Optically Isolated Sigma-Delta Modulator

The ACPL-7970 miniature isolation amplifier is a 1-bit, second-order amplifier based on sigma-delta modulator technology that converts an analog input signal into a high-speed data stream through integration of galvanic isolation based on optical coupling technology. The ACPL-7970 miniature isolation amplifier uses a 5 V power supply with dynamic range of 78 dB and an appropriate digital filter. The differential inputs of ±200 mV (full scale ±320 mV) are best suited for direct connection to shunt resistors or other low-level signal sources in applications such as motor phase current sensor.

The miniature isolation amplifier analog input is continuously measured by the sigma-delta modulator through over-sampling using an on-board clock. The signal information is contained in the modulator data, as a density of ones with data rate of 10 MHz. Data is then encoded and transmitted across the isolation boundary where it is recovered and decoded into high-speed data stream of digital ones and zeros. The original signal information can be reconstructed with a digital filter. The serial interface for data and clock has a wide supply range of 3 V to 5.5 V.

Combined with superior optical coupling technology, the sigma-delta modulator delivers high noise margins and excellent immunity against isolation-mode transients. With 0.5 mm minimum distance through insulation (DTI), the ACPL-7970 miniature isolation amplifier provides reliable reinforced insulation and high working insulation voltage, which is suitable for fail-safe designs. This isolation performance is superior to alternatives including devices based on capacitive or magnetic coupling with DTI in micro-meter range. Offered in a DIP-8 package, the isolated ADC delivers the reliability, small size, superior isolation and over-temperature performance motor drive designers need to accurately measure current. This design alternative delivers a lower price when compared to traditional current transducers.

The ACPL-7970 miniature isolation amplifier product is also available from Avago in an SSO-8 package version, the ACPL-C797 device. And, an external clock version of this miniature isolation amplifier, which comes in an SO-16 package, is also available - the ACPL-796J device.

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