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Resistors for Medical Applications

tags2r7 smd resistor

As medical science develops new technologies and electronic engineering provides new solutions, electronic content for medical application equipment continues to grow steadily. At the same time, these devices have got rid of the limitations of hospitals and serve the growing community-based and family-based healthcare market.

Many developments are based on the ever-increasing computing power of digital systems. However, the human body is still simulated, and high-reliability passive components will always play an important role in the following three broad areas:

The first area, CONTACT, includes all equipment that is electrically connected to the human 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 respiration or plethysmograph monitoring.

The IMAGE area includes X-ray, MRI and ultrasound, all of which have special requirements for resistor components. Finally, the analysis covers the areas of in vitro diagnostics and laboratory instruments.

This application note aims to guide the designer in choosing the best components for all these areas. It should be read together with the complete product data sheet.

Contact: Defibrillator charging control

A simplified schematic diagram of the defibrillator charging circuit is shown in Figure 1. The stable and repeatable measurement of the charging voltage is the key function of the circuit, because it determines the amount of electrical energy delivered to the patient. Since the energy delivered is proportional to the square of the voltage, a tolerance requirement of 1% for energy will require a tolerance of 0.5% for voltage control.

In this circuit, R1 is a high-value resistor, usually in the range of 5M to 50M. It forms a voltage divider with the commercial chip resistor R2 for voltage feedback.

The key characteristics of this high voltage resistor are linearity (represented by voltage coefficient (VCR) and temperature coefficient (TCR)) and long-term stability under voltage stress. In addition, the rapid growth of the non-professional AED field has increased the requirements for a compact footprint and suitable for a wider range of environmental conditions including high humidity. The error budget of the resistance value can be expressed as:

Error = tolerance + solder thermal stability + TCR error at Tmin / Tmax + VCR error at Vmax + environmental stability

The first two terms are usually eliminated by calibration after assembly. The remaining three will be explained in detail below.

The most suitable resistor technology for this application is thick film. The linear limit is expressed as the temperature and voltage coefficient of the resistance, which are the limits of the reversible resistance change, as shown in Figure 2. The temperature characteristic is usually a "U" shape, represented by TCR, and the typical value is ±100ppm/ºC. By choosing the largest possible ohmic value to reduce self-heating, and by avoiding layouts close to heating components, TCR errors can be minimized.

In contrast, the voltage characteristic only has a negative gradient, and its limit is represented by the VCR, which is usually -1 to -5 ppm/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 under high voltage increases, only operating the resistor at 75% of the highest rated voltage can reduce the VCR error. The resistance is usually adjusted and measured under the voltage Vmeasure 100V, but TT Electronics can also provide a resistance with an adjusted offset to minimize the VCR error under a given higher rated operating voltage.

Environmental stability describes the limit of irreversible resistance change under a given load and environmental conditions. The most demanding condition is high humidity, but TT Electronics can choose to use a specially formulated powder coating to seal the resistance element to achieve a typical resistance change within ±0.1% during humidity resistance and bias humidity tests.

High voltage planar SIL resistor

HVP series

High voltage chip resistor

HVC series 

Electrical data HVC series and HVP series

                                                          HVC1206 HVC2010 HVC2512 HVP precautions

Rated power @ 70°C Watt 0.3 0.5 1 0.4-2

Limit component voltage volt 1000 2000 3000 2000-20,000 DC or AC peak

Overload voltage (2s) 1500 3000 4000

Resistance range ohms 10K to 1G 1K to 10G consult factory

                                                                                                                                                                          Out of range value

Resistance tolerance% 0.5, 1, 2, 5, 10 0.25, 0.5, 1, 5 For the applicable value range, please refer to the data sheet

TCR ppm /°C 50, 100 25, 50, 100

Ambient temperature range °C -55 to +155

Any value that can be ordered with E24 and E96 values ​​preferred

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

More data                                            

Contact: Defibrillator pulse protection

Any directly connected monitor may be exposed to defibrillation pulses, so it is necessary to prevent damage to the sensitive input stage of such devices. It is more important to avoid transferring the patient's defibrillation energy. This is achieved by adding resistance to the monitor input circuit, which is usually pulse resistant. It is built into the lead frame, whether on the probe connector or in the yoke, where a single cable is divided into individual probe wires. The display itself can provide additional protection.

The proportion of the total defibrillation energy received by the protection resistor depends on its ohmic value, which should be minimized using the maximum value consistent with the function of the monitor. Depending on the test circuit used, and for IEC601, there are differences depending on how many leads are in the lead set.

TT Electronics can suggest the exact rated energy required, but the typical value of lead set protection is to reduce 25J at 1K to 2.5J at 10K, which requires the use of composite technology products, such as the CC series. For PCB mounting resistors that provide secondary protection, use thick film products that can withstand pulses, such as the PWC and DSC series.

High energy component resistance

CC series

Pulse resistant chip resistor

PWC&DSC series

Electrical data CC, PWC and DSC series

                                                               CC series PWC2512 DSC2512

Rated power watts 1 and 2 1.5 1.5

Rated energy (10ms pulse) Joule 20 to 30 1 2

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

Resistance tolerance% 10,20 0.5,1,5

More data

Contact: ECG high gain amplifier

When ECG monitors and analytical instruments need a sensitive first stage to amplify small signals, the feedback resistors need high ohmic values ​​(Figure 4). TT Electronics has long been committed to providing values ​​beyond the usual available range, and some professional products have been extended to 100TΩ (1014Ω). These are glass sealed resistors with protective tape to reduce the effects of leakage. In the flat chip format, the value is extended to 50G, and a special size is provided to minimize the parallel capacitance and the capacitance to the ground plane.

Ultra high value resistors

3810 series

High value chip resistors

HR series 

Electrical data 3810 series and HR series

                                                            3810 3811 3812 HR0805 HR1206

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

Limit element voltage volt 500 1000 100 200

Image: X-ray supply

The X-ray system requires a stable and accurate high-voltage power supply to provide acceleration voltage for generating X-rays. The voltage is usually in the range of 50kV to 100kV, and the circuit is usually assembled in an oil-filled room. This reduces the gap and creepage constraints on components and layouts, thereby realizing a compact X-ray head design.

TT Electronics has extensive experience in providing ultra-high pressure thick film solutions for partial pressure for this application, and can provide a variety of standard and customized formats. One design method is to use a series combination of high-voltage resistors, such as the HVP series (for data, see page 3), each component can provide up to 20kV. Another solution is to use T series axial resistors. In oil-filled or SF6 filled components, a single element can provide up to 100kV. The terminations are wires or screws and can be easily stacked into multiple resistor assemblies. In order to eliminate the possibility of bubbles, the sleeveless version should be selected (as shown in the figure). It is possible to provide these parts in a matched manner to provide precise ratio tolerances or offset to provide very low TCR. Finally, a customized divider network can be designed using track layouts and component outlines to provide an optimized solution for highly compact sprinkler designs.

Ultra high voltage resistance

T series

Custom resistor and voltage divider network

Electrical data T series

                                                            T43 T44 T48 Note

Limit element voltage volt 4000/8000 14,000 / 28,000 50,000 / 100,000 in air/oil

Rated power @ 20°C Watt 1.5 / 2.3 3.5 / 5.3 10/15

Resistance range 1K to 50G

Tolerance %1,2,5 0.5% is within the limit

TCR ppm /°C 25, 50, 100

More data                                

Image: Medical Scanner

Ultrasonic transducers need a termination network that can operate at high frequencies and provide multi-channel resistive termination. The typical requirement is 128 channels, and the performance is up to 15MHz. TT Electronics can provide standard and customized thin-film resistor networks in a variety of packages including SOIC, QSOP and TSSOP. The latest addition is the BGA component, which can provide up to 32 terminator in a 6.4 x 2.5mm footprint with a return loss of less than -20dB to 3GHz. In the space of four 2512 chips, the above requirements can be met.

MRI scanners require a control circuit, which is not sensitive to extremely high magnetic field strengths, and therefore requires components that do not contain iron alloys and nickel. These are materials that are usually installed in the termination caps at the ends of most types of axial resistors and are used as anti-leakage barriers for chip resistors. TT Electronics can provide non-magnetic bondable chip GCR series for this.

Analysis: Precision resistance

For a variety of laboratory analysis equipment, precision resistors are required to have strict tolerances, low temperature sensitivity and high stability. The input stage of instruments with resistance sensors (for example, thermistors in precision temperature monitoring circuits) consists of a resistance bridge whose values ​​must be closely matched (Figure 5). In this case, what matters is the ratio between the values, not the absolute value itself. Similarly, the maximum difference between TCRs, that is, tracking TCR is more important than absolute TCR. In this case, TT Electronics can provide two solutions. First, most through-hole precision resistors are available with matching kits with specified ratio tolerance and tracking TCR, and this solution provides the best accuracy. Secondly, SMD film products with multiple elements combine high precision and compact one-component solutions.

Precision through-hole resistors range from semi-precision PR series to popular precision RC series to ultra-precision MAR, which match the performance of expensive metal foil technology using advanced metal film technology.

Precision SMD products include conventional thin-film chip resistors using nickel-chromium alloy components and ultra-high stability versions using the self-passivation characteristics 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

                      RC 0.25 to 1W 0.05 0.02 5 2

                       RCP 0.5 to 1.5W 0.05 0.02 5 2

                       Automotive 0.25 to 0.5W 0.01 0.005 5 1

                       MAR 0.3W 0.005 0.005 2 1

SMD PCF 0.063 to 0.25W 0.1 10

                      WIN 0.1 to 0.33W 0.05 15

                  PFC divider 0.25W total 0.02 5      

For our complete product portfolio, internal and local design support/distribution partners, please visit:

TT Electronics,

4222 South Staples Street,

Corpus Christi, Texas, USA 78411

Weilin Electronic Park

Bedlington, Northumberland, UK, NE22 7AA

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