What problem do today’s circuits address? The very general task of improving performance, through the application of negative feedback, of a great many of the circuits we have met to this point.

In our own version of this course, only a minority of the busy students choose to do projects. But a project can be heaps of fun. To help you conceive of one, here is some information on gadgets and ideas that might inspire a project builder, along with sketches of some great projects of yester-term.

Here, we’d like you to show you how to do one task many ways. This is a favorite device of exam-writers; this kind of question lets the teachers feel that there’s some coherence in the digital material. Students may not feel the same way about these questions.

In this chapter you will configure the Timer/Counter peripheral to interrupt the CPU at a constant rate to output a sampled sine wave from the DAC. On the way, you will land on the Moon.

Deliberate rolling-off of op-amp gain as frequency rises: used to assure stability of feedback circuits despite dangerously-large phase shifts that occur at high frequencies.

In the previous lab, you configured the SAMD21’s internal 10-bit Digital-to-Analog converter to output an analog voltage to an I/O pin and then used the DAC to synthesize a 128 point sine wave. While this worked, updating the DAC in a loop did not provide precise control over the frequency of the output signal and the process of sending data values to the DAC consumed all the processor CPU cycles. This design also did not ensure that samples were output at constant time intervals.

Prehistory: before the microprocessor: Yes, there was a time when computers roamed the Earth, but were not based on microprocessors. In the 1930s electromechanical computers were built using relays; some were true “Turing machines,” fully programmable.

The notion of multiplexing, or time-sharing, is more general and more important than the piece of hardware called a multiplexer (or “mux”). You won’t often use a mux, but you use multiplexing continually in any computer, and in many data-acquisition schemes.

Until now, as we have said in Chapter 8N, we have treated positive feedback as evil or as a mistake: it’s what you get when you get confused about which op-amp terminal you’re feeding. Today you will qualify this view: you will find that positive feedback can be useful: it can improve the performance of a comparator; it can be combined with negative feedback to make an oscillator (“relaxation oscillator”: positive feedback dominates there); or to make a negative impedance converter (this we will not build, but see AoE §4.107, Fig. 4.104: there, negative feedback dominates).

Adjust frequency so as to get a useful image: too high, and you won’t allow time enough to see the waveform move far; too low, and you’ll see the full waveform, but using just a small portion of the scope screen, and thus your time measurements will be only approximate.

The sort of problem we mean to solve with the most important of today’s circuits is the conversion of a sinusoidal power supply voltage – AC coming from the wall supply (often called “line” voltage) – to a constant DC level.

These are just lines that make lots of stops, picking up and letting off anyone who needs a ride. The origin of the word is the same as the origin of the word for the thing that rolls along city streets.

How many bits does the converter need? We can tolerate slices that are two parts in 10,000 wide, or 1/5k. 12 bits give 4K slices (4096), and give an error of 1/8K or 0.012%: this does not quite satisfy the specification.