“This article first introduces the detailed concept of Common Mode Transient Immunity (CMTI) and its importance in the system. We will discuss a new family of isolated sigma-delta modulators, their performance, and how it improves and enhances system current measurement accuracy, especially for offset error and offset error drift. Finally, the recommended circuit solution is presented.
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This article first introduces the detailed concept of Common Mode Transient Immunity (CMTI) and its importance in the system. We will discuss a new family of isolated sigma-delta modulators, their performance, and how it improves and enhances system current measurement accuracy, especially for offset error and offset error drift. Finally, the recommended circuit solution is presented.
Isolated modulators are widely used in motors/inverters that require high accuracy current measurement and galvanic isolation. As motor/inverter systems move towards higher integration and higher efficiency, SiC and GaN FETs are starting to replace MOSFETs and IGBTs due to their smaller size, higher switching frequency and lower heat generation. However, isolation devices need to have high CMTI capability, in addition to the need for more accurate current measurements. Next-generation isolated modulators greatly increase CMTI capabilities and improve their own accuracy.
What is Common Mode Transient Immunity?
Common mode transient immunity specifies the rate of rise and fall of transient pulses applied to insulation critical conditions. Exceeding this rate may result in corruption of data or clocks. Both the rate of change of the pulse and the absolute common mode voltage are recorded.
The new isolated modulator was tested under static and dynamic CMTI conditions. Static testing detects single bit errors from the device. Dynamic testing monitors the filtered data output to observe changes in noise performance in random applications of CMTI pulses. The detailed test block diagram is shown in Figure 1.
Figure 1. Simplified CMTI Test Block Diagram
CMTI is important because high slew rate (high frequency) transients can disrupt data transfer across the isolation barrier. Understanding and measuring the effect of these transients on the device is critical. ADI’s test method is based on the IEC 60747-17 standard, which covers the Common Mode Transient Immunity (CMTI) measurement method for magnetic couplers.
How to Test the CMTI Characteristics of Isolated Modulators on a Platform
The simplified CMTI test platform includes the following items, as shown in Figure 1:
• Battery power for VDD1/VDD2.
• High common voltage pulse generator.
• Oscilloscope for monitoring data.
• A data acquisition platform for analyzing data and a decimation-by-256 sinc3 filter for isolating modulators.
• Isolation modules (usually using optical isolation).
• Isolated modulator.
Static and dynamic CMTI tests use the same platform, just with different input signals. The platform can also be used to test the CMTI performance of other isolation products. For isolated modulators, the bitstream data is decimated and filtered for transmission into the control loop in the motor control system, making dynamic CMTI test performance more comprehensive and useful. Figures 2 and 3 show the time-domain and frequency-domain CMTI dynamic test performance at different CMTI levels. As can be seen from Figure 2, for the same isolated modulator, the spurs become larger when higher VCM transients are applied. When VCM transients exceed the isolated modulator specifications, very large spurs appear in the time domain (as shown in Figure 2c). This has serious consequences in motor control systems, resulting in large torque ripple.
Figure 2. Time Domain Dynamic CMTI Performance
Figure 3. Frequency Domain Dynamic CMTI Performance
Figure 3 shows the FFT domain performance under different frequency transients (i.e. maintaining the VCM transient level by changing the transient period). The results in Figure 3 show that the harmonics are highly correlated with the transient frequency. Therefore, the higher the CMTI capability of the isolated modulator, the lower the noise level in the FFT analysis. The next-generation ADuM770x devices increase the CMTI capability from 25 kV/μs to 150 kV/μs compared to the previous generation of isolated modulators, greatly improving system transient immunity, as detailed in the comparative data in Table 1.
Table 1. Comparison of key specifications
System-level compensation and calibration techniques
In a motor control or inverter system, the higher the accuracy of the current data, the more stable and efficient the system will be. Offset and gain errors are common sources of DC errors in ADCs. Figure 4 shows how offset and gain errors affect the ADC transfer function. These errors can affect the system in the form of torque ripple or speed ripple. For most systems, in order to limit the effect of errors, these errors can be eliminated by calibrating at ambient temperature.
Figure 4. Offset and Gain Error of ADC Transfer Function
Otherwise, offset drift and gain errors over temperature become problematic because they are more difficult to compensate. With known system temperature, for converters with linear and predictable drift curves, compensation for offset and gain error drift can be achieved (albeit at a high cost) by adding a compensation factor to the curve to make the offset drift curve as flat as possible and time-consuming). See application note AN-1377 for details on this compensation method. This approach can reduce the drift values specified in the AD7403/AD7405 data sheets by up to 30% in offset drift and up to 90% in gain error drift. This method can be applied to any other conversion device when it is desired to improve offset and gain error drift at the system level.
How to use chopping techniques
There is also a design called chopping which is more efficient and convenient for the system designer, and the chopping function is also well integrated with the silicon itself to minimize offset and gain error drift . Chopping Scheme As shown in Figure 5, the solution implemented on the ADC is to chop the entire analog signal chain to remove all offset and low frequency errors.
Figure 5. Chopping
The differential inputs to the modulator are alternately inverted (or chopped) on the input multiplexer, performing an ADC conversion for each phase of the chop (the multiplexer switches to a 0 or 1 state). The modulator chopping is inverted in the output multiplexer, which then feeds the output signal into a digital filter.
If the offset in a sigma-delta modulator is expressed as VOSthen when the chopper is 0, the output is (AIN(+) − AIN(−)) + VOS; when chopping is 1, the output is −[(AIN(−) − AIN(+)) + VOS]. Error voltage VOSCancellation by averaging these two results in a digital filter gives (AIN (+) − AIN (−)), which is equal to the differential input voltage without any offset term.
The latest isolated modulators improve performance related to offset and gain errors by optimizing the internal analog design and using the latest chopping techniques, which greatly simplifies system design and reduces calibration time. The latest ADuM770x devices have very high isolation and excellent ADC performance. An LDO version is also available, which simplifies the power supply design of the system.
Recommended circuit and layout design
A typical current measurement circuit for a motor system is shown in Figure 6. Although three phase current measurement circuits are required in the system, only one is shown in the block diagram. The other two phase current measurement circuits are similar and represented by blue dashed lines. As can be seen from the phase current measurement circuit, one side of the RSHUNT resistor is connected to the input of the ADuM770x-8. The other side is connected to the high voltage FET (can be an IGBT or MOSFET) and the motor. Overvoltage, undervoltage, or other voltage instability conditions are always present when high voltage FETs change states. Correspondingly, RSHUNTThe voltage fluctuation of the resistor is passed to the ADuM770x-8 and the relevant data is received on the DATA pin. Layout and system isolation design can improve or worsen voltage instabilities that affect phase current measurement accuracy.
Figure 6. Typical Current Measurement Circuit in a Motor System
The recommended circuit setup is shown in Figure 6:
• VDD1/VDD2 decoupling requires 10 μF/100 nF capacitors, which should be placed as close as possible to the corresponding pins.
• A 10 Ω/220 pF RC filter is required.
• An optional differential capacitor is recommended to reduce the noise impact of the shunt. Place this capacitor close to the IN+/INC pins (0603 package recommended).
• When the digital output line is long, an 82 Ω/33 pF RC filter is recommended. For good performance, shielded twisted pair cable should be considered.
• For higher performance requirements, consider using a 4-pin shunt resistor.
A good layout is also essential for optimal performance. The recommended layout is shown in Figure 7. Differential pair routing is recommended between the shunt resistor and the IN+/INC input pins for enhanced common mode rejection. The 10 Ω/220 pF filter should be placed as close as possible to the IN+/INC input pins. The 10 μF/100 nF decoupling capacitors should be placed close to the VDD1/VDD2 supply pins. It is recommended to place part of the ground plane GND1 below the input related circuits to improve signal stability. For the separate GND1 line (shown in purple and parallel to the differential pair trace), a star connection is required from the shunt resistor to the ADuM770x-8 GND pin to reduce the effects of supply current fluctuations.
Figure 7. Recommended PCB Layout for ADuM770x-8 Circuit
in conclusion
The latest ADuM770x isolated sigma-delta modulators increase CMTI to 150 kV/µs levels and improve temperature drift performance, which is very beneficial for current measurement applications. It will be helpful to use the recommended circuit and layout during the design phase.
