Portable medical equipment signal adjustment

Today's medical portable devices facilitate diagnosis and treatment, patients themselves, or even to move around freely, and you use the device. Portable electronic medical devices to achieve the "portable" features, you must have the miniaturization and low-power characteristics.

In addition, these devices also have a very high accuracy to ensure patient safety.

Medical equipment using a variety of different sensors to monitor the patient's health status, then the sensor signals into electrical signals for the analysis of the use of electronic equipment. Because of medical devices for transmission of signals are relatively weak, and it will be a lot of noise sources of interference, signal path design for portable medical equipment is very important. This article is for ECG and blood glucose meter both device applications, discusses how the sensors and matching PowerWise amplifiers are connected in order to extend battery life and improve the diagnosis and treatment of security.

ECG machine operation principle

Electrocardiogram (ECG) can record a patient's heart beat.

The heartbeat is through three connected to the patient body electrode measurement. Figure 1 shows the output for one of the electrodes, the figure of the ECG signal contains five of the measuring points, respectively, P, Q, R, S, and T, these measuring points can be used to diagnose patients suffering from heart disease.

From the electrode signal is collected in the 400 μ V to highest 5mV, and 0.05Hz and 100Hz with 3dB corner frequency.

Such signals are also quite a number of General interference, such as electrode contact noise, power line noise (50MHz), breathing, muscle activity, as well as interference from other electronic devices.

ECG signal adjustment

As mentioned above, the ECG signal path must be able to adjust the noise from different sources.

Against direct current noise, you can use a highpass filter. However, most headaches are 50 Hz noise because it coincides with our collection of signal frequency is in the same range. To eliminate this common mode noise, need to establish a measurement instrumentation amplifier. This configuration is ideal because it can suppress common-mode voltage while zoom useful differential signal, which facilitate the weak effective signal from noise in separated out. As shown in Figure 2, the amp is LMP2234 (four-channel, micro-power, high precision of RRO operational amplifier) and high-precision resistors (0.1%) implementation.

A1, A2, A3, A4: LMP2234 R1, R2: 2 M Ω R3, R5: R4: 40 k Ω 20 k Ω R6, R7: 10 k Ω R8, R9: 10 k Ω R10, R11: 20 K ω LMP2234 VIP50 process technology is used, i.e. the insulation Silicon BiCMOS process.

This process of making out ultra-low power amplifier is ideal for battery-powered low-power applications. The process of having 1.8 to 5.5V operating voltage range and 36 A quiescent current of μ, conducive to extend laptop battery life in the system. LMP2234 is LMP high precision amplifier family, its high impedance CMOS input so that it becomes the measuring instruments and other sensor interface applications the ideal choice.

As from the electrode signal amplitude is extremely low, amplifiers DC parameters it is very important.

The maximum offset LMP2234 voltage 150V (typical for 10 V), and offset voltage drift temperature coefficient and bias current respectively 0.3 μv/° c and ± 20 fA. These high-precision specifications enable LMP2334 strictly to maintain system stability constant accuracy and long term aspects quite well.

This measuring instrument amplifier contains a two-level.

The last level (i.e. output level) is a differential amplifiers, it can refuse to effect both DC level as well as two input of interference and noise voltage source. Two amplifier consisting of first level (that is, input level) is configured as an input isolation buffer. However, based on the mismatch between the amplifier so that they are not interconnected, so between the two amplifiers retrofitted balancing resistors. The two-level can be obtained by multiplying the gain instrumentation amplifier gain. In theory, the common mode rejection ratio (CMPR) should be infinite, but due to resistance mismatch, causing the output level of a non-zero-mode gain very little. In the circuit of input, electricity flowing through all resistance is the same. This will depend on LMP2234 high input impedance and low input bias current features.

Output voltage is defined as:

The input signal for only the most significant 5mV, but in order to establish a gain, we must take into account that DC offset voltage electrode, it sometimes can be up to ± 300mV.

LMP2234-rail‐to‐rail output from the power supply voltage rail swing to 15mV, thus increasing the dynamic range of the system. In addition, the R10 and R11 composition of bias voltage divider can provide a voltage, the voltage is exactly equal to being confirmed in paragraph patient signs the required supply voltage.

As shown in Figure 3, the Council adopts a high pass filter to suppress can lead to a gain stage saturation DC devices.

The high-pass filter cutoff frequency for 0.5Hz. The filter uses the second order Sallen-Key Butterworth topology technology. As for the second level is a low-pass filter, which owns the cutoff frequency and 100Hz 100 gain, and similarly to the topology to implement Sallen Key. Called Sallen Key class analog filter is built around with resistors and capacitors and the establishment of operational amplifier. Which did not use inductor because it was huge, heavy and not perfect.>

C1: 1 μ F R3: 5.36 k Ω C2: 220 nF R4: 14.3 k Ω R1: 1.24 M Ω C3: 33 nF R2: 365 k Ω C4: 1 μ F R5: 10 k Ω R6: 1 M Ω two filters are based on low-power operational amplifier LMV552, but this amplifier is a United States National Semiconductor VIP50 process.

Have the bandwidth, while 3MHz each amplifier only consume 34 μ A current, bandwidth/watt are siblings in the highest operational amplifier. LMV552 has a rail-to-rail output stage and an extension to the ground following input common-mode range, operating power supply voltage range is 2.7V to 5.5V.

Battery-operated system needs a DC/DC boost to provide signal path required 3 .3V.

LM2623 is a high efficiency step-up of universal-type DC-DC switching regulator, specifically apply to the use of battery-powered low input voltage system. The voltage regulator 8V to acceptable input voltage is 14V and convert them into stable 1.24V output voltage is 14V. Combined with the efficiency of the system LM2623 can be as high as 90%.

If the system requirements have patient safety isolation, through the use of Galileo cutting, isolation (galvanic isolation), electro-optical coupling capacitor and magnetic coupling.

Although this article focuses on the ECG's front-end, but in the development of medical electronic systems, safety standards should not be overlooked.

Bluetooth transmission

Heartbeat records you can use wireless transfer to PC, mobile phone or PDA.

Bluetooth technology is stable and reliable, easy-to-use, cost-effective, and coverage, patient monitoring aspects become widely used wireless technology.

United States National Semiconductor's LMX9838 Bluetooth serial port module is a highly integrated device that contains Bluetooth baseband controller, 2.0 2.4GHz radio machine, Crystal, antenna, LDO and discrete components.

Complete the Bluetooth function, and the node small (10 mm x 17 mm x 2.0 mm).

In the full programme including the hardware and software, apply to have the full height of the Bluetooth stack layers of antenna, universal access practice (GAP), service discovery application specification (SDAP) and the serial port (SPP), and other applications.

The module contains a configurable service database to meet the master of the additional service specification requirements. In addition, LMX9838 adopted the Bluetooth product certification, and no longer need to pass any test or payment technology licence fees can be used directly to the final application.

Based on the United States National Semiconductor's 16-bit processor CompactRISC architecture and digital smart radio technology, LMX9838 as an optimization solution can fully satisfy the necessary Bluetooth node data processing and link management requirements.

Another example: blood glucose meter

Blood glucose meter is designed for patients suffering from diabetes patients of important device.

Blood glucose control will undoubtedly help to reduce the risk of disease, improve the quality of life for patients. Blood glucose measurement usually has two methods: reflex photometer measurements and electrochemistry. In optical methods, blood sugar and paper on another compounds produce reaction produces a color mark, a color depth of the concentration with glucose. Reflectance spectroscopy measurement principle is by measuring the reflectance of light LED to quantify color depth: the higher the blood glucose concentration, reflected light is weak. LED will capture light and convert it into electronic signals.

However, most of the glucose meter are used to oxidation-based electrochemical method.

In this method, the blood glucose sensor is a picture contains enzymes and three electrodes Test Strip. One of the two electrodes for the measurement and the other is used as a reference. When blood samples placed in paper, electronic will be transmitted to the electrode. Electrochemical will quantify the number of electrons: number of electrons and the blood sugar concentration is proportional. When a reference voltage (typically 200mV) applied to the test strip, the e-mail will be converted to a blood glucose concentration is proportional to the current, this approach is called the current method. However, the current rather weak, generally between A μ to nA-level, and need to be converted into a voltage available for processing.

Figure 5 illustrates a complete programme of block diagrams, the system is battery operated and can be used to provide LM2623 glow power supply voltage.

Through the electrodes after the current measured through an operational amplifier is converted into a voltage, the voltage is then sent to microcontrollers, finally appears in the LCD screen.

R1= 47 kΩ and R2= 3 kΩ, RF= 25 kΩ

Next, and then look at this from a transimpedance to current to voltage conversion.

This topology will gain to output voltage and input current ratios to measure, meaning the feedback resistor Rf must be large enough to be detected by a small electric current. However, the amplifier input capacitance (Cin) plus the feedback resistor (Rf) will produce a phase lag behind, and this can lead to gain peaks and affects the stability of the circuit. Capacitor Cf is used to produce an extremely and increase the angular stability margin. In addition, this capacitor can also be used to limit the bandwidth to enable noise suppression. As previously mentioned, the sensor output current is very weak, this means that the amplifier must have extremely high precision specification. First, use the current in amplifiers must be very low noise, comes with 10 fA/sqrt (Hz)By LMP2232 is the best choice.

Another thing to note is the amplifier bias current, especially when the feedback resistor is large, this parameter is very important.

In addition, the impact of offset voltage is equally important that come from these two parameters of the error can be calculated into Verror = Vos + Ib x Rf. LM2232 in operating temperature range of offset voltage and offset current 230 V respectively μ (typical value is 10 μ V) and 50pA (typical for 20fA). Therefore, it applications this class LM2232.

As noted above, this amplifier is a micro-power components, dual-channel models only only consume 13 μ A current.

Conclusions of the

Whether blood glucose measuring ECG or application, portable medical devices, the most important of all is to ensure patient safety and convenience of mobility.

In the design of the system, ultra-low power consumption and high precision are non-negotiable requirements. These two examples show the United States National Semiconductor products in precision, low power consumption, size and flexibility advantages.