Medical Equipment Design

By John Di Cristina, Director of Strategic Marketing, Maxim Integrated Products Inc., Chelmsford, MA, USA


Operation and design considerations for a digital stethoscope

A stethoscope, whether acoustic or digital, is used mainly to listen to heart and lung sounds in the body as an aid to diagnosis. Listening, or auscultation, has been done with acoustic stethoscopes for almost two hundred years, but recently electronic digital stethoscopes have been developed and adopted. The goal of a basic digital stethoscope is to have it retain the look and feel of an acoustic stethoscope but to improve listening performance. In addition, high-end digital stethoscopes offer sophisticated capabilities such as audio recording and playback. They also provide data to visually chart results by connecting to an off-instrument display such as a computer monitor. This advanced functionality increases the physician's diagnostic capability. Maintaining the existing acoustic stethoscope form (i.e., that "look and feel") while improving the performance digitally requires the use of small, low-power solutions. Power management: Key to long use Most digital stethoscopes use either one or two AAA 1.5V primary batteries. This design requires a step-up, or boost, switching regulator to increase the voltage to 3.0V or 5.0V, depending on the circuitry utilized. If a single 1.5V battery is installed, the switching regulator will probably be on all the time, making low quiescent current a critical factor for long battery life. The longer the battery life, the more convenient the digital stethoscope is to use and the closer the experience will be to an acoustic stethoscope. When using two 1.5V batteries in series, the switching regulator can be left on all the time or shut down when not in use. If the circuit operates from 3.6V down to 1.8V, then a switching regulator may not be needed. Cost will be reduced and space saved. A low-battery warning is required so that a patient's examination need not be interrupted to replace the battery. In the stethoscope, rechargeable batteries can be used; the best choice is a single-cell Li+ battery. If a rechargeable battery is used, a battery charger is required either in the digital stethoscope or in a charging cradle. The charger can be built around a chip such as the MAX8900A or B, or if the battery charger can charge from the USB interface, a device such as the MAX1811 can be used. A fuel gauge such as the MAX17043 or 044 is a good solution to accurately determine the remaining battery life. If the battery is removable, then battery authentication is also required for safety and aftermarket management. For example, the DS2784 includes a SHA-1 encryption capability for cell authentication. The authentication is used by the stethoscope vendor to prevent counterfeit batteries from damaging the stethoscope. Audio signal path The essential elements of a digital stethoscope are the sound transducer, the audio codec electronics, and the speakers (Figure 1). The sound transducer, which converts sound into an analog voltage, is the most critical piece in the chain. It determines the diagnostic quality of the digital stethoscope and ensures a familiar user experience to those accustomed to acoustic stethoscopes. The analog voltage needs to be conditioned and then converted into a digital signal using an audio analog-to-digital converter (ADC) or audio codec. Some digital stethoscopes have noise cancellation that requires a secondary sound transducer or microphone to record the ambient noise so that it can be removed digitally. In this approach, two audio ADCs are required.

Once in the digital domain, a low-power microcontroller such as the MAXQ2000 or MAXQ2010, and/or a digital-signal processor (DSP) performs signal processing, including ambient noise reduction and filtering, to limit the bandwidth to the range for cardiac or pulmonary listening. The processed digital signal is then converted back to analog by a low-power audio digital-to-analog converter (DAC) or audio codec such as the MAX9856 or similar device. A headphone or speaker amplifier conditions the audio signal before outputting to a speaker. A single speaker can be used below where the stethoscope tube bifurcates, with the amplified sound traveling through the binaural tubes to the ears. Alternatively, two speakers can be used, with one speaker at the end of each earpiece. The frequency response of the speaker is similar to that of a bass speaker because of the low-frequency sound production needed. Depending on the implementation, one or two speaker amplifiers are used. A stethoscope must be most sensitive to cardiac sound in the 20Hz to 400Hz range and to pulmonary sound in the 100Hz to 1200Hz range. Note that the frequency ranges vary by manufacturer, and the DSP algorithms filter out sound beyond these optimal ranges. Data storage and transfer Once the captured sound is converted to an analog voltage, it can be sent out through an audio jack and played back on either a computer or through the digital stethoscope. The captured sound can also be manipulated digitally. It can be stored in the stethoscope using internal or removable nonvolatile (NV) memory like EEPROM or flash, and then played back through the stethoscope's speakers; or it can be transferred to a computer for further analysis. Adding a real-time clock (RTC) facilitates tagging the recording with time and date. The captured sound can also be transferred to a host system with a wired interface, such as USB, or with a wireless interface like Bluetooth® or another proprietary wireless interface. Display and backlighting Some digital stethoscopes have a small, simple LCD display due to the limited space available; others have only buttons and LED indicators. Backlighting for the LCD is required because the ambient lighting during medical procedure is often at a low level. The small display requires just one or two white light-emitting diodes (WLEDs) controlled by an LED driver. Alternately, an electroluminescent (EL) panel controlled by an EL driver or other display technology can be used. With the growing popularity of touch-screen technology and the lower associated costs, many of the user-interface buttons can be eliminated by using a touch-screen display and controller.