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AD698 bảng dữ liệu(PDF) 10 Page - Analog Devices |
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AD698 bảng dữ liệu(HTML) 10 Page - Analog Devices |
10 / 12 page REV. B –10– AD698 Determining LVDT Sensitivity LVDT sensitivity can be determined by measuring the LVDT secondary voltages as a function of primary drive and core posi- tion, and performing a simple computation. Energize the LVDT at its recommended primary drive level, VPRI (3 V rms for the E100). Set the core displacement to its mechanical full-scale position and measure secondary voltages VA and VB. Sensitivity = VSECONDARY VPRI × d From Figure 19, Sensitivity = 0.72 3 V × 100 mils = 2.4 mV /V mil d = –100 mils d = 0 1.71V rms 0.99V rms d = +100 mils V SEC WHEN V PRI 3V rms V A V B Figure 19. LVDT Secondary Voltage vs. Core Displacement Thermal Shutdown and Loading Considerations The AD698 is protected by a thermal overload circuit. If the die temperature reaches 165 °C, the sine wave excitation amplitude gradually reduces, thereby lowering the internal power dissipa- tion and temperature. Due to the ratiometric operation of the decoder circuit, only small errors result from the reduction of the excitation ampli- tude. Under these conditions the signal-processing section of the AD698 continues to meet its output specifications. The thermal load depends upon the voltage and current deliv- ered to the load as well as the power supply potentials. An LVDT Primary will present an inductive load to the sine wave excitation. The phase angle between the excitation voltage and current must also be considered, further complicating thermal calculations. APPLICATIONS Most of the applications for the AD598 can also be imple- mented with the AD698. Please refer to the applications written for the AD598 for a detailed explanation. See AD598 data sheet for: – Proving Ring-Weigh Scale – Synchronous Operation of Multiple LVDTs – High Resolution Position-to-Frequency Circuit – Low Cost Setpoint Controller – Mechanical Follower Servo Loop – Differential Gaging and Precision Differential Gaging AC BRIDGE SIGNAL CONDITIONER Bridge circuits which use dc excitation are often plagued by er- rors caused by thermocouple effects, 1/f noise, dc drifts in the electronics, and line noise pickup. One way to get around these problems is to excite the bridge with an ac waveform, amplify the bridge output with an ac amplifier, and synchronously de- modulate the resulting signal. The ac phase and amplitude in- formation from the bridge is recovered as a dc signal at the output of the synchronous demodulator. The low frequency system noise, dc drifts, and demodulator noise all get mixed to the carrier frequency and can be removed by means of a low- pass filter. The AD698 with the addition of a simple ac gain stage can be used to implement an ac bridge. Figure 20 shows the connec- tions for such a system. The AD698 oscillator provides ac excitation for the bridge. The low level bridge signal is amplified by the gain stage created by A1, A2 to provide a differential in- put to the A Channel of the AD698. The signal is then synchro- nously detected by A Channel. The B Channel is used to detect the level of the bridge excitation. The ratio of A/B is then calcu- lated and converted to an output voltage by R2. An optional phase lag/lead network can be added in front of the A compara- tor to adjust for phase delays through the bridge and the ampli- fier, or if the phase delay is small, it can be ignored or compensated for by a gain adjustment. This circuit can be used for resistive bridges such as strain gages, or for inductive or capacitive bridges that are commonly used for pressure or flow sensors. The low level signal outputs of these sensors are susceptible to noise and interference and are good candidates for ac signal processing techniques. Component Selection Amplifiers A1, A2 will be chosen depending on the type of bridge that is conditioned. Capacitive bridges should use an amplifier with low bias current; a large bleeder resistor will be required from the amplifier inputs to ground to provide a path for the dc bias current. Resistive and inductive bridges can use a more general purpose amplifier. The dc performance of A1, A2 are not as important as their ac performance. DC errors such as voltage offset will be chopped out by the AD698 since they are not synchronous to the carrier frequency. The oscillator amplitude and span resistor for the AD698 may be chosen by first computing the transfer function or sensitivity of the bridge and the ac amplifier. This ratio will correspond to the A/B term in the AD698 transfer function. For example, sup- pose that a resistive strain gage with a sensitivity, S, of 2 mV/V at full scale is used. Select an arbitrary target value for A/B that is close to its maximum value such as A/B = 0.8. Then choose a gain for the ac amplifier so that the strain gage transfer function from excitation to output also equals 0.8. Thus the required am- plifier gain will be [A/B]/ S; or 0.8/ 0.002 V/V = 400. Then select values for RS and RG. For the gain stage: |
Số phần tương tự - AD698 |
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Mô tả tương tự - AD698 |
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