EMI Robust Signal Path for Medical Diagnostic Equipment
29 March 2010
Controlling EMI is critical in a medical environments, but becoming more tricky as more portable electronics finds its way into the hospitals. Carine Alberti discusses how to safely get round these issues, using the development of an ECG machine as an example.
In a hospital environment many types of diagnostic and clinical equipment are used to monitor and control patients’ health. Due to the fact that increasing numbers of electronic devices are transmitting data, the electromagnetic interference (EMI) between the different pieces of equipment can cause malfunctioning of the medical devices, and this is becoming a serious concern. This article will describe the general sources of EMI and the issues they are causing. Using the example of an electrocardiogram (ECG) system it will then be demonstrated how the new LMP2021 EMI hardened operational amplifier can be used to design a robust signal path.
Causes of Electromagnetic Interference
In a medical environment electromagnetic interference can be caused by a visitor’s mobile phone for example, or by the radio transmitters used by an ambulance. Other interference effects can be due to patients’ electric razors or the call systems for the nurses. Furthermore, devices used by the medical team such as cell phones, walkie-talkies, personal digital assistants, and two way pagers can be a source of emissions. Finally the radiating medical instruments such as X-Ray machines, CAT (Computer Axial Tomography) scanners and MRI (Magnetic Resonance Imaging) machines are also potential sources of electromagnetic interference.
Effects of Electromagnetic Interference
EMI can cause medical equipment to fail with more or less severe consequences. It can cause loss of control or create errors in a patient’s data readings such as pulse rate or gas concentration. Even more dangerously it can produce errors in the amount of medicine administered to a patient by an infusion system, for instance.
To avoid such concerns the medical equipment is tested for susceptibility and has to comply with various standards. Common methods of preventing the harmful effects of electromagnetic interference are cable shielding and filtering. In addition regulations are enforced in medical environments to reduce the risk of EMI problems such as checking if the equipment is compliant to standards or restricting the use of mobile phones or other wireless equipment.
To further protect the patient, EMI hardened components should be used to design the signal path. This will be illustrated using the example of an electrocardiogram (ECG) signal conditioning system.
Signal Conditioning of an ECG
An electrocardiogram is a recording of the heart’s electrical activity over time. This signal is measured by connecting three small electrodes to the human body. An ECG signal is presented in Figure 1; it is characterized by five specific points P, Q, R, S, T which permit the diagnosis of possible heart diseases.
The signal collected through the electrodes can range from a 400µV to 5mV peak, with 3dB corner frequencies at 0.05Hz and 100Hz. Due to its very low amplitude this signal is generally disrupted by various types of interference such as electrode contact noise, power-line noise (50Hz), respiration, muscle activity, and as previously mentioned, interference from other electronic devices. This implies that the signal path has to deal with different sources of noise. 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 the doctors. The use of an instrumentation amplifier has proven highly effective in eliminating common mode noise. Indeed, this configuration is ideal because it rejects common mode voltage while amplifying differential voltages, which allows the separation of the low level signal from the background noise.
As illustrated in Figure 2, the instrumentation amplifier is implemented using the LMP2021 which is a precision operational amplifier offering ultra low input offset voltage (0.4μV typical) and near zero input offset voltage drift (0.004μV/°C), very low input voltage noise and very high open loop gain. The proprietary continuous correction circuitry enables impressive CMRR and PSRR performance and removes the flicker noise component. This auto correction eliminates the need for calibration in many circuits. This amplifier has only 260nVpp (0.1Hz to 10Hz) of input voltage noise and no 1/f noise component. This makes it suitable for low frequency applications such as non-invasive low frequency medical instrumentation applications.
In the signal path, the part the most sensitive to EMI is the interface between the sensor and the operational amplifier. In this section the signal is analogue, with very low amplitudes and the wires are long, resulting in an increased susceptibility to interference. The interface between the operational amplifier and the analogue to digital converter (ADC) is less sensitive, as the signal has higher amplitude levels due to the amplification and the wires are generally shorter.
As the critical point is the interface between the sensor and the operational amplifier, National Semiconductor has introduced product solutions with integrated electromagnetic interference (EMI) filters that maintain the accuracy of analogue systems by reducing the effects of radio frequency (RF) interference.
To allow for the characterisation of EMI hardened operational amplifiers, a new parameter in the datasheet is needed that provides a quantitative description of the EMI performance of op amps. This quantitative measure shows how well the operational amplifier rejects the EMI, and enables the comparison and also ranking of op amps based on their EMI robustness and performance. Much like CMRR, EMI Rejection Ratio, EMIRR, is specified as the ratio of the change in an applied RF signal to the resulting change in the offset voltage.
With the increased use of wireless data transmission, the presence of cellular phones, Bluetooth modules and other computer peripherals, EMI is a growing concern in many medical applications. The LMP2021 uses an on chip filter to reject unwanted signals such as RF injections at the inputs of the amplifier, thereby preventing the high frequency noise from propagating through the amplifier and to the circuit. The use of the integrated EMI filter offers several advantages and helps maintain signal integrity.
In addition it saves space on the PC board by avoiding the use of external filtering which also adds to the cost.
Figure 4 shows the block diagram of the complete signal path. Filters are then implemented to suppress the unwanted signals. The analogue high and low pass active filters can be implemented using the Sallen-Key topology, built around operational amplifiers with resistors and capacitors.
The analogue signal is then converted to digital with the ADC161S626 a 16 bit successive-approximation register analogue to digital converter from the PowerWise family. The ADC161S626 has a minimum signal span accuracy of ±0.003% over the temperate range of −40°C to +85°C. The converter features a differential analog input with an excellent common-mode signal rejection ratio of 85dB, making the ADC161S626 suitable for noisy environments.
The data converter operates with a single analogue supply (VA) and a separate digital input/output (VIO) supply. VA can range from +4.5V to +5.5V and VIO can range from +2.7V to +5.5V. This allows a system designer to maximise performance and minimise power consumption by operating the analogue portion of the ADC at a VA of +5V while interfacing with a +3.3V controller.
When building the complete system, isolation is needed for the patients’ security, this can be done with either galvanic isolation, photo-opto coupling, capacitive coupling or magnetic coupling. This article focuses on the EMI hardened ECG front-end as part of a complete medical electronic system which has to observe all safety standards. In addition, EMI hardened amplifiers are presented as a way to make the systems more robust against the effects of interference.
When physiological electrical activity of the body is measured, we have seen that EMI is a major challenge, as the signal sensed has a very low amplitude. Using the example of an ECG system we have demonstrated in this article that National Semiconductor’s product solutions enable the design of a very precise and robust system to guarantee patients’ safety. Indeed, with the use of EMI hardened components such as the operational amplifier LMP2021 the signal path is less susceptible to electromagnetic interferences.
Carine Alberti works for Systems Marketing Healthcare, National Semiconductor
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