最显著的特征是一个方形电容阵列。看到这么多电容，我猜它是一个开关电容滤波器。芯片管芯提供了另一个线索：零件编号 HF-10。有了这些信息，我们很快发现该芯片是 Harris 公司生产的标准 MF10 开关电容滤波器芯片的变体。

Switched-capacitor filters were a popular way to implement analog filters in the 1980s. Rapidly switching capacitors in and out of a circuit enabled the construction of single-chip filters that were easy to use and performed well. The MF10, introduced by National Semiconductor in 1981, provides two flexible filters on a chip; each filter acts as a low-pass filter, band-pass filter, or a high-pass filter. The filter’s characteristics are simple to control with a few external resistors.

开关电容滤波器是 20 世纪 80 年代实现模拟滤波器的一种流行方式。通过在电路中快速切换电容的接入与断开，可以构建出易于使用且性能良好的单芯片滤波器。美国国家半导体公司（National Semiconductor）于 1981 年推出的 MF10 在单个芯片上提供了两个灵活的滤波器；每个滤波器都可以作为低通、带通或高通滤波器使用。只需几个外部电阻，即可轻松控制滤波器的特性。

Since I had the chip under the microscope, I took the opportunity to analyze it more closely. The white lines are the metal wiring that connects the chip’s circuitry. Under the metal layer are two layers of polysilicon (reddish) and the underlying silicon (gray). The top and bottom halves of the chip are mostly mirror images, corresponding to the chip’s two filters. The distinctive reddish squares in the middle of the chip are 72 tiny capacitors, constructed from polysilicon. Above the capacitors, CMOS switches turn on and off at the clock frequency, switching capacitors in and out of the circuit. Each filter uses three operational amplifiers (op amps), outlined in red. At the right are the three outputs from the three op amps: high pass, band pass, and low pass.

既然芯片已经在显微镜下，我便借此机会对其进行了更深入的分析。白线是连接芯片电路的金属布线。在金属层之下是两层多晶硅（微红色）和底层的硅（灰色）。芯片的上下两部分基本呈镜像对称，分别对应芯片的两个滤波器。芯片中间独特的微红色方块是 72 个由多晶硅构成的小电容。在电容上方，CMOS 开关以时钟频率开启和关闭，从而将电容切换进出电路。每个滤波器使用三个运算放大器（运放），图中以红色勾勒出。右侧是三个运放的输出端：高通、带通和低通。

Switched capacitor circuits. The filter is based on switched-capacitor circuits. A switched capacitor can replace a resistor in certain circuits, as shown below. The switches are controlled by a clock signal; the switches alternately close in clock phase 1 and phase 2 (ϕ1 and ϕ2). In phase 1, the capacitor is charged to the input voltage. In phase 2, the capacitor passes charge to the output. By rapidly toggling the switches, charge is (almost) steadily passed to the output. The larger the capacitance, the more charge that is passed through. Likewise, a higher frequency passes more charge. It can be shown that the circuit matches a resistor with resistance of 1/(fC): a higher capacitance and frequency correspond to lower resistance.

开关电容电路。该滤波器基于开关电容电路。在某些电路中，开关电容可以替代电阻，如下图所示。开关由时钟信号控制；开关在时钟相位 1 和相位 2（ϕ1 和 ϕ2）交替闭合。在相位 1，电容充电至输入电压。在相位 2，电容将电荷传递到输出端。通过快速切换开关，电荷（几乎）稳定地传递到输出端。电容越大，传递的电荷就越多。同样，频率越高，传递的电荷也越多。可以证明，该电路等效于一个电阻值为 1/(fC) 的电阻：更高的电容和频率对应更低的电阻。

Why would you replace a simple resistor with this complicated switching circuit? In an integrated circuit, resistors are inaccurate and inconveniently large, especially high-value resistors. Replacing a large resistor with a small capacitor saves space on the die. Moreover, it is easy to generate an extremely accurate clock frequency with an inexpensive quartz crystal, making the filter’s frequency highly accurate. Finally, the equivalent resistance can be changed simply by changing the clock frequency, making it easy to tune or sweep the filter.

为什么要用这种复杂的开关电路来代替简单的电阻呢？在集成电路中，电阻不仅精度不高，而且体积庞大，尤其是高阻值电阻。用小电容代替大电阻可以节省芯片管芯上的空间。此外，使用廉价的石英晶体很容易产生极其精确的时钟频率，从而使滤波器的频率非常准确。最后，只需改变时钟频率即可改变等效电阻，这使得调节或扫描滤波器变得非常容易。

On-chip capacitors are fairly inaccurate, with the capacitance typically varying by 20% from chip to chip due to variations in manufacturing conditions. However, this isn’t a problem in the MF10 because the circuitry was designed to depend on the ratio between capacitances, which is stable. Specifically, the MF10 uses 72 identical square capacitors, which will have almost identical capacitances. Careful examination shows that some of the capacitors are separate, while others are connected in groups of 8 to form larger capacitors. This yields a highly accurate ratio of 8:1 between the grouped capacitors and the individual capacitors, even though the absolute capacitance will vary from chip to chip.

片上电容的精度相当低，由于制造条件的变化，不同芯片之间的电容值通常会有 20% 的偏差。然而，这在 MF10 中并不是问题，因为电路的设计依赖于电容之间的比率，而这个比率是稳定的。具体来说，MF10 使用了 72 个相同的方形电容，它们的电容值几乎完全一致。仔细观察可以发现，有些电容是独立的，而另一些则以 8 个为一组连接成更大的电容。这使得分组电容与单个电容之间形成了 8:1 的高精度比率，尽管绝对电容值在不同芯片间会有所波动。