Resistors advance diverse medical diagnostic and therapeutic capabilities
Medical devices are leaving the confines of hospitals and serving the growing community-based and home-based healthcare markets. The extension of capability and access is supported by a growing level of electronics development, much of which is based on the increasing computing power of digital systems. However, as the human body remains analog, there will always be an important role for high reliability passive components in contact, imagine and analysis applications.
Contact applications include all devices with electrical connection to the body, such as defibrillation, ECG or EEG, and respiratory or plethysmographic monitoring. The image area encompasses X-ray, MRI and ultrasound. And finally, analysis covers the area of IVD and laboratory instruments.
Contact
In an effort 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 this is being implemented by providing automatic external defibrillators (AEDs) for use by police, first aid volunteers and even members of the general public.
Two of the main improvements required to adapt hospital-based equipment were the miniaturization of the defibrillator to a small portable device, and its ability to withstand harsher temperature and humidity environments without effecting reliability.
Defibrillators need stable and repeatable measurement of the charging voltage, as this determines the amount of electrical energy delivered to the patient.
Figure 1: A simplified defibrillator-charging circuit
Figure 1 shows a simplified defribrillator-charging circuit which uses a high-voltage resistor.. In this example, R1 is a high-value resistor, normally in the range 5MΩ to 50MΩ which, together with a low-value resistor R2, forms a potential divider for voltage feedback.
The critical features of this high voltage resistor are linearity (expressed by voltage coefficient or VCR, temperature coefficient (TCR), and long-term stability under voltage stress. The error budget for resistance value can be expressed as:
Error =
Tolerance + Solder heat stability + TCR error at Tmin / Tmax
+ VCR error at Vmax + Environmental Stability
The first two terms are commonly eliminated by post-assembly calibration. The remaining three are described in detail below.
The resistor technology best suited to this application is thick-film. The linearity limits are expressed as the temperature and voltage coefficients of resistance, which are limits on the reversible resistance changes illustrated in Figure 2.
The temperature characteristic is typically “U” shaped with limits expressed by the TCR, which is normally in the range ±25 to ±100ppm/ºC. The TCR error may be minimized by choosing the highest-possible ohmic value, so as to reduce the self-heating, and by a layout which avoids proximity to heat-generating components.
The voltage characteristic, by contrast, only ever has a negative gradient, with a limit expressed by the VCR, which is typically -1 to -5ppm/V. High-voltage resistors use special design techniques to minimize VCR, but ultimately there is a trade-off between VCR and product size. It should be noted that, as the gradient increases at high voltage, VCR error can be reduced by only operating the resistor at up to 75% of the full rated voltage. Furthermore, if the nominal VCR is known, compensation is relatively simple.
Figure 2: Linearity limits are expressed as the temperature and voltage coefficients of resistance.
The environmental stability describes the limits of non-reversible resistance change under given loading and environmental conditions. The most-demanding condition is high humidity, but materials are available which seal the resistive element to achieve typical resistance changes below 0.25% after 56 days at 95% RH and 40ºC.
The possibility of exposure to defibrillation pulses exists for any directly connected monitors, such as ECG, respiratory and plethysmographic monitors. It is therefore necessary to guard against damaging the sensitive input stages of such equipment.
Moreover, it is even more important to avoid diverting the defibrillation energy from the patient. This is achieved by adding resistance to the monitor input circuit, usually in the form of a pulse-withstanding resistor.
This is built into the leadset, either at the probe connector or in the yoke where the single cable breaks out into individual probe leads. Additional protection may be provided within the monitor itself. The standard test circuits are shown in Figure 3.
Figure 3: Standard test circuits for protection within the monitor.
The proportion of the total defibrilation energy received by a protection resistor depends on its ohmic value, and the highest value consistent with monitor function should be used to minimize this. There is also a variation depending on which test circuit is used and, in the case of IEC601, on how many leads are in the leadset.
Vendors such as TT electronics can advise on the exact energy rating required, but 25J at 1K falling to 2.5J at 10K are typical values for leadset protection. This calls for a composition technology product such as CC series or high-energy wirewound parts like NAS series. For PCB mounted resistors offering secondary protection, pulse withstanding thick-film products such as PWC, DSC and CHP series are used.
Where ECG monitors and analytical instruments require sensitive first stages for amplification of small signals, high ohmic values are required in the feedback resistor. TT electronics has long specialized in providing values outside the range normally available, with glass-sealed resistors extending to 100TΩ (1014Ω) and flat chips extending to 50GΩ.
Imaging
Imaging applications include X-ray, ultrasound and MRI systems, each with its own requirements for resistive components. X-ray systems require stable and accurate high-voltage supplies to provide the accelerating voltage for X-ray generation.
Voltages are typically in the 50kV to 100kV range, and the circuit is often assembled in an oil-filled chamber. This reduces clearance constraints on the layout, thus enabling a compact X-ray head design.
Figure 4: Specialty resistors such as these T Series axial devices are needed for the high voltage of X-ray equipment.
One design approach is to use an ultra-high voltage thick-film solution, such as the T Series axial resistors from TT electronics (Figure 4). These devices provide up to 100kV in a single element in an oil-filled assemby. Terminations are either wire or screw, allowing stacking into multiple resistor assemblies. In order to eliminate the possibility of air pockets an unsleeved version should be selected. It is possible to supply these parts in matched sets to give accurate ratio tolerance or to give very low TCR by means of cancellation.
Ultrasound transducers require termination networks capable of operating at high frequency and providing multiple channels of resistive termination. A typical requirement is 128 channels and performance up to 15MHz. Standard or custom thin-film resistor networks can be produced to meet the application requirements.
MRI scanners require control circuits that are insensitive to extremely high magnetic-field strengths, requiring components free of ferrous alloys and nickel. These are the materials commonly used in the termination caps fitted to the ends of most types of axial resistor and as an anti-leaching barrier in chip resistors. Capless through-hole resistors and nickel-free chip resistors are available for such requirements.
Analysis
For a broad range of laboratory analysis equipment, precision resistors are required with tight tolerance, low temperature sensitivity, and high stability. The input stage of an instrument with a resistive sensor, such as a thermistor in a precision temperature-monitoring circuit, consists of a bridge of resistors which must be closely matched in value.
In such a case it is the ratio between values which matters rather than the absolute values themselves. Likewise, the maximum difference between TCRs, that is, the tracking TCR, is more important than the absolute TCR.
There are two possible solutions in these cases. First, discrete precision resistors may be supplied in matched sets with specified ratio tolerance and tracking TCR. Second, thin-film networks with multiple elements provide a combination of high precision and an integrated solution. These are available with conventional nichrome elements as well as high stability versions which exploit the self-passivating properties of tantalum nitride film.
When evaluating the long-term stability of resistors, designers should consider several environmental tests. Some of these are initial factors, like exposure to solder heat, and some are reversible, like TCR. Most are long-term factors and, in general only, one of these figures should be used; the one that most closely reflects operating conditions.
The Shelf Life figure applies where loading is negligible and the environment is benign. The Load figure applies where power dissipation is the main factor, the Long Term Damp Heat figure where humid environments may be encountered. In all these tests, the majority of the value change happens within the period of the test, as the value will tend to stabilize.
For example, the 1000-hour Load figure is a good guide to the change predicted over a longer period of service. For greater precision, mathematical models exist to extrapolate from tested stability levels to long-term stability under application conditions.
The figure of most interest to designers is the maximum total error in resistance value at the end of product life, or before scheduled re-calibration, if applicable. This is termed the total excursion, and is the root of the sum of the squares (RSS) of applicable, statistically independent short-term and long-term factors.
This is best illustrated by an example: An RC55 Series resistor with 0.05% tolerance and TCR of 15ppm/°C is to be soldered to a PCB for use in a laboratory-based product. The voltage across the resistor will be low, and the power dissipation negligible. The operating temperature range inside the equipment is 20 to 50°C, and there is no temperature compensation.
Initial and reversible factors:
Tolerance: ±0.05%
TCR: ±0.045% (±15ppm/°C x 30°C)
VCR: Negligible
Soldering: ±0.06% maximum ±0.03% typical
Long-term factors:
Shelf Life (no load) ±0.1% maximum ±0.03% typical
Max = √ (0.052 + 0.0452 + 0.062 + 0.12) = 0.13,
Typical = √ (0.052 + 0.0452 + 0.032 + 0.032) = 0.08
Total Excursion (12 months):
Maximum = ±0.13%
Typical = ±0.08%
Clearly, initial calibration can be used to eliminate tolerance and soldering process induced errors, and this gives:
Calibrated Total Excursion:
Maximum = ±0.11%
Typical =±0.05%
For designs with extremely low error budgets, the MAR Series resistor can improve on this by nearly an order of magnitude. Repeating the above analysis for a MAR42H gives a calibrated total excursion of 0.012% maximum and 0.007% (70ppm) typical. This pushes metal-film performance into the range normally associated only with costly metal-foil parts.
The medical sector, perhaps more than any other, generates challenging requirements for passive component performance and reliability. Component manufacturers such as TT electronics, support this area with new product development.
About the author
Stephen Oxley is a Senior Applications Engineer specializing in Fixed Resistors for Welwyn’s parent company, TT electronics. After graduating from Bath University, UK with a Master of Engineering degree in Electrical and Electronic Engineering in 1987, Stephen worked as a development engineer with Marconi before joining Welwyn Components Ltd near Newcastle, UK in 1992.
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