PLLs and Analog Blocks on Silicon

Most digital chips contain analog blocks: PLLs that make the clocks, references and regulators that set voltages, sensors, and data converters. Their behaviour cannot be fully predicted before silicon, and many need calibration on every part. This article describes what is measured on PLLs and analog blocks, how converters are characterized, and how trimming with fuses adjusts each part to its target.

6 min read | Intermediate

What You'll Learn

• List the key measurements of a PLL on silicon
• Describe the main specifications of a data converter
• Explain trimming and why many analog blocks need it
• Use on-chip analog test access

PLLs

A phase-locked loop multiplies a reference clock up to the chip's operating frequencies. Validation measures:

• Lock: whether it locks at every required frequency, and how long it takes.
• Frequency range: the lowest and highest frequencies it can reach, across voltage, temperature and process.
• Jitter: how much the output edges wander, which eats directly into every timing path's margin.
• Spurs and supply sensitivity: unwanted tones in the output and how much supply noise reaches the clock.

Many chips bring a divided clock out to a pin or measure it with an on-chip counter, so these can be measured without probing the die.

Data Converters

Analog-to-digital and digital-to-analog converters are characterized with static and dynamic measurements:

• Differential nonlinearity (DNL): how much each code step differs from the ideal step width. A DNL of -1 means a missing code.
• Integral nonlinearity (INL): how far the transfer curve bends away from a straight line.
• Signal-to-noise ratio and effective bits: measured with a pure sine wave input. The effective number of bits is (SNR - 1.76) / 6.02, with SNR in decibels.

Converters are sensitive to supply noise and substrate noise from the digital logic around them, so they are measured both alone and with the digital part of the chip busy, as the Physical Design article Analog and Mixed-Signal Blocks in a Digital Chip explains.

Trimming

Many analog values vary too much from part to part to be fixed in design: a bandgap reference voltage, an oscillator frequency, a temperature sensor's offset, a regulator's output. These blocks are made adjustable, and each part is trimmed.

1. Measure the value on the part.
2. Compute the trim code that brings it closest to the target.
3. Program the code into fuses or non-volatile memory.
4. Measure again to verify.

Validation decides whether the trim range and step size are enough: does every part, across the process, land within specification after trimming? Production test then trims every part.

Analog Test Access

Analog nodes inside the chip are hard to measure. Many chips include an analog test bus: switches that route a chosen internal node, such as a reference voltage or a regulator output, to a pin where an instrument can measure it. Some I/O libraries provide analog buses through the pad ring for the same purpose, and digital readouts of on-chip sensors let software see analog values without any instrument at all.

In Practice

Analog validation is usually done by the analog designers themselves, with specialized instruments and a lot of statistics across parts and temperatures. Its results feed trim programs, datasheet limits and sometimes changes to the digital side: for example, quieting nearby logic during a converter's sampling window.

Common Mistakes