Diagnosing Delta-Sigma Modulator Bitstream Using C2000™ Configurable Logic Block (CLB)

Abstract

In many industrial and automotive applications, an increasingly important requirement is the ability to continually monitor systems and enter appropriate fail-safe states. Particularly in real-time control applications, being able to detect potential faults and perform necessary actions within the system can prevent damage to the system and help support functional safety. This application note describes a technical diagnostic application involving a delta-sigma modulator interfaced with a C2000 microcontroller. The configurable logic block (CLB) peripheral in C2000 real-time MCUs can be utilized for test and measurement applications to provide a greater insight into the status of current and voltage feedback systems making use of delta-sigma modulators.

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1 Introduction

Enhanced monitoring and diagnostic features integrated within control systems enable increased system reliability and risk mitigation. In the scheme described within this report, C2000 real-time microcontroller devices' advanced features are used to monitor the status of a delta-sigma modulator. Specifically, the output bistream, which encodes the digital conversion output of an AMC1306x modulator and is connected to a TMS320F28004x MCU for filtering, is monitored to determine if the high-side of the isolated modulator is powered.

The delta-sigma modulator outputs a bitstream of 1s and 0s, where the density of 1s is proportional to the input analog voltage. A differential input signal of 0 V ideally produces a stream of 1s and 0s that are high 50% of the time and low 50% of the time. A large density of 1s corresponds to a greater input voltage. It features additional fail-safe conditions, described in AMC1306 Bitstream & Fail-Safes, which can be used to identify potential problems in the system. For more information about the AMC1306x device, see the AMC1306x Small, High-Precision, Reinforced Isolated Delta-Sigma Modulators With High CMTI Data Sheet.

The configurable logic block on C2000 devices is a peripheral composed of various submodules that assist in performing logic operations independent of the CPU. This peripheral can be used in place of an external FPGA or CPLD to execute complex device logic. The clock and bitstream, which are necessary inputs into the SDFM module when in Mode 0 operation, can be routed to the CLB peripheral purely through software, and CLB can diagnose the output bitstream of delta-sigma modulators. For more information about the TMS320F28004x device, see the TMS320F28004x Data Sheet.

2 System-Level Application

Delta-sigma modulation is used to measure current and voltage levels in power systems such as the AC motor drive shown in Figure 2-1. Shunt-based isolated delta-sigma modulators are often used for current and voltage sensing in 3-phase inverters due to their high accuracy and high bandwidth, while offering lower system cost.

Figure 2-1 Current and Voltage Measurement in Three-Phase Inverters

Delta-sigma modulators are ADCs that allow for continuous oversampling of analog inputs through a delta-sigma modulator and require a digital/decimation filter. The voltage drop across a shunt resistor can be measured by a delta-sigma modulator to enable current sensing. A delta-sigma modulator converts an analog input signal into a high-frequency stream of single bits with out-of-band noise. The advantages of moving the quantization noise to higher frequency bands include simple anti-aliasing filtering, low-cost solution by eliminating cost on drivers, and scalable performance. This leads to reduced overall system cost while providing high performance.

Figure 2-2 shows a block diagram showcasing shunt resistor and delta-sigma modulator for phase current measurement in a three-phase inverter design.

Figure 2-2 Delta-Sigma Modulator in Three-Phase Inverter

The AMC1306 device requires a low-side supply which can be supplied by the 3.3 V rail supplying the C2000 device. The AMC1306 also requires an isolated 3.3 V or 5 V high-side supply (AVDD) which is generated from the isolated high-side gate driver supply by an LDO. The AMC1306 takes as inputs the differential voltage across the shunt resistor on the high-side and the clock supplied by the C2000 device on the low-side. The output of the modulator can be fed into the SDFM peripheral of the C2000 device.

The AMC1306 device's fail-safe conditions, which are described in greater detail throughout this report, provide indication into potential system faults.

One of the fail-safe conditions occurs when the high-side power supplied into the AMC1306 device is not present. Presence of this fail-safe could indicate an issue with the high-side gate driver supplying AVDD. It could also indicate an issue with the LDO used to step-down the voltage into the device.

Another fail-safe condition, known as common mode overvoltage, can be used to indicate a missing or disconnected shunt resistor. If AINN or AINP is disconnected from the shunt resistor, the input bias current of the AMC1306 drives the disconnected terminal towards the positive supply rail, and the common-mode input voltage increases. A similar effect happens when there is no DC current path between AINN, AINP, and AGND. This fail-safe condition is useful to identify interconnect problems on the board.