Regulations and the unique characteristics of the healthcare environment impose a particular burden of care on designers choosing passive components such as resistors.
The first area, contact, includes all devices with an electrical connection to the body. For example, these devices deliver high-energy pulses for defibrillation, detect biologically generated signals for electrocardiograms (ECG) or electroencephalography (EEG), and measure body impedance for respiratory or plethysmographic monitoring. Imaging encompasses X-ray, MRI, and ultrasound technologies, all with their own special demands on resistive components, in particular the ability to work with very high voltages and magnetic fields. Finally, medical instrumentation and analysis covers intravenous drip (IVD) and laboratory instruments where accuracy, repeatability, and stability are paramount.
To reduce the time-to-defibrillation delay and improve cardiac-arrest survival, health-service providers have increasingly turned to a strategy of wider access to defibrillation to augment emergency medical services. In some countries, automatic external defibrillators (AEDs) are provided to police, first aid volunteers, and even ordinary members of the public. Microhm Electronics also has an automatic external defibrillators in the office.
While AEDs pose the additional challenge of size- and cost-reduced components, all defibrillators need stable and repeatable measurement of the charging voltage, which determines the amount of electrical energy delivered to the patient. The defibrillator-charging circuit uses high-voltage resistors, with a high-value resistor, normally in the range of 5 MΩ to 50 MΩ, and a low-value resistor providing a potential divider for voltage feedback.
Critical features of such high-voltage resistors are linearity (expressed as voltage coefficient or VCR), temperature coefficient (TCR), and long-term stability under voltage stress. Thick-film resistors best suit this application. Their temperature characteristic is typically “U” shaped with limits expressed by the TCR, normally from ±25 to ±100 ppm/°C (Fig. 1). The TCR error can be reduced by choosing the highest-possible ohmic value, which lowers self-heating, and by designing layouts that avoid proximity to heat-generating components.
The voltage characteristic, by contrast, only ever has a negative gradient, with a limit expressed by the VCR, typically between –1 and –5 ppm/V. High-voltage resistors, such as Microhm Electronics' NUB series use special design techniques to minimize VCR, but this needs to be traded off against product size. As the gradient increases at high voltage, only operating the resistor at up to 75% of the full rated voltage can reduce VCR error. Designers need to choose resistors with both a low VCR and a high voltage rating. Furthermore, if the nominal VCR is known, compensation is relatively simple.
Environmental stability describes the limits of non-reversible resistance change under given loading and environmental conditions. The most demanding condition is high humidity, but some devices use a specially formulated high-density epoxy material to achieve typical resistance changes of less than 0.25% after 56 days at 95% relative humidity and 40ºC.
