ULTRASONIC SENSOR ICs MEASUREMENT OF AMBIENT TEMPERATURE AN 0084

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1 1 Description This application note demonstrates a simple approach to measure the ambient temperature in an ultrasonic sensor module. This information can be used to compensate the temperature dependent shift of the transducer frequency. In order to keep the additional effort as small as possible, a package with a dual sensor diode is used. 2 Scope If it is not specially marked, this document is valid for the following Elmos Ultrasonic Sensor ICs: E / E / E / E Circuit setup The Elmos IC has an integrated 8bit ADC which is mainly used for ultrasonic signal processing. But through a spe - cial register the ADC can be used also for other purposes. One option is to measure the voltage at GPIO4. Thus, the only place to connect an analog temperature sensor to the IC is pin GPIO4. There are two possibilities to con - nect a temperature sensor: One possibility is to use a temperature dependent resistor with a temperature independent bias resistor. Another approach, which is described in this application note, is to use a silicon diode biased with a con - stant current. The figure 5 is taken from the data sheet of BAV99 [3]. It can be seen that a lower bias cur - rent gives a higher sensitivity to temperature changes. Because the analog supply is 3.3 V, some diodes can be stacked, so it gives a better adjustment to the ADC input voltage range. Care should be taken when the diodes are mounted far away from GPIO4. Because of the small bias current the line is sensitive to any noise coupled into it. For use in the application a calibration is needed. During end of line calibration process, also the temperature measurement should be done and the result of this measurement stored into EEPROM as room temperature ref - erence value. The temperature slope of the diodes forward voltage depends only on the bias current and can therefore assumed to be constant for the precision required for ultrasonic applications. In this application note the BAV99 is used, because the small SOT23 package contains two diodes in the right configuration. The GPIO of the Elmos IC has a pull-up option giving a pull-up current of about 30 μa (see [1], GPIO characteristics). This current is low enough to avoid any heating of the diode above the ambient temperature. The overall effort to add temperature measurement to this device is one external component, a dual diode from GPIO4 to ground. The modification is shown in the schematics (figure 4). 1/8

2 4 Software setup The functions for controlling the ADC are already included in s2g_util. Using these functions a routine was con - structed to read the ADC 8 times and put the results into a LIN frame. /* */ static bool_t loc_linsendtemperature( volatile lin_msgbuffer_t *msg) /* LIN callback */ /* */ { uint8_t i; /* fill 8 bytes answer with temperature measurements */ GPIO_CFG->val &= (uint8_t) 0x77; /* Pullup for GPIO4 on */ gpio_set_pullup(0x08); util_adcselectinput (util_adc_gpio4); for ( i = 0; i < msg->msglen; i++ ) { /* fill LIN buffer with error memory */ msg->msg.std.data[ i] = util_adcmeasure ( ); } /* tell the SendAnswer routine how much byte you want to send out */ msg->buflen = msg->msglen; /* switch to answer mode at LAST operation of your routine */ msg->state = lms_answering; /* if do not return TRUE, the answer will not be send an an error state will be assigned */ return(true); } If shown in this example only a small number of code lines is needed to implement the temperature measure - ment function. Please be aware that the ADC is not reset to the previous function, so starting an ultrasonic measurement should set up the ADC properly. This routine is called when LIN identifier 4 appears on the bus and the message length is 8 bytes. The routine has to be entered in the LIN Message Table and then everything is done. 2/8

3 5 Temperature Chamber Measurements To check the function, described in chapter 4, the ambient temperature is measured continuously in a temperature chamber. Therefore three boards with diodes mounted on every board were prepared. These boards were programmed with different identifiers, so all modules are connected to the same bus without making use of pincoding or auto addressing. After the chamber has reached a constant temperature, the measurement data was taken automatically. About 50 data frames of each sensor were logged for each temperature. The results of the complete measurement cycle are shown in the following figures 1, 2 and 3. Shown is the average value (LSB at the ADC), the minimum measurement and the maximum measurement at temperatures between -40 C and 100 C. The calc -curve is the linear approximation of the measurement results. Temperature Measurement Device ID ADC Value [LSB] Temperature [ C] Diodes Diodes_min Diodes_max Diode_calc. Figure 1: Temperature Measurement Device ID 1 3/8

4 Temperature Measurement Device ID ADC Value [LSB] Temperature [ C] Diodes Diodes_min Diodes_max Diode_calc. Figure 2: Temperature Measurement Device ID 2 Temperature Measurement Device ID 3 ADC Value [LSB] Temperature [ C] Diodes Diodes_min Diodes_max Diode_calc. Figure 3: Temperature Measurement Device ID 3 4/8

5 5.1 Measurement results As shown in figure 1, the temperature range is from -40 C to 100 C and the measured temperature varies (LSB of the ADC) from 93 LSB to 36 LSB (= 57 LSB). Δ ADC LSB Δt = 57LSB =0.407 LSB / C 140 C The slope of the temperature sensor is LSB/ C. Also the two other boards provide the same result. The internal ADC has a 3.3V supply, thus the temperature difference corresponds to 12.9mV 57 = 734mV and the temperature coefficient of a single diode can be calculated as follows: ΔV Diode = 734mV =2.62mV / C Δ t C This value is close to the expected value calculated from the graph in figure 5. 6 Summary The adding of external diodes generates a good temperature sensor. The slope of this temperature sensor varies in a small range between and LSB/K. Also the difference in offset at 0 C is below 1 bit. Thus, this approach gives a stable result to measure the ambient temperature of the ultrasonic sensor module. Also, if the tolerance is acceptable, any end of line calibration is not needed. 5/8

6 Figure 4: Modified evaluation board with dual diode on GPIO 4 6/8

7 Figure 5: Diode forward voltage as function of temperature and bias current Sample measurement with 30µA (blue curve) 7 References [1] 25DS0075E.xx Data sheet E / E / E / E (Elmos Semiconductor AG) [2] 25AN0083E.xx Application note Temperature behavior of transducer (Elmos Semiconductor AG) [3] Data sheet BAV99 (Infineon) 7/8

8 WARNING Life Support Applications Policy Elmos Semiconductor AG is continually working to improve the quality and reliability of its products. Nevertheless, semiconductor devices in general can malfunction or fail due to their inherent electrical sensitivity and vulnerability to physical stress. It is the responsibility of the buyer, when utilizing Elmos Semiconductor AG products, to observe standards of safety, and to avoid situations in which malfunction or failure of an Elmos Semiconductor AG Product could cause loss of human life, body injury or damage to property. In the development of your design, please ensure that Elmos Semiconductor AG products are used within specified operating ranges as set forth in the most recent product specifications. General Disclaimer Information furnished by Elmos Semiconductor AG is believed to be accurate and reliable. However, no respons - ibility is assumed by Elmos Semiconductor AG for its use, nor for any infringements of patents or other rights of third parties, which may result from its use. No license is granted by implication or otherwise under any patent or patent rights of Elmos Semiconductor AG. Elmos Semiconductor AG reserves the right to make changes to this document or the products contained therein without prior notice, to improve performance, reliability, or manufacturability. Application Disclaimer Circuit diagrams may contain components not manufactured by Elmos Semiconductor AG, which are included as means of illustrating typical applications. Consequently, complete information sufficient for construction purposes is not necessarily given. The information in the application examples has been carefully checked and is believed to be entirely reliable. However, no responsibility is assumed for inaccuracies. Furthermore, such information does not convey to the purchaser of the semiconductor devices described any license under the patent rights of Elmos Semiconductor AG or others. Elmos Semiconductor AG Headquarters Heinrich-Hertz-Str Dortmund Germany Phone + 49 (0) Fax + 49 (0) sales@elmos.com 8/8

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