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Monolithic component integrates isolation plus ADC for AC line-voltage monitoring

AUSTIN, Texas—Monitoring of the AC line is an important requirement for systems such as smarter residential and commercial power, solar-power inverters, uninterruptible power supplies (UPS), but there are two challenges: the line voltages are much higher than electronic circuits like to see directly, and there are also serious system- and user-safety issues. The first is easily solved by a resistor network, but the latter requires more advanced techniques to achieve the mandatory galvanic isolation.

To solve the isolation issue while also addressing the data-acquisition function, the Si890x trio from Silicon Laboratories Inc., incorporates capacitive-based digital isolation along with a 10-bit analog/digital converter (ADC); Figure 1 shows an overview of the isolated signal path. Their small size, low profile, and standard IC package bring desired attributes to the overall system and PCB design.


Figure 1: Basic function of the Silicon Laboratories Si890x analog channel (click on image to enlarge).

The ICs, with 2.5 and 5 kV isolation rating, meet applicable safety standards, including IEC 60950-1, 61010-1, and 60601-1, and have UL, CSA, and VDE certifications. Designed primarily for 120/220 Vac mains, the 1200 V working voltage rating provides an isolation barrier rated for more than 60 years operation, using standard test and data-extrapolation techniques.

The basic function of the Si890x devices (Figure 2)  provides a three-channel analog multiplexer feeding an isolated ADC, thus allowing the device to monitor up to three differential channels; typically, this would be current, voltage, plus a spare channel. The ADC reference can either be the IC's supply rail or a separate, user-supplied reference.


Figure 2: Detailed block diagram of the Si8900 member of the family; the other are similar except for their bus interface (click on image to enlarge).

The ADC offers ±1 LSB integral and differential nonlinearity (INL/DNL) and ±2 LSB offset and full-scale errors (both are maximum specifications), along with a ×0.5/×1 programmable gain amplifier (PGA); the ADC can be set to convert on-n demand or in burst mode. Also especially relevant to this application are transient immunity (>35 kV/µs) as well as E-field and M-field immunity (>300 V/m and >1000 A/m, respectively). The ICs run from an input-side supply of 2.7 to 3.6 V and output-side supply of 2.7 to 5.5 V (recall that these are isolated devices, which mandates separate supplies for each side).

Digital interface: The three members of the family differ in only one respect—their bus-interface digital I/O. Users can choose among  234-ksps UART with automatic baud-rate selection algorithm (Si8900), Figure 2, 240-ksps I2C (Si8901), or 2.5-Msps SPI (Si8902) interfaces. Note that the pin-out for all three versions is identical except for the bus-interface pin assignments, so changing from one interface choice (and IC) to another requires minimal PC board re-routing.

Support and evaluation:  The Si890x PWR-KIT evaluation kit is a complete PC board with all three versions, along with microcontroller, I/O connections, and protective enclosure (very important here, of course), Figure 3.


Figure 3: The PWR-KIT evaluation kit includes all three family members, microcontroller, power, and digital interfaces, all housed in a protective enclosure (click on image to enlarge).

Packaging, pricing, and availability:
All three versions are housed in a 16-pin wide-body SOIC package, with choice of 2.5 or 5 kV isolation. Pricing is $1.65 each in 10,000 unit lots; all are available now. For more information, go to http://www.silabs.com/pr/isolation.

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