TI Designs: TIDA-01576 Isolated 16-Channel AC Analog Input Module Reference Design Using Dual Synchronous Sampling ADCs

System Description

Power system failures are a common problem for power generation, transmission, and distribution companies, leading to revenue loss and reduced generation capacity. Power utilities are implementing reliable power solutions through secondary protection, control, monitoring, and measurement systems to enhance power system efficiency and reliability.

1.1 Introduction to Protection Relays

Digital protection relays detect defective lines, equipment, or other abnormal power system conditions. They locate faults by measuring electrical quantities, distinguishing between normal and abnormal states. Accurate measurement of AC voltage and current inputs over a wide range is critical for these relays. Key functional components include data acquisition (analog filtering and sampling), measurement (phasor estimation), and logic (tripping, alarming, carrier send).

1.1.1 Multifunction Protection Relay for Generation, Transmission, or Distribution Applications

The primary inputs for protection relays are AC voltages and currents. Protection algorithms are based on amplitude, frequency, and phase. The AC AIM captures outputs from voltage and current transformers. The number of analog inputs can range from 4, 8, 12, or 16, depending on the monitored equipment and protection function.

1.1.2 Stand-Alone Merging Unit

A merging unit captures voltage and current inputs from connected equipment and provides digital data to different IEDs using the IEC 61850-9-2 protocol. The number of analog inputs can be 4, 8, 12, or 16.

1.1.3 Substation Bay Controller and Terminal Unit

A bay controller monitors analog and digital inputs from primary equipment in a substation. The number of analog inputs varies from 8 to 24 based on configuration.

1.2 Key System Specifications

The following table outlines the key system specifications for the isolated, high-accuracy, 16-channel AIM reference design.

System Overview

This reference design accurately measures AC voltage and current inputs using a precision 16-bit successive approximation (SAR) analog-to-digital converter (ADC) over a wide input range, covering protection and measurement requirements (including IEC 61850-9-2 sampling). It simplifies system design and improves trip time performance and reliability. The Analog Input Module (AIM) is isolated from the host processor using a digital isolator with an integrated power converter. A complete AC AIM can be designed using only TI products, optimizing system cost and size. The alarm feature identifies AC analog input faults on a sample basis for faster fault detection. The ADC includes an auxiliary channel to diagnose the digital isolator's supply output. Two ADCs are connected together to provide 16 analog input channels using daisy chain mode or dual SDO output mode with common input signals.

2.1 Block Diagram

This reference design showcases several configurations for improved system performance:

• ADS8688A-based, 16-channel input with dual SDO for AC or DC analog input measurement.
• ADS8688A-based, isolated, 16-channel input with dual SDO and integrated power.
• ADS8688A-based, isolated, 16-channel input with dual SDO using a 6-channel digital isolator.

The design architecture should be chosen based on accuracy and board size requirements.

2.1.1 ADS8688A-Based, 16-Channel AC AIM With Dual SDO

This configuration includes:

• Two ADS8688A ADCs connected for 16 channels of AC analog input (±10.24-V input range).
• External reference for improved measurement accuracy between ADCs.
• LDOs for analog and digital power supply generation.
• Host interface with dual SDO outputs.
• MOSFET-driven LEDs for alarm indication.
• Host interface and GUI for ADC performance evaluation.

Diagram Description: A block diagram showing two ADS8688A ADCs connected in a daisy-chain configuration, receiving analog inputs and communicating via SPI to a host interface. It includes power supplies, reference, and alarm indicators.

2.1.2 ADS8688A-Based, Isolated, 16-Channel AC AIM With Dual SDO and Digital Isolator With Integrated Power

This design incorporates:

• ADS8688A-based, 16-channel input with dual SDO for AC or DC analog inputs (±10.24-V range).
• Host interface isolation using a 4-channel digital isolator with integrated power and a 2-channel digital isolator.
• Provision for SCKL loop back from ADC to host for improved performance.
• DC/DC converter and LDO for analog and digital power supply.
• Onboard analog temperature sensor for ambient temperature compensation.
• Reference with integrated buffer for improved ADC performance.
• Host interface and GUI for evaluation.

Diagram Description: A block diagram illustrating the isolated configuration using ADS8688A ADCs, a digital isolator with integrated power (ISOW7841), and associated power supply components.

2.1.3 ADS8688A-Based Isolated 16-Channel Input With Dual SDO Using ISO7763 Six-Channel Digital Isolator

This variant features:

• ADS8688A-based, 16-channel input with dual SDO for AC or DC analog inputs (±10.24-V range).
• Host interface isolation using a six-channel digital isolator.
• Isolated power generated via a transformer driver.
• Provision for SCKL loop back.
• DC/DC converter and LDO for power supply.
• Analog temperature sensor for compensation.
• Reference with integrated buffer.
• Host interface and GUI for evaluation.

Diagram Description: A block diagram similar to the previous one but utilizing the ISO7763 six-channel digital isolator for isolation.

2.1.4 Precision ADC With External Reference

The core subsystem is the ADC (ADS8688 or ADS8688A) for measuring up to 16 inputs. While the internal ADC reference can be used, external references like REF5025 or REF6025 with buffers are recommended for improved performance.

2.1.5 Digital Isolator

Digital isolators provide the isolated interface. The ADCs use SPI signals (/CS, CLK, SDI, SDOA, SDOB). Options include the ISOW7841 (4-channel, reinforced, integrated power) or 6-channel isolators with external power. ISO7841 or ISO7763 can provide the necessary isolation.

2.1.6 Power Supply

The ISOW7841 provides a 3.3V output for the ADC digital interface (DVDD). A 5V analog supply is generated using the REG71055 boost converter and TPS71750 LDO. The SN6505 transformer driver can supply isolated power when using a 6-channel digital isolator.

2.2 Design Considerations

2.2.1 AC Current and Voltage Measurement Module

The data acquisition function for protection relays is handled by the AC AIM, comprising several subsystems:

2.2.1.1 Current Sensor Input

Various current sensors can be used, including current transformers, shunts, Rogowski coils, Hall effect sensors, and optical current transformers. Isolation for shunt-based current sensing is typically provided by an isolation amplifier or an isolated delta-sigma modulator.

2.2.1.2 Voltage Sensor Input

Voltage sensors include potential transformers, potential dividers, and capacitor voltage transformers. For potential divider measurements, isolation can be provided by an isolation amplifier or an isolated delta-sigma modulator.

2.2.1.3 Signal Conditioning

A signal conditioning circuit scales the sensor output to the ADC range. The circuit type (e.g., precision operational amplifier, instrumentation amplifier, programmable gain amplifier) depends on the application and requirements for accuracy and temperature drift.

2.2.1.4 Host Interface

The ADC interfaces with a host system that captures digital values and computes electrical parameters for protection, measurement, and control applications.

2.2.1.5 ADC

Accurate voltage and current measurements are critical for grid infrastructure. ADC selection is key to protection relay performance, accuracy, monitoring, and control. Important parameters include architecture, resolution, sampling method, input type/range, power supply, clock, and reference. The sampling rate must meet IEC 61850-9-2 standards.

2.2.2 Need for Isolation, Challenges, and Solutions

Power system equipment failures contribute significantly to outages. Predicting failures can reduce unplanned outages. Isolation is achieved using isolation amplifiers, delta-sigma modulators, or transformers. Key isolation requirements include type (basic or reinforced), jitter, integrated or external power supply, and interface type (I2C, SPI, UART).

2.2.3 Reference Design Advantage

This reference design offers several advantages:

• Meets ADC dynamic specifications (ENOB, SNR, THD) with an isolated interface.
• Measures 16 analog input channels with programmable input ranges.
• Provides interface isolation using digital isolators with integrated or external power.
• Simplifies design with a 6-channel digital isolator in a 16-pin package.
• Enables clock loop-back for improved measurement performance.
• Simplifies power supply design and improves efficiency.
• Features programmable input ranges and scalable performance (>19 bits).
• Supports bidirectional inputs up to ±10.24 V.
• Includes power supply diagnostics and alarm indication.
• Offers simple SPI interface with dual SDO or daisy-chained modes.

System Design Highlights

2.3.1 Precision Dual ADC

The design utilizes precision ADCs like the ADS8688A for accurate analog-to-digital conversion. The block diagram illustrates the interface and configuration.

Diagram Description: A schematic showing the connection of two ADS8688A ADCs, highlighting their input ranges, SPI interface, reference voltage connections, and power supplies.

2.3.1.1 16-Bit ADC ADS8688

The ADS8688 is an 8-channel, 16-bit SAR ADC operating at 500 kSPS. It features an integrated analog front-end with overvoltage protection (up to ±20 V), an 8-channel multiplexer, and a 4.096V on-chip reference. It supports bipolar input ranges of ±10.24 V, ±5.12 V, and ±2.56 V, with software-programmable input ranges. It offers a 1-MΩ input impedance and a simple SPI interface, supporting daisy-chaining.

2.3.1.2 16-Bit ADC ADS8688A With Alarm Output

The ADS8688A is similar to the ADS8688 but includes an alarm output and supports additional input ranges: ±1.28 V and ±0.64 V.

2.3.1.3 Low-Noise, Low-Drift, High-Precision Reference REF5040

The REF5040 is a high-precision voltage reference with low noise and low drift, offering excellent line and load regulation and temperature stability (3 ppm/°C).

2.3.1.4 High-Precision Voltage Reference With Integrated High-Bandwidth Buffer REF6041

The REF6041 provides a voltage reference with an integrated buffer, essential for driving the REF pin of precision ADCs while maintaining linearity, distortion, and noise performance.

2.3.2 Interface Isolation Using Digital Isolator With Integrated Power ISOW7841 Family

This section discusses digital isolator options for AC AIM designs, focusing on the ISOW7841 family.

2.3.2.1 Digital Isolator With Integrated Power ISOW7841

The ISOW7841 is a quad-channel reinforced digital isolator with an integrated high-efficiency power converter (up to 650 mW), eliminating the need for a separate isolated power supply in space-constrained designs.

2.3.2.2 Digital Isolator ISO7820

The ISO7820 is a dual-channel digital isolator with 8000-VPK isolation voltage, offering high electromagnetic immunity and low emissions. It provides default high/low outputs on signal loss.

2.3.3 Interface Isolation Using Digital Isolator ISO7763 Family

The ISO7763 family offers six-channel digital isolators with 5000-VRMS isolation, high electromagnetic immunity, and low emissions.

2.3.4 Power Supply

The ISOW7841 can generate the required isolated power supply. Alternatively, a digital isolator combined with a transformer driver can provide isolated power.

2.3.5 Diagnostics and Protection

This section details the diagnostics and protection features implemented in the design.

2.3.5.1 Load Switch TPS22944

The TPS22944 load switch protects systems and loads from overcurrent conditions using a 0.4Ω P-channel MOSFET. It includes thermal shutdown protection.

2.3.5.2 Power-on Reset TPS3836K33

The TPS3836 family provides circuit initialization and timing supervision, asserting RESET when VDD exceeds a threshold.

2.3.6 Board Layout With ISOW7841

The ISOW7841 simplifies design and reduces board area. Its high-frequency switching may increase radiated emissions, but on-chip techniques help mitigate this.

2.3.7 Design Enhancements

2.3.7.1 Non-Simultaneous Sampling ADC Phase Compensation

The design uses a 16-bit multiplexed SAR ADC. Phase delay between channels, inherent in non-simultaneous sampling, is compensated in software.

2.3.7.2 Multiplexed ADS86xx ADC Selection

Measurement accuracy can be improved using precision gain amplifiers or higher resolution ADCs like the ADS8698 (18-bit SAR ADC). Table 3 lists suitable SAR ADCs.

2.3.7.3 Gain Amplifier Selection

Gain amplifiers scale low sensor outputs to improve measurement accuracy and dynamic range. Table 4 lists recommended op amps.

2.3.7.4 Dual Output Power Supplies for Gain Amplifier

Gain amplifiers require dual power supplies. Table 5 lists suitable DC/DC converters and LDOs.

2.3.7.5 Isolated Interface Approaches and Advantages

Two primary approaches for isolated power and data interface are presented:

2.3.7.5.1 Isolated Interface With Transformer Driver and Digital Isolator

This approach requires multiple components: a digital isolator, transformer driver, isolation transformer, and LDO. Table 6 lists various digital isolator families.

2.3.7.5.2 Isolated Interface Using ISOW7841

This integrated solution simplifies design and reduces cost. It is recommended when the interface matches the ISOW784x family. Figure 8 illustrates this integrated approach.

2.3.8 Achieving Higher Output Efficiency With Transformer Driver SN6505B

Transformer drivers like SN6505B offer higher efficiency (60-75%) compared to integrated power solutions. Table 7 details efficiency at various output currents.

2.3.9 Achieving Higher Efficiency With DC/DC Converters

Table 8 lists DC/DC converters that can generate efficient split-rail outputs for AC AIMs.

2.3.10 Interface With High-Precision ADCs With Serial Interface

Digital isolators like ISOW7841 or ISO7741 interface with ADCs via SPI. Table 9 provides ADC interface options.

2.3.10.1 Voltage Supervisor Selection and Options

External programmable-delay supervisory circuits are recommended for overload conditions. Table 10 lists voltage supervisor options.

2.3.11 ADC Current Consumption

Table 11 summarizes the current consumption of various ADCs, indicating the ISOW7841's efficiency in powering them.

2.3.12 Load Switch Selection

The TPS22944 load switch, along with the ISOW7841, protects against overloads. Table 12 lists load switch selection options.

2.3.13 ADC Measurement Performance With Higher Clock Frequency and Digital Isolator

In isolated AC AIMs, SCLK delay across the isolation barrier can affect performance. Rerouting SCLK (SCLK_RET) mitigates skew. Figure 9 illustrates the ADS8681 interface using multiple digital isolators.

2.3.14 Temperature Compensation of Measured Analog Input for Improved Measurement Accuracy

The auxiliary ADC channel can measure ambient temperature for real-time compensation. Two approaches are available: onboard sensors (Table 15) and remote sensors like RTDs (Table 16).

2.3.15 Improving Analog Input Measurement Accuracy Using 18-Bit ADC

Accuracy is enhanced by using precision gain amplifiers or higher resolution ADCs like the ADS8698.

2.3.16 Design of Wide Input AC or DC Digital, Contact, or Binary Input Module

Reference designs like TIDA-00847 showcase DC BIMs. The ADS8668 can implement 16-channel isolated DC BIMs.

2.3.17 DC Transducer Input Module With Unidirectional or Bidirectional Signal Input

This module measures unipolar/bipolar inputs (e.g., 0-20mA, ±10V) and can be simplified with integrated digital isolators and power converters.

Hardware, Testing Requirements, and Test Results

3.1 Required Hardware

This section describes the setup for performance testing of the AC AIM.

3.1.1 AC AIM

Interface connectors for power, host, and analog inputs are detailed.

3.1.1.1 16-Channel ADS8688A and ISOW7763-Based AC AIM

Table 17 lists the connectors used for input and power supply evaluation.

3.1.1.2 PHI Board Connections

The PHI board facilitates quick ADC performance evaluation and connects via a breakout board. Table 18 details PHI board connections for daisy chain configuration, and Table 19 for dual SDO output.

3.2 Testing and Results

3.2.1 Test Setup

Figure 11 illustrates the test setup, comprising a DC power supply (3.3V), the TIDA-01576 board, a function generator for AC inputs (±10.24V max), a PHI breakout board, and a PHI card/GUI for HMI and data capture. Analog inputs must not exceed ±10.24V.

Diagram Description: A test setup diagram showing connections from a DC power supply, function generator, and external voltage/current sources to the TIDA-01576 board, with interfaces to a PHI module and GUI.

3.2.2 Test Results

Test conditions include using a function generator or programmable AC source, with a GUI for evaluation.

3.2.2.1 Functional Testing

Table 20 summarizes the functional tests performed, confirming OK status for various parameters like isolated supply output, digital isolator functionality, ADC input measurements, frequency measurement, alarm function, and host interface.

3.2.2.2 ADC Performance

This section details performance tests and results analyzed using a custom GUI. Figures 12 and 13 show Time Domain and Spectral Analysis.

3.2.2.2.1 ADS8688A Time Domain Measurement Performance for Dual SDO With Isolated Interface

Figure 12 displays the time domain analysis from the GUI, showing waveforms and RMS values.

3.2.2.2.2 ADS8688A Time Domain Measurement Performance for Dual SDO With Isolated Interface

Figure 13 presents spectral analysis results for DUT A and DUT B.

3.2.2.2.3 ADS8688A Measurement Performance for Dual SDO With Isolated Interface

Tables 21 and 22 provide detailed performance test results for the ADS8688A ADC across different input ranges and boards.

3.2.2.2.4 Measurement With DUTB Configured as External Reference

Table 23 shows test results when DUT B uses an external reference, demonstrating that the AIM meets performance requirements.

3.2.2.2.5 Measurement With Non-Isolated Interface and Daisy

Table 24 presents performance results for a non-isolated interface and daisy chain configuration.

3.2.2.2.6 Measurement With ADC Interfaced to ISOW7841

Table 25 details performance test results for the ADS8688 interfaced with the ISOW7841 digital isolator.

3.2.2.2.7 Voltage Measurement

Table 26 shows RMS values for analog inputs calculated using the GUI.

3.2.2.2.8 ADC Measurement Accuracy

Table 27 details measurement errors observed for different input voltages, indicating accuracy within ±0.2%.

3.2.2.3 ADS8688A Synchronization Testing and Results

This section covers the synchronization test procedure and results.

3.2.2.3.1 Synchronization Procedure

The procedure corrects phase shift during multiplexed sampling. It involves calculating theoretical phase difference, estimating signal frequency, calculating phase angles, and compensating for the difference.

3.2.2.3.2 Synchronization of 16 Analog Input Channels

Two ADCs can provide 16 channels via daisy chaining (one SDO) or separate SDOs for increased throughput. Phase shift compensation details can be found in related documents.

3.2.2.3.3 GUI for Synchronization

A GUI is available for performance analysis, including phase analysis shown in Figure 14.

Diagram Description: A screenshot of the GUI displaying phase analysis results with and without compensation, using DUT A Channel 0 as a reference.

3.2.2.3.4 Synchronization Test Results for ADCs Connected in Daisy Chain

Table 28 provides phase error results for ADCs in a daisy chain configuration relative to DUT A channel 0.

3.2.2.3.5 Synchronization Test Results for ADCs With Dual SDO Output

Table 29 presents synchronization test results for ADCs configured for dual SDO output.

3.2.2.4 ISOW7841 Isolated Power Supply Testing

3.2.2.4.1 ISOW7841 Load Regulation Testing

Table 30 shows load regulation test results, indicating load regulation is less than ±1%.

3.2.2.4.2 ISOW7841 Line Regulation (Input versus Output Voltage Variation) Testing

Table 31 details line regulation results, showing line regulation is less than ±3 mV/V.

3.2.2.4.3 ISOW7841 Ripple Measurement

Figure 15 illustrates the DC output ripple measurement for the isolated supply.

3.2.2.4.4 ISOW7841 Input Switching Current

Figure 16 shows the input switching current measured for a DC input current of 160 mA.

3.2.2.4.5 ISOW7841 Device Hotspot Monitoring

Figure 17 displays hotspot measurements on the ISOW784x evaluation module under load.

3.2.2.5 Test Results Summary for AC AIM

Table 32 summarizes the tests and observations for the AC AIM, confirming satisfactory performance across various aspects.

Design Files and Related Documentation

4 Design Files

Schematics, Bill of Materials (BOM), PCB Layout Recommendations, Altium Project, Gerber Files, and Assembly Drawings are available for download at TIDA-01576.

5 Related Documentation

Additional Texas Instruments design guides are listed, covering topics such as high-voltage data acquisition systems, isolator optimization, binary input modules, low-emission designs, and high-accuracy AIM reference designs.

5.1 Trademarks

E2E and multiSPI are trademarks of Texas Instruments. All other trademarks are the property of their respective owners.

Terminology

• AC: Alternating current
• AIM: Analog input module
• DC: Direct current
• PHI: Precision host interface
• RMS: Root mean square
• RTD: Resistance temperature detectors

About the Authors

KALLIKUPPA MUNIYAPPA SREENIVASA is a systems architect at Texas Instruments, responsible for developing reference design solutions for the industrial segment. He has experience in high-speed digital and analog systems design.

AMIT KUMBASI is a systems architect at Texas Instruments Dallas, focusing on subsystem solutions for Grid Infrastructure. He has expertise in product definition, business development, and board-level design using precision analog and mixed-signal devices.

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