Discussion on the Technology of Analog Signal Path in Portable Medical Equipment

With the development and continuous improvement of the monitoring function of medical equipment, telemedicine providers can provide better diagnostic tools for home patients, emergency room paramedics and hospitals in several important areas of human health. Sphygmomanometers, blood glucose meters, defibrillators and other monitoring instruments need clear analog signals for accurate measurement, otherwise they may be life-threatening. The excellent design of analog signal path can help designers reduce the interference of external noise, expand the dynamic range and enhance the accuracy. In addition, in terms of component selection, designers must also choose carefully to meet the performance requirements of the final product.

High performance requirements in small packages

Previously, it was generally believed that medical equipment in hospitals and clinics was more accurate than portable instruments used at home. However, new technological trends are rapidly reversing this view. The users of the new portable medical devices are not only ordinary consumers, but also patients with deep scientific and technological understanding. Therefore, the needs of customers are no longer limited to taking body temperature, ECG and blood pressure. What customers need is a full range of nursing and measurement functions.

In order to meet people's urgent demand for home medical diagnostic instruments, equipment suppliers are relying on advanced inventory management and innovative design to enhance market competitiveness, and equip products with more functions to win more users. In the field of developing home medical instruments, one factor is very important, which is the development time required from the initial design to the real launch of the product. Shortening the time to market can enable manufacturers' products to seize the market. Whether the development cycle can be shortened depends on whether the design of system designers is flexible and cost-effective.

Process technology affects system design

Although electrical specifications are the main factor for designers to select components, the process used to manufacture integrated circuits is also important. For example, a typical blood glucose meter usually needs to be equipped with an operational amplifier with very low input bias current. Most designers will choose JFET amplifier. However, they should consider the temperature before making a decision.

Because JFET has a very low initial input bias, it is vulnerable to temperature changes. For every 10 C rise, the input bias will approximately double. To calculate the drift of the input bias, use the following formula (reference 1).

Ib(T)Ib(T0) x 2(T-T0)/10

For example, a JFET Input Operational Amplifier (such as National Semiconductor's lf411) has an input bias current of 50pA at 25 C, while a better option is National Semiconductor's lmp7731, which is a bipolar input operational amplifier with an input bias current of 1.5na. Through the above formula, we can quickly calculate that the input bias current of lf411 becomes 3.2na at 85 C, which is twice that of lmp7731.

Trade off of evaluation system

Speed, noise and power consumption may be equally important for some designs. A low-noise device will consume more current, while a low-power device can only provide limited bandwidth. One way to overcome these problems is to use inverse compensation amplifiers in appropriate applications. Compared with the stable unit gain and high speed, the advantage of the anti compensation amplifier is that it can provide a large bandwidth without affecting the power consumption.

Inverse compensation operational amplifier is most suitable for current voltage conversion (transimpedance) circuit. In medical instruments, one of the most common applications is to measure the oxygen content in blood cells, which is called SpO2 or saturated or peripheral oxygen. Figure 1 shows the block diagram of SpO2 module, in which the anti compensation amplifier (TIA) is used to convert the current from the photodiode into voltage.

Figure 1 typical block diagram of SpO2 module

Use shortcuts to shorten design time

The most important parameter of medical instruments is noise, which can cause serious interference to the circuit itself and nearby equipment. Calculating noise is a tedious task, especially when you want to calculate the overall impact of signal path on signal-to-noise ratio from power supply, amplifier, data converter and external components.

Generally speaking, medical instrument circuits tend to work at a lower frequency, so the designers of these systems usually pay more attention to the noise in the frequency band of 0.1 to 10Hz, also known as peak to peak noise. Unfortunately, some data sheets do not provide time domain noise (peak to peak) values, but only typical charts of voltage or current noise density. In addition to waiting for the circuit supplier to provide measurement data, there is a fast method to help calculate the peak to peak noise.

Suppose you intend to use lmp7731 of national semiconductor to calculate the noise of peak to peak (0.1 to 10Hz) voltage. First, select a point in the frequency range within the specified frequency band, such as 1Hz, and the value in the comparison curve is 5.1nv / Hz (Fig. 2). Then use the following formula to calculate the root mean square value (RMS) of noise: Formula 1: ENRMS = "enf" ln (10 / 0.1), Where enf is the noise at 1Hz. The total root mean square noise of 10.9nv can be obtained through the above formula. If you want to calculate the peak to peak noise, you only need to multiply this root mean square value by 6.6 to obtain 72.2nv. The result of this estimation is quite good, which is close to the specification 78nv listed in the data sheet.

Fig. 2 Relationship between input voltage noise and frequency of lmp7731 frequency and voltage noise

If the voltage noise density diagram in the data sheet does not represent the noise value at 1Hz, you can use the following simple equation (equation 2) to calculate the value at a certain frequency.

Where ENB is broadband noise (usually the value at 1kHz), and FCE is the 1 / F inflection point. As for F is the frequency of interest, in our case, it is 1Hz.