Medical equipment uses sensors to monitor body condition that transform physical signals into electrical signals that will be converted to the digital world. The signal-path design in medical applications is critical due to the low magnitude of the signals and the presence of many sources of noise. This article will discuss, through the example of an ECG (electrocardiogram), how to build the sensor interface with a suitable amplifier from the PowerWise family in order to increase battery life and patient security.
Principle of the ECG
An ECG is a recording of the heart's electrical activity over time. This signal is measured by connecting three small electrodes to the human body and is characterised by five specific points - P, Q, R, S and T - that allow diagnosing possible heart diseases.
Signal conditioning of an ECG
The ECG signal can range from a 400µV to 5mV peak, with 3dB corner frequencies at 0.05 and 100Hz. This signal is generally disturbed by interference, such as electrode contact noise, power-line noise (50Hz), respiration, muscle activity, and interference from other electronics devices. To reject DC noise, high-pass filters can be implemented. The main concern is the 50Hz noise, which is in the same frequency range as the signal of interest. To eliminate this common-mode noise, it is useful to build an instrumentation amplifier. This configuration rejects common-mode voltage while amplifying differential voltage, which allows separation of the low-level signal from the background noise. As illustrated in Figure 1, the instrumentation amplifier is implemented using the LMP2234 quad micropower, precision, RRO, operational amplifier and precision resistors, featuring 0.1% precision.
Figure 1: A block diagram of an instrumentation amplifier.
The LMP2234, which is part of the LMP precision amplifier family, is built on VIP50 process technology, a silicon-on-insulator BiCMOS process. This amplifier is designed for battery-powered applications. The 1.8 to 5.5V operating-voltage range and quiescent-current supply of 36µA help extend battery life in portable systems. The high-impedance CMOS input makes the amplifier suitable for instrumentation and other kinds of sensor-interface applications.
Due to the very low amplitude of the signal coming from the electrodes, the DC parameters of the amplifier are very important. The LMP2234 has a maximum offset voltage of 150μV (10μV typical) and 0.3 μV/°C offset drift along with a low bias current of only ±20fA.
There are two stages in this instrumentation amplifier. The output stage is a differential amplifier that has the ability to reject dc levels, interference and noise voltage common to both inputs. The two amplifiers of the first stage are configured as buffers to isolate the inputs. The product of the two stages of gain will yield the gain of the instrumentation amplifier.
The output voltage is defined as Vout = (Vin2 -Vin1) x (1 + 2 x R3:R4) (R8/R6).
The maximum amplitude of the input signal is 5mV, but to set up the gain we have to consider the electrode DC offset that can be up to ±300mV. The gain will be set to 5 to match the output range of the amplifier, which is supplied with 3.3V. The LMP2234 has a rail-to-rail output that swings 15mV from the supply voltage, which increases system dynamic range.
Figure 2: An ECG-system block diagram.
As shown in Figure 2, a high-pass filter is then implemented in order to suppress the DC components, which can cause saturation in the next gain stage. The cut-off frequency for the high-pass filter is 0.5Hz. The filter is implemented using the second-order Sallen Key Butterworth topology. The next stage is a low-pass filter with a cut-off frequency of 100Hz and a gain of 100, also implemented with the Sallen Key topology. Analogue active filters such as Sallen Key are built around operational amplifiers with resistors and capacitors.
Figure 3: HP and LP filters.
Both filters presented in Figure 3 are implemented with the LMV552 low-power operational amplifiers, which utilises National's advanced VIP50 process. They feature 3 MHz of bandwidth while consuming only 34μA of current per amplifier, which is an exceptional bandwidth-to-power ratio in this op amp class. The LMV552 features a rail-to-rail output stage and an input common-mode range that extends below ground and has an operating supply-voltage range from 2.7 to 5.5V.
For reasons of patient security, isolation is needed; this can be achieved through galvanic isolation, photo/opto-coupling, capacitive coupling or magnetic coupling. This article is focused on the ECG front-end, but when developing a medical-electronic system, all safety standards must be respected.
