Resistors for Medical Applications

TT Electronics Mar 24, 2017

The electronic content of devices for medical applications continues to rise steadily as medical science develops new techniques and electronic engineering supplies new solutions. At the same time, these devices are leaving the confines of hospitals and serving the growing community-based and home-based healthcare markets.

Much of the development is based on the increasing computing power of digital systems. However, the human body remains analogue, and there will always be an important role for high reliability passive components in three broad areas:

CONTACT
IMAGE
ANALYSIS

The first area, CONTACT, includes all devices with electrical connection to the body. Examples include the delivery of high energy pulses for defibrillation, the detection of biologically generated signals for ECG or EEG and the measurement of body impedance for respiratory or plethysmographic monitoring.

The IMAGE area encompasses X-ray, MRI and ultrasound, all with their own special demands on resistive components. Finally, ANALYSIS covers the area of IVD and laboratory instruments.

This application note aims to guide the designer in selecting the optimum components for all these areas. It should be read in conjunction with the full product data sheets.

High-voltage compact solutions for defibrillator charge control
Pulse withstanding resistors for defibrillation pulse protection
High ohmic value parts for high gain amplifiers in ECG and similar monitors
Ultra high-voltage resistors for X-ray head supply dividers
Thin-film termination networks for ultrasound transducers
Non-magnetic resistors for MRI scanners
Precision resistors for high stability in IVD analysis equipment

CONTACT: Defibrillator Charge Control

A simplified schematic of a defibrillator charge circuit is shown in Figure 1. The stable and repeatable measurement of the charging voltage is a critical function of this circuit, as this determines the amount of electrical energy delivered to the patient. As the energy delivered is proportional to the square of the voltage, a 1% tolerance requirement for the energy will demand a 0.5% tolerance on the voltage control.

In this circuit R1 is a high value resistor, normally in the range 5M to 50M, which, together with a commodity chip resistor R2, forms a potential divider for voltage feedback.

The critical features of this high voltage resistor are linearity, expressed by voltage coefficient (VCR) and temperature coefficient (TCR), and long term stability under voltage stress. In addition to this, the rapid growth in the non-professional AED segment adds the requirement for compact footprint and suitability for a wider range of environmental conditions, including high humidity. 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 normally “U” shaped with limits expressed by the TCR, which is typically ±100ppm/ºC. The TCR error may be minimised 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 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 minimise 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. Resistors are normally adjusted and measured at voltages Vmeasure 100V, but TT Electronics can also supply resistors with an adjustment offset which minimises the VCR error at a given higher nominal operating voltage.

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 TT Electronics optionally seals the resistive element using a specially formulated powder coat material to achieve typical resistance changes within ±0.1% on both Moisture Resistance and Biased Humidity tests.

High Voltage Planar SIL Resistors

HVP Series

Compact design with minimal footprint
Sealed by high-density epoxy protection
Low TCR and VCR
Leadframe standoff permits thorough washing
Custom sizes, ratings and HV screening
Integrated divider version available as HVD Series
Optional powder coated version

High Voltage Chip Resistors

HVC Series

Standard SMD chip format
Continuous rating to 3kV
Low voltage coefficient
Tolerance to ±0.5%
TCR to ±50ppm/°C

Electrical Data HVC Series & HVP Series

HVC1206 HVC2010 HVC2512 HVP Notes

Power rating @70°C watts 0.3 0.5 1 0.4 - 2

Limiting element voltage volts 1000 2000 3000 2000 - 20,000 DC or AC peak

Overload voltage (2s) volts 1500 3000 4000

Resistance range ohms 10K to 1G 1K to 10G Consult factory for
out of range values

Resistance tolerance % 0.5,1,2,5,10 0.25, 0.5, 1, 5 See datasheet for applicable value ranges

TCR ppm/°C 50, 100 25, 50, 100

Ambient temperature range °C -55 to +155

Values E24 & E96 preferred Any value to order

VCR ppm/V -15 -5 -1.5 -0.5 to -1 Typical

Further data http://www.ttelectronics.com/sites/default/files/resistors-datasheets/HVC.pdf http://www.ttelectronics. com/sites/default/files/resistors-datasheets/HVP.pdf

CONTACT: Defibrillator Pulse Protection

The possibility of exposure to defibrillation pulses exists for any directly connected monitors, and it is necessary to guard against damaging the sensitive input stages of such equipment. 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 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 minimise 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.

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, and this calls for a composition technology product such as CC series. For PCB mounted resistors offering secondary protection, pulse withstanding thick-film products such as PWC and DSC series are used.

High Energy Composition Resistors

CC Series

Carbon composite element
Energy rating to 30J
High voltage epoxy coating

Pulse Withstanding Chip Resistors

PWC & DSC Series

Standard SMD chip with guaranteed pulse performance
Special materials and adjustment technique
DSC is double sided for double energy capacity

Electrical Data CC, PWC and DSC Series

CC Series PWC2512 DSC2512

Power rating watts 1 & 2 1.5 1.5

Energy rating (10ms pulse) joules 20 to 30 1 2

Resistance range ohms 100R - 50K 1R -10M 1R -4M7

Resistance tolerence % 10, 20 0.5,1,5

Further data

http://www.ttelectronics.com/sites/default/files/ resistors-datasheets/CC.pdf

http://www.ttelectronics.com/sites/default/files/ resistors-datasheets/PWC.pdf

http://www.ttelectronics.com/sites/default/files/ resistors-datasheets/DSC.pdf

CONTACT: ECG High Gain Amplification

Where ECG monitors and analytical instruments require sensitive first stages for amplification of small signals, high ohmic values are required in the feedback resistor (Figure 4). TT electronics has long specialised in providing values outside the range normally available, with some specialist products extending to 100TΩ (1014Ω). These are glass sealed resistors with the option of a guard-band for mitigation of leakage effects. In the flat chip format values extend to 50G and special sizes are available to minimise shunt capacitance and capacitance to groundplane.

Ultra-High Value Resistors

3810 Series

Values up to 100T
precision to 1% at 1T
Silicone coated glass body

High Value Chip Resistors

HR Series

Standard SMD chip up to 50G
Precision to 5% at 1G
Solder or wirebond terminations

Electrical Data 3810 Series & HR Series

3810 3811 3812 HR0805 HR1206

Resistance range ohms 100M-1T 1T-100T 100M to 50G

Limiting element voltage volts 500 1000 100 200

Further data

http://www.ttelectronics.com/sites/default/files/resistors-datasheets/3810.pdf http://www.ttelectronics.com/themes/ttelectronics/datasheets/resistors/literature/3810_AN.pdf

http://www.ttelectronics.com/sites/default/files/resistors-datasheets/HR.pdf

IMAGE: X-Ray Supply

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 and creepage constraints on the components and layout, thus enabling a compact X-ray head design.

TT Electronics has extensive experience of supplying ultra-high voltage thick-film solutions for voltage division in this application and can supply various standard and custom formats. One design approach is to use a series combination of high voltage resistors such as HVP Series (see Page 3 for data) which provides up to 20kV per element. Another solution is to use T Series axial resistors which provide up to 100kV in a single element in an oil-filled or SF6-filled assemby. Terminations are either wire or screw, allowing easy stacking into multiple resistor assemblies. In order to eliminate the possibility of air pockets the unsleeved version (pictured) 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. Finally, custom divider networks can be designed with track layouts and component outlines which give optimised solutions for highly compact head designs.

Ultra-High Voltage Resistors

T Series

LEV to 100kV
Values up to 50G
Precision to 0.5% 25ppm
Matched sets available

Custom Resistor & Divider Networks

Good ratio tolerance and TCR tracking
Optimised size and shape
Ideal for compact head designs

Electrical Data T Series

T43 T44 T48 Notes

Limiting element voltage volts 4000 / 8000 14,000 / 28,000 50,000 / 100,000 In air / in oil

Power rating @20°C watts 1.5 / 2.3 3.5 / 5.3 10 / 15

Resistance range ohms 1K to 50G

Tolerance % 1, 2, 5 0.5% in restricted range

TCR ppm/°C 25, 50, 100

Further data http://www.ttelectronics.com/sites/default/files/resistors-datasheets/T40.pdf

IMAGE: Medical Scanners

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. TT Electronics can provide standard and custom thin-film resistor networks in a wide variety of packaging styles including SOIC, QSOP, TSSOP. The latest addition is a BGA part which can offer up to 32 terminators in a 6.4 x 2.5mm footprint with return loss below -20dB to 3GHz. This meets the requirement above within the space of four 2512 chips.

MRI scanners require control circuits which are insensitive to extremely high magnetic field strengths, and this calls for components which are 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. TT Electronics can supply the non-magnetic glueable chip, GCR Series, for this requirement.

ANALYSIS: Precision Resistors

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, for example a thermistor in a precision temperature monitoring circuit, consists of a bridge of resistors which must be closely matched in value (Figure 5). 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. TT Electronics can offer two solutions in these cases. Firstly, most through-hole precision resistors are available in matched sets with specified ratio tolerance and tracking TCR, and this solution gives the best available precision. Secondly, SMD thin-film products with multiple elements provide a combination of high precision and a compact single-component solution.

The precision through-hole resistor offering ranges from semiprecision PR Series through the popular precision RC Series to ultraprecision MAR which matches the performance of costly metal foil technology using advanced metal film techniques.

The precision SMD offering includes conventional thin-film chip resistors using nichrome elements as well as ultra high stability versions which exploit the self-passivating properties of tantalum nitride film (WIN Series).

Format Product Absolute Tolerance % Ratio Tolerance* % Absolute TCR ppm/°C Tracking TCR ppm/°C Through-hole PR 0.1 25 http://www.ttelectronics.com/sites/default/files/resistors-datasheets/PR.pdf

RC 0.25 to 1W 0.05 0.02 5 2 http://www.ttelectronics.com/sites/default/files/resistors-datasheets/RC.pdf

RCP 0.5 to 1.5W 0.05 0.02 5 2 http://www.ttelectronics.com/sites/default/files/resistors-datasheets/RCP.pdf

CAR 0.25 to 0.5W 0.01 0.005 5 1 http://www.ttelectronics.com/sites/default/files/resistors-datasheets/CAR.pdf

MAR 0.3W 0.005 0.005 2 1 http://www.ttelectronics.com/sites/default/files/resistors-datasheets/MAR.pdf

SMD PCF 0.063 to 0.25W 0.1 10 http://www.ttelectronics.com/sites/default/files/resistors-datasheets/PCF.pdf

WIN 0.1 to 0.33W 0.05 15 http://www.ttelectronics.com/sites/default/files/resistors-datasheets/WIN.pdf

PFC-DIVIDER 0.25W total 0.02 5

http://www.ttelectronics.com/sites/default/files/resistors-datasheets/PFC-D.pdf

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Corpus Christi, TX 78411, USA