基于硬件的智能 DAC 医用警报设计

本资源的原文使用英文撰写。为方便起见，TI 提供了译文；由于翻译过程中可能使用了自动化工具，TI 不保证译文的准确性。为确认准确性，请务必访问 ti.com 参考最新的英文版本（控制文档）。

Introduction

This application brief details an IEC60601-1-8 medical alarm design using the 10-bit, DAC53701 smart digital-to-analog converter (DAC), and shares primary alarm compliance testing data. The proposed design provides all the required components for a medical alarm subsystem and implements key features such as the primary alarm, visual alarm, and backup alarm (supplied by a supercapacitor). Figure 1 shows the IEC standard requirements that the design incorporates and the additional features implemented. The prototype board shown in Figure 1 stacks onto an MSP430 launchpad, allowing for the MSP430 microcontroller (MCU) to demonstrate control of the primary medical alarm.

Figure 1 Medical Alarm Design using DAC53701 Smart DAC

Design

Figure 2 shows the block diagram for this design. The MCU triggers the primary alarm high, medium, or low state. The audio waveform is then generated by the DAC53701 devices, along with the buffering op-amp (OPA363). DAC 1 is used to create the alarm envelope and burst, while DAC 2 is used to create a waveform of the desired pulse frequency. The enable pin of the op-amp is repeatedly toggled on and off by the pulsed output of DAC 2. The input to this buffering op-amp is the DAC 1 envelope and burst. As the DAC 1 output passes through the op-amp, the DAC 2 toggling enable operation overlays the proper audio frequency. A digital potentiometer (digipot) is then used to scale the audio signal for volume control. Finally, the audio signal is sent to the audio amplifier to play on the speaker. For example, Figure 3 shows the high alarm state audio signal waveform.

Figure 2 Block Diagram

Note that the DAC53701 is capable of producing the primary medical alarm even when the MCU is powered down or not present. The IEC compliant medical alarm tones are stored in the DAC53701’s nonvolatile memory. The general purpose input (GPI) pin on the DAC53701 can be used to trigger the alarm without requiring an MCU.

Another important aspect of an alarm system is the backup alarm. This alarm is typically a buzzer that is powered by a supercapacitor or battery backup. The backup alarm is triggered by a variety of cases. The first case is when the MCU or processor stops responding. In this case the watchdog timer triggers the alarm. The second case is when there is a loss of power, leading to the backup alarm being powered from the supercapacitor backup circuit. The third case is when an external coincidence detection circuit monitoring for primary alarm speaker faults triggers the alarm. If any of these conditions are met, the NAND gate output engages the backup alarm. The alarm continues to play for more than 3 minutes, or until the alarm is disabled.

Figure 3 High Alarm State Waveform

Primary Alarm Compliance Testing

To meet the IEC6060-1-8 requirements, the primary medical alarm must follow specific harmonic requirements. Specifically, at least 4 harmonics must be ±15 dB from the amplitude of the fundamental frequency. These harmonics allow for enhanced spacial localization of the alarm sound. Following the IEC testing requirements this design was internally tested to meet these harmonic requirements using a variety of speakers and microphone measurement locations. Figure 4 shows an example of measured harmonic content for the high alarm state. More than 6 harmonics are measured within the required range for this high alarm state test.

Figure 4 Harmonic Test High Alarm State

Conclusion

The design presented provides a full hardware based implementation of a medical alarm system including the primary alarm, backup alarm, and visual alarm. Features including a software free implementation and supercapacitor backup functionality make this design compelling for medical alarm systems.

References

Texas Instruments, Demystifying medical alarm designs, part 1: IEC60601-1-8 standard requirements, blog.
Texas Instruments, Demystifying medical alarm design, part 2: Design inputs and existing techniques, blog.
Texas Instruments, Demystifying Medical Alarm Designs With Smart DACs, application brief.
Texas Instruments, MSPM0-Based Medical Alarm Design, application brief.
Texas Instruments, DACx3701 10-Bit and 8-Bit, Voltage-Output Smart DACs With Nonvolatile Memory and PMBus™ Compatible I2C Interface With GPI Control, data sheet.
Texas Instruments, TPS61094 60-nA Quiescent Current Boost Converter with Supercap Management, data sheet.
Texas Instruments, TPA6211A1 3.1-W Mono Fully Differential Audio Power Amplifier, data sheet.

重要声明和免责声明

TI 均以“原样”提供技术性及可靠性数据（包括数据表）、设计资源（包括参考设计）、应用或其他设计建议、网络工具、安全信息和其他资源，不保证其中不含任何瑕疵，且不做任何明示或暗示的担保，包括但不限于对适销性、适合某特定用途或不侵犯任何第三方知识产权的暗示担保。

所述资源可供专业开发人员应用TI 产品进行设计使用。您将对以下行为独自承担全部责任：(1) 针对您的应用选择合适的TI 产品；(2) 设计、验证并测试您的应用；(3) 确保您的应用满足相应标准以及任何其他安全、安保或其他要求。所述资源如有变更，恕不另行通知。TI 对您使用所述资源的授权仅限于开发资源所涉及TI 产品的相关应用。除此之外不得复制或展示所述资源，也不提供其它TI或任何第三方的知识产权授权许可。如因使用所述资源而产生任何索赔、赔偿、成本、损失及债务等，TI对此概不负责，并且您须赔偿由此对TI 及其代表造成的损害。

TI 所提供产品均受TI 的销售条款 (http://www.ti.com.cn/zh-cn/legal/termsofsale.html) 以及ti.com.cn上或随附TI产品提供的其他可适用条款的约束。TI提供所述资源并不扩展或以其他方式更改TI 针对TI 产品所发布的可适用的担保范围或担保免责声明。IMPORTANT NOTICE

邮寄地址：上海市浦东新区世纪大道 1568 号中建大厦 32 楼，邮政编码：200122