How Do Mixed-Signal Chips Connect Two Worlds?

• Analog circuitry
• Converter linking the two parts
• Digital circuitry

What you will learn

• Say which parts a mixed-signal chip contains.
• Follow a signal through an ADC and a DAC.
• See why two kinds of circuit on one die are hard to design.

Before you start

It helps to read the article on analog and digital chips first. If you have not, you only need to know that an analog signal changes continuously and a digital signal is a set of separate codes.

What does a mixed-signal chip contain?

According to the IEEE page, mixed-signal integrated circuits are semiconductor devices that contain both analog and digital circuitry on a single die. That lets a chip acquire, convert, process and output signals that span both the continuous analog domain and the discrete digital domain.

IEEE gives two examples from the category: data converter chips that translate analog voltages to digital codes and back, and system-on-chip designs that integrate a processor alongside radio frequency and sensor interfaces.

• Data converter chip
• System-on-chip

Why put both kinds of circuit on one chip?

Because the real world is analog and computing is digital. IEEE says temperature, pressure, light and sound are inherently analog, while computation and communication are inherently digital. Any system that must measure a real quantity and then compute with it needs both parts.

The IEEE page on analog-digital circuits describes them as placing the interface between the continuous physical world and discrete digital processing on a single chip or within a single package. Close together, the signal's path stays short and tidy.

How does a signal travel through the chip?

The path has three stages, with converters at both ends. According to IEEE, data conversion is the central function of most mixed-signal circuits. An ADC samples a continuous input signal at regular intervals and quantizes each sample into a binary code. The digital part processes those codes. Then a DAC reconstructs an analog waveform from a sequence of digital codes.

Analog Devices' electronics text describes the same loop: in many cases digitized data is processed and then reconverted back to analog signals to drive external transducers.

• Analog signal in
• ADC converter
• Digital processing
• DAC converter
• Analog signal out

Why is it hard to design both on one die?

Because the two domains want opposite things. IEEE writes that the two domains impose conflicting demands on the manufacturing process, layout geometry, power supply and substrate. Engineers must apply techniques from both disciplines at the same time.

Real-world example

Think of a hypothetical electronic thermometer. A sensor gives an analog voltage; an ADC turns that voltage into a code; the digital part calculates and shows the result. If all three parts sit in one chip, that chip is a mixed-signal chip. This is an illustrative model, not a description of any particular product.

Common misunderstandings

A mixed-signal chip is two chips glued together. According to IEEE, both kinds of circuit sit on a single die.

Only an ADC counts as a mixed-signal circuit. An ADC is a classic example, but IEEE also places system-on-chip designs with a processor and sensor interfaces in the category.

Putting them together is easy. The two domains place conflicting demands on manufacturing and layout, so design is harder.

Frequently asked questions

What is a mixed-signal chip?

A chip that contains both analog and digital circuitry on a single die.

What do the ADC and DAC do in such a chip?

The ADC turns an analog signal into binary codes; the DAC rebuilds an analog signal from a sequence of digital codes.

Is a system-on-chip a mixed-signal chip?

Some are. IEEE lists system-on-chip designs with a processor plus radio frequency and sensor interfaces in the mixed-signal category.

Summary

• A mixed-signal chip has analog and digital circuits on one die.
• The ADC brings the real signal into the digital part; the DAC sends results back out.
• The two domains demand conflicting things in manufacturing and layout, which makes design hard.