DESIGN
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temperature in the temperature-controlled chamber rises to 125°C in the experiment described in the section 'Measuring I B+ with Known Input Capacitance' (page six of AN-1373), we use materials that can withstand such a temperature. RG-316U was used as the material for the coaxial cable. Furthermore, the non-inverting inputs of the ADA4530-1 on the evaluation board are triaxial connectors. For this reason, a triax-to-coaxial conversion connector (BJ-TXP-1 from the Axis Company) was used. In this configuration, the guard terminal on the triax side was left floating. As a result of the measurement, C p = 73.6pF was obtained, which is a relatively large value since the actual measurement, according to AN-1373, is about 2pF. The reason for this is related to the cable length from the test box – which looks more like a test board – to the non-inverting input. Measuring I B+ with known input capacitance Finally, we start to measure the bias current. The circuit configuration is shown in Figure 1, and the mounted test box is shown in Figure 4. Note that the input resistor used in the section 'Measuring Total Input Capacitive with an Input Series Resistor' is removed. As described in AN-1373 (the capacitive integration measurement method, page seven), short circuit the SW to GND, then open it and monitor the output voltage fluctuation with a digital multimeter (DMM) for a few minutes (We used the 34401A DMM from Keysight Technologies). Finally, calculate the I B+ by substituting V OUT into Equation 1. The results of three measurements under the same conditions are shown in Figure 5. The lower part of the figure shows the output voltage fluctuation of the ADA4530-1 measured by the DMM, and the upper part shows the current value calculated using Equation 1. The figure shows that for all three instances, there is no repeatability in the measured voltage values. Therefore, the waveform of the calculated current value
and substituting it into Equation 1.
Figure 3. C p measurement setup: (a) inside the temperature-controlled chamber - the evaluation board of ADA4530-1 is shown - and (b) setup of the test box side.
Measuring total input capacitance with an input series resistor To calculate C p , this experiment adopts a method using series resistance. Figure 2 shows a simple circuit diagram. The value of the series resistance is based on the measurement guidelines found on page 6 of AN-1373, and the actual value is R s = 8.68M Ω . An SW is also mounted in the test box for later experiments (SW is open at this time). The frequency at which the waveform from the function generator is attenuated to –3dB can be measured, and the input capacitance can be calculated using Equation 2.
Figure 3 shows the setup. Since the
Figure 4. Setup of the capacitive integration measurement.
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