Harmonizing Circuit Design and EMC DesignPart 6 Product Specifications (3) Examples of General EMC Evaluation Indexes

Hello! I’m Inagaki, at ROHM.

This sixth article is my third discussion of product specifications. In the previous two columns, I think the reader was able to acquire a broad understanding of product specifications. Some of frequently asked questions related to product specifications is what rate and how often semiconductor integrated circuits (LSIs and ICs) are tested for the specified values described in the product specifications. For example, I get the following kinds of questions.

 Q：Are outgoing tests performed on one in 1000 products? Or one in 100?
 Q：Are tests performed for all specification items? Or only for the main items?
 Q：Are tests performed only when a product lot is new?

My answer:

 A：We perform tests of all products, for all specification items.

All products are tested on shipment, one at a time, using an automated test equipment. The number of specification items to be tested varies depending on the type of product, ranging from hundreds to thousands of specification items, but tests are performed for all items. These also include high-temperature tests.

To go into somewhat more detail, 100% tests of all specification items are performed in the state in which the semiconductor integrated circuits (LSIs, ICs) are on the original silicon wafer (the LSIs or ICs are fabricated on the disc-shaped silicon substrate). Then, the wafer is scribed to separate the devices, which are encapsulated in packages (resin encapsulation or the like), and 100% tests of all specification items are again performed. In this way, only those devices that conform to multiple tests are shipped as products. In product quality assurance, there are various methods and approaches, but at ROHM, the basic stance is 100% tests of all products, covering all specification items.

Well, let’s move on to discuss electromagnetic compatibility (EMC). The specification items for which 100% tests are performed include evaluation indexes relating to EMC. Among those one sees frequently are PSRR (Power Supply Rejection Ratio), CMRR (Common Mode Rejection Ratio), and EMIRR (EMI Rejection Ratio).

The PSRR is the voltage fluctuation rejection ratio for a power supply, that is, a ratio that indicates the extent of fluctuation occurring in the output when the power supply voltage is changed. The CMRR, or common mode rejection ratio, is the ratio of the fluctuation occurring in the output when a differential input is changed by a common potential.

Both the PSRR and the CMRR are DC characteristics defined for differential operational amplifiers (op-amps) and the like, and because they have frequency characteristics, they are measured over frequency ranges from 10 Hz to about 1 MHz. At low frequencies such as 10 Hz or so, many devices exhibit impressive performance of 100 dB and higher, and observed fluctuations are roughly 1/100,000 or lower (the electrical characteristic is a declining curve, with better characteristic values at lower frequencies).

Finally, the EMIRR, also called EMI immunity, is the electromagnetic interference rejection ratio. It is an index that indicates the amount of high-frequency noise that can be tolerated. A slightly more complicated quantity, it indicates, in decibels, the ratio of the peak voltage when a RF (high-frequency) signal has been applied to the input terminal (+ side) to the input-referred offset voltage shift amount.

The EMIRR is measured over the frequency range from 1 MHz to several GHz; high values mean a better characteristic. The EMIRR is about 100 dB or so at several GHz, and the characteristic is an upward-sloping curve. In differential operation amplifiers and the like, the frequency response performance of a semiconductor integrated circuit is worse at higher frequencies, and so the EMIRR value is better as a matter of course.

By examining and comparing these values, some of the EMC characteristics of a product can be assessed. There are numerous stipulations of EMC characteristics in the IEC international standards (including CISPR) and the ISO international standards, which is why I use the qualifier “some of”.

Thank you very much for your attention.