Abstract: Position loop, speed loop, and current loop constitute a three-loop control system for a servo motor . For the current acquisition stage of the current loop control, a current signal acquisition circuit based on the AD7403 was designed. Its key advantage lies in the fact that the AD7403 chip can closely approximate the actual AC current path, converting the analog signal into a digital output stream nearby. The original information can then be reconstructed through a DSP's digital filter. This minimizes noise pickup, reduces EMI/RFI effects, and improves system accuracy. Moreover, compared to traditional optocoupler-based solutions that are susceptible to long propagation delays, current sensing based on an isolated modulator using ADI's iCoupler® technology allows for precise timing of both the isolated modulator response and the low propagation delay of the gate driver. Short-circuit overcurrent protection is achieved through a fast coarse-tuning digital filter.
0 Introduction
Servo motors requiring high control precision typically employ a three-loop control system consisting of a position loop, a speed loop, and a current loop. This three-loop structure allows the servo system to achieve better dynamic tracking performance and anti-interference capabilities. Current sensing is crucial for high-performance closed-loop motor control and is difficult to achieve high-fidelity measurements in harsh, noisy environments. In higher-power systems, isolated current sensors (such as current transformers or Hall effect sensors) with built-in isolation are used; while in lower-power systems, the trend is towards using shunt resistors with isolated Σ-Δ modulators (such as the ADI AD7403). Previous systems often used desaturated gate drivers for short-circuit overcurrent protection. ADI's iCoupler® technology enables isolated modulator-based current sensing to achieve both low propagation delay of the isolated modulator response and gate driver while maintaining accurate timing through a fast coarse-tuning digital filter, superior to traditional optocoupler-based solutions.
1 System Overall Design
The sampling circuit designed in this paper converts the detected current signal into a voltage signal and inputs it to the voltage test terminal of the AD7403 device. The AD7403 outputs a single 5~20MHz bit stream according to the clock signal given by the DSP. The DSP then calculates the corresponding 16-bit ADC value through the sinc3 filter and stores the data into the corresponding circular queue via DMA. It can also communicate with the host computer via USB or RS485 to view the acquisition effect of the AD7403 and perform corresponding data analysis.
2. Current Detection Circuit Design
First, the current in the current loop is converted into a corresponding voltage signal through a shunt resistor. The voltage signal is then appropriately amplified and adjusted by an integrated operational amplifier before being fed into a modulator to convert it into a digital stream. The integrated operational amplifier used in this paper is the AD8639. When adjusting the voltage signal, a dual non-inverting comparator circuit is employed, with a very high input resistance to minimize signal loss and distortion. The modulator used is the AD7403, a high-performance second-order Σ-Δ modulator. Its on-chip digital isolation utilizes Analog Devices' iCoupler® technology, enabling the conversion of analog input signals into high-speed single-bit data streams. The AD7403 operates on a 5V (VDD1) power supply and can accept differential signals of ±250mV (full scale ±320mV). The analog input is continuously sampled by the high-performance analog modulator and converted into a digital output stream with a data rate of up to 20MHz and a density of 1. The original information can be reconstructed using appropriate digital filters to achieve a signal-to-noise ratio (SNR) of 88dB at 78.1kSPS. The serial input/output can be powered by either a 5V or 3V supply (VDD2); the circuit presented here uses +5V_ISO. This differential input signal is ideal for monitoring shunt voltages in high-voltage applications requiring current isolation. The serial interface employs digital isolation. By combining high-speed complementary metal-oxide-semiconductor (CMOS) technology with monolithic transformer technology, on-chip isolation provides superior operating characteristics compared to other components such as traditional optocouplers.
The external current loop is converted into a corresponding voltage signal through a shunt resistor. The gain of the AD8639 is adjusted by adjusting RFA/R1 and RFB/R1 to maximize the input range advantage of the AD7403. The AD8639 is a dual-channel, wide-bandwidth, self-stabilized zero-gain amplifier with rail-to-rail output swing and low noise characteristics. These amplifiers have extremely low offset, drift, and bias current. A single 5V to 16V supply (or a dual ±2.5V to ±8V supply) is used. The circuit designed in this paper uses a ±2.5V isolated supply, with R1 and RFA/RFB forming two sets of in-phase amplifiers to set the amplifier gain. If the amplifier gain is set to 10, the sensing resistor is 1mΩ, and the AD7403 input is a ±300mV differential signal, then this circuit can detect a peak current of up to ±30A. To prevent input overvoltage from damaging the amplifier, a protection ring consisting of two sets of Schottky diodes and two 10Ω resistors is used at the current test terminal to protect it from transient overvoltage and ESD. Since the AD8639 has a dual external power supply of ±2.5V, the protection circuit is connected to the ±2.5V power supply separately to prevent overvoltage. R2, R3, and C1 form a low-pass filter to reduce noise fed into the modulator. The test signal is connected to the VIN+ and VIN- terminals of the modulator. The MDAT port shifts serial data to the DSP one-by-one on the rising edge of the MCLKIN input and is valid on the next rising edge of MCLKIN. The DSP inputs the host clock logic to the MCLKIN pin, with an operating frequency range of 5MHz to 20MHz. The VDD1 pin is the power supply voltage for the AD7403's internal isolation terminal, +5V_ISO, with GND1 as the reference ground. A 10µF capacitor and a 1nF capacitor are connected in parallel to decouple the power supply pin to GND1. The VDD2 pin provides a power supply voltage of +5V to the internal non-isolated terminal, with GND2 as the reference ground. A 100nF capacitor is used to decouple this power supply to GND2.
3 Power Supply Configuration Circuit
In the aforementioned current sensing circuit, the AD7403 requires both +5V isolated and non-isolated power supplies, while the AD8639 requires ±2.5V isolated power. Many mature three-terminal regulators can provide a +5V non-isolated power supply. To obtain a +5V isolated power supply, this circuit uses an internal 625kHz PWM signal from the ADuM5000 to provide a 5V DC power supply across the isolation gate. The ADuM5000 is a 5V isolated DC-DC converter used as the power supply for the isolation terminals, providing complete isolation at both ends, a 2500Vrms isolation value (1 minute, compliant with UL1577), and the system uses only one power supply. The ADuM5000 operational amplifier AD8639 is supplied with ±2.5V. The +2.5V_ISO can be provided by the low-noise ADP121-2.5 low-dropout regulator, which is powered by the +5V_ISO output from the ADuM5000's VISO pin. To provide a -2.5V ISO power supply, this paper uses a TPS60400 charge pump inverter. The TPS60400 has an input voltage range of 1.6V to 5.5V and can invert the input voltage to -1.6V to -5.5V, i.e., VOUT = -VIN. The maximum output current is 60mA, and the entire circuit only requires three filter capacitors. Connecting the output voltage of the ADP121-2.5 to the IN pin of the TPS60400 will provide the -2.5V ISO power at the OUT pin.
4. Conclusion
In actual circuit design, the DSP or FPGA is usually some distance from the AC path, and may even be located on different circuit boards. If the DSP's internal A/D converter is used, the acquired analog signal will travel a certain distance before being sent to the DSP, inevitably affecting the signal and thus the accuracy of the entire system. The signal acquisition circuit based on the AD7403 designed in this paper is a fully isolated current sensing circuit with its own isolated power supply. This circuit has extremely strong robustness; the AD7403 can closely approximate the actual AC current path, converting the analog signal into a digital output stream nearby. The original information can then be reconstructed through the DSP's digital filter. This minimizes noise pickup, reduces EMI/RFI effects, and improves system accuracy. Safety is achieved through a 20µm polyimide film isolation barrier.
