Externally Clocked, Optically Isolated Sigma-Delta Modulator Precisely Measures Currents and Voltag
By Chen Hong Lei, Avago Technologies
Introduction
One of the challenges in motor drive applications is the sensing of motor phase current, bus current and bus voltage, and feeding back to the controller. Avago Technologies’ ACPL-796J isolated ADC is an ideal choice for such designs.
The ACPL-796J is a 1-bit, sigma-delta modulator has the capability to convert an analog input signal into a high-speed data stream with galvanic isolation based on optical coupling technology. Operating from a single 5 V power supply, Avago’s optically isolated sigma-delta modulator provides 80 dB dynamic performance, when used with an appropriate digital filter.
The differential analog inputs of the ACPL-796J are implemented with a fully-differential, switched-capacitor circuit. The ACPL-796J accepts a signal of ±200 mV (full scale ±320 mV), which is ideal for direct connection to shunt based current sensing or other low-level signal sources
applications such as motor phase current measurement. An internal voltage reference determines the full-scale analog input range of the modulator (±320 mV); an input range of ±200 mV is recommended to achieve optimal performance. Users are able to use higher input range, for example ±250 mV, as long as within full-scale range, for purpose of over-current or overload detection. Figure 1 shows the functional block diagram.
Figure 1. ACPL-796J functional block diagram.
Input signal information is contained in the modulator output data stream, represented by the density of ones and zeros. The density of ones is proportional to the input signal voltage, as shown in Figure 2. For example, an input of 0 V produces a data stream of ones 50 percent of the time and zeros 50 percent of the time, ideally. A differential input of –200 mV corresponds to 18.75 percent density of ones, and +200 mV is represented by 81.25 percent density of ones. Input of +320 mV results in ideally all ones, while input of -320 mV will result in all zeros ideally.
Figure 2. Modulator data output vs. analog input.
Features and Benefits
Based on an advanced low power CMOS process, Avago’s ACPL-796J uses an external clock that allows synchronized data conversion between the current sensor and controller to help motor control designers eliminate complex design processes in data and clock reading. Moreover, this optically isolated modulator is designed with digital inputs and outputs that are compatible with systems powered by 5- or 3-volt power supplies and operates over a wide industrial temperature range. Key features include:
- 5 MHz to 20 MHz external clock input range allow user configurable conversion speed
- 16-bit resolution - no missing codes (12 bits ENOB) provides high accuracy data conversion
- 74 dB minimum SNR
- 3.5 mV/°C maximum offset drift
- ±1% maximum gain error
- 3 V to 5.5 V wide supply range for digital interface compatible with 3.3V or 5V controller
- –40°C to +105°C operating temperature range
- SO-16 package saves PCB space
- 25 kV/ms common-mode transient immunity ignores high common-mode switching noise ensuring smooth motion control
- Safety and regulatory approval:
IEC/EN/DIN EN 60747-5-5: 1140 Vpeak working insulation voltage UL 1577
Applications
The targeted applications that can benefit from the use of an isolated sigma-delta modulator include current sensing and voltage sensing in AC and brushless DC motor drives, industrial inverters, air conditioners, washing machines, switching power supplies, power inverters used in renewable energy systems such as solar panels and wind turbines. Offered at a much better price/performance, this device is an excellent alternative solution for traditional current transducers.
In addition to power converter applications, the ACPL-796J can be used as an isolated A-D converter in data acquisition systems and general voltage isolators, transmitting signals such as temperature, pressure, flow, in industrial process control environments.
Typical Connections
Avago’s ACPL-796J is designed for current and voltage sensing in electronic motor drives and uses advanced sigma-delta A/D converter technology to allow designers to accurately measure motor phase currents in servo and high-end inverter motor drives. Figure 3 shows an overview of a motor drive system using the ACPL-796J for current sensing and voltage sensing.
Figure 3. Motor control application block diagram
Figure 4 shows a typical application circuit for motor phase current sensing. By choosing the appropriate shunt resistance, the range of current can be monitored (from less than 1 A to more than 100 A). In other applications, the motor can be replaced by a load such as a main grid in the solar energy system. Note that the ACPL-796J is connected in a single-ended input mode in this circuit. Better performance can be achieved with a differential input connection. Refer to ACPL-796J data sheet for details.
Figure 4. Typical application circuit for motor phase current sensing.
Latch-up Consideration
Latch-up risk of CMOS devices needs careful design considerations, especially in applications with direct connection to a signal source that is subject to frequent transient noise. The analog input structure of the ACPL-796J is designed to be resilient to transients and surges, which are often encountered in highly noisy application environments such as motor drive and other power inverter systems. Other situations that could cause transient voltages to the inputs include short circuit and overload conditions. The ACPL-796J is tested with DC voltage of up to –2 V and 2-second transient voltage of up to –6 V to the analog inputs and there is no latch-up or damage to the device.
Conclusion
Combined with superior optical coupling technology, the modulator delivers high noise margins and excellent immunity against isolation-mode transients. With 0.5 mm minimum distance through insulation (DTI), the ACPL-796J provides reliable double protection and high working insulation voltage, which is ideal for fail-safe designs. The isolation performance provided is superior to many other alternatives including devices based on capacitive- or magnetic-coupling with DTI in the micro-meter range. Offered in a 16-lead small outline package, the isolated A/D converter delivers the reliability, small size, superior isolation and over-temperature performance motor drive designers need to accurately measure current at a much lower price than traditional current transducers.
Evaluating the ACPL-796J
The evaluation kit is available for designer to quickly test the ACPL-796J (see Figure 5). The kit comes with a PCB board that consists of three portions that can be separated easily to provide distance between them when needed for field testing. Two shunt resistors (surface mount) with different values are provided along with the board for the designer to choose from based on the current rating. Besides a surface mount shunt or an axial type shunt is also acceptable.
Figure 5. The ACPL-796J evaluation kit.
The user can choose to use the onboard clock generator, or an external clock source. The onboard clock generator provides selectable clock frequencies from 5 MHz to 20 MHz. An HCPL-0872 is used as the digital filter to down-sample the modulator output to a lower speed data rate, which is sent to an MCU. The MCU communicates with the computer using a HyperTerminal through an RS-232 port. The board may also be used for general voltage isolation (isolated A-D converter). In this case shunt resistor are not required.
Reference
[1] ACPL-796J Data Sheet: http://www.avagotech.com/docs/AV02-1670EN
[2] Application Note 1078 – Designing with Avago Technologies Isolation Amplifiers: http://www.avagotech.com/docs/5965-5976E
[3] Reference Guide – Isolation Products in Motor Control Systems: http://www.avagotech.com/docs/AV00-0074EN