編集後記

Welcome to the October issue of Analog Dialogue.

Isolation is one of the main requirements for providing safety in electronic systems. In the past, when the power supply was mainly created using an ac transformer, the transformer provided isolation between the grid and the lower voltage signals. Later, when primary switching power supplies rose to prominence, the transformer was removed. This change required a different method to achieve isolation. While optocouplers provide isolation, they are slow and power hungry. iCoupler® digital isolators were developed to overcome these limitations. If you’d like some insights on the functionality, here is a short video. These isolation devices support applications such as motor control, ac-to-dc power converters for solar and windmill systems, medical systems, and many more.

Back to the articles from the Analog Dialogue.

Isolated analog-to-digital converters (ADCs) combine an ADC and an isolation barrier, and they are the preferred method for phase current measurement in high performance motor and servo drives. With every new generation of ADCs, performance is enhanced even further, but to fully utilize the potential of the latest ADCs, the rest of the motor drive needs to be designed accordingly. Because the topic of optimized, isolated sigma-delta (Σ-Δ) current measurement for motor control applications is such a wide area, we will address the subject in two parts. Part 1 discusses demodulation of sigma-delta coded data using sinc filters in a motor control application. Part 2 (to feature in November) will propose a new sinc filter structure that improves measurement performance in motor control applications, followed by a discussion on implementation of sinc filters with HDL code for optimum performance. And, finally, measurement results from an FPGA based 3-phase servo drive will be presented. The article is based on the ADuM7701 16-bit, sigma-delta modulator.

Our second article this month concentrates on a not so common specification of an operational amplifier. The input capacitance of these devices is quite often ignored by the designer. However, it could be a key specification for high impedance and high frequency op amp applications. Notably, when, for example, a photodiode junction capacitance is small, the op amp input capacitance can dominate noise and bandwidth issues in an application. The op amp input capacitance and feedback resistor create a pole in the amplifier’s response, impacting stability and increasing the noise gain at higher frequencies. As a result, stability and phase margin could degrade and output noise could increase. The article describes a new direct way to measure CDM (capacitance—difference mode).

In case the CDM article is too much analog for you, the next article will better address your digital needs. High efficient, low electromagnetic interference (EMI) buck regulators are often used for high power density digital integrated circuits (ICs), such as digital graphics processor units (GPUs) and field programmable gate arrays (FPGAs). A new Silent Switcher® 2 family uses special design and packaging techniques to enable >92% efficiency at a high 2 MHz switching frequency while easily passing the CISPR 25 Class 5 peak EMI limits. An internal unique construction uses copper pillars in lieu of bond wires, while the addition of internal bypass capacitors and an integrated substrate ground plane further reduce EMI.

Our following article addresses the question: how is inductor current measured? Switched-mode power supplies commonly use inductors for temporarily storing energy. In the evaluation of these power supplies, it is often useful to measure the inductor current to gain a complete picture of the voltage conversion circuit. What is the best way to measure the inductor currents? A small auxiliary cable is inserted in series with the inductor. It is used to attach a current probe and to display the inductor current with an oscilloscope.

The ADALM2000 series continues as Doug Mercer and Antoniu Miclaus introduce the voltage dependency of a PN junction. Increasing the reverse bias voltage, VJ, across a PN junction leads to the redistribution of charge away from the interface, leaving a depleted region or layer. This depleted layer acts like the insulator between the two conducting plates of a capacitor. The PN junction capacitance is divided into two components, the barrier capacitance and the diffusion capacitance. The barrier capacitance is the dominant source of capacitance for reverse and small positive bias voltages. The dependence of the junction capacitance to the applied bias voltage, is called the capacitance voltage (CV) characteristic of the junction. In this ADALM2000 exercise we will measure and plot this characteristic for various PN junctions.

And as we have for 53 years, we invite you to be part of the dialogue in Analog Dialogue. You can get in touch through our blog, Facebook page, or email. Let us know how we’re doing and what you’d like to see from us in the coming months.