Medical IC design process: bipolar and CMOS

Implantable, digestible, interactive, interoperable, and supporting the Internet, these medical devices' unique needs now and in the future require suitable IC process technology and packaging. This article compares the bipolar and CMOS processes used in medical semiconductor devices, and explains some packaging issues that require special attention.

Developers of medical applications must balance power consumption, noise, linearity, reliability, and cost, and need to carefully select the process and design architecture based on these requirements.

This article will compare bipolar devices with CMOS devices to help users judge the suitability of each device. This article will use high-performance ultrasonic equipment as an example to discuss how to balance noise, power consumption, chip footprint and integration.

Power consumption is very important in many battery-powered applications. In such applications, the CMOS process is an excellent choice. However, the balance between leakage and performance is also critical and determines the choice of technology. In addition, in such applications, mixed signal integration is also an important requirement.

Efficient use of some packaging technology can meet the needs of implementing a large number of functions in a single integrated circuit, such as when supporting dense digital functions while requiring low noise. Such contradictory requirements can sometimes be easily satisfied with multi-chip modules.

This article will also discuss the future development trend of medical equipment, including direct measurement of biological signals and self-powered equipment. These trends will drive improvements in existing process technologies to meet energy harvesting characteristics and other non-standard sensor functions.

Simulation performance

First, take the ultrasonic equipment as an example to discuss the simulation performance requirements. Through this example, this article will introduce how to balance the performance, power consumption, size and integration, and test the applicability of bipolar and CMOS process technology. Figure 1 is a system block diagram of a typical ultrasonic machine, showing the two parts of transmission and reception. These two parts are responsible for driving the sensors and the digital processing part (not shown), thus forming a complete ultrasonic device.

Figure 1: Block diagram of an ultrasound system

Issues to be considered when designing this type of receiving module include input noise, linearity, gain, and power consumption. The number of receiving channels for a given package size determines the degree of integration. The signal received from the sensor can support amplitude changes of more than 100dB. Therefore, the input noise on the low-level signal (about 10uV) and the linearity of the large input signal (about 1V) are very important performance parameters. To adapt to this large dynamic range, the channel gain can be adjusted through a voltage controlled attenuator (VCA) and programmable gain amplifier (PGA). Figure 3 shows how the overall gain through the device changes with the voltage on the VCA under several PGA settings.

Figure 2: Detailed block diagram of the part of Figure 1 that performs the receiving function

Figure 3: Curve of receiver module gain with voltage control

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