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AD204JY Datasheet(PDF) 10 Page - Analog Devices |
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AD204JY Datasheet(HTML) 10 Page - Analog Devices |
10 / 12 page REV. D AD202/AD204 –10– High Compliance Current Source. In Figure 19, an isolator is used to sense the voltage across current-sensing resistor RS to allow direct feedback control of a high voltage transistor or FET used as a high compliance current source. Since the isolator has virtually no response to dc common-mode voltage, the closed- loop current source has a static output resistance greater than 10 14 W even for output currents of several mA. The output current capability of the circuit is limited only by power dissipa- tion in the source transistor. AD202 OR AD204 100k +15V –15V 470pF 10k +5V REF + – VC 20k LOAD IL = VC RS –10V TO +250V RS 1k MPS U10 1k Figure 19. High Compliance Current Source Motor Control Isolator. The AD202 and AD204 perform very well in applications where rejection of fast common-mode steps is important but bandwidth must not be compromised. Current sensing in a fill-wave bridge motor driver (Figure 20) is one example of this class of application. For 200 V common-mode steps (1 ms rise time) and a gain of 50 as shown, the typical response at the isolator output will be spikes of ±5 mV ampli- tude, decaying to zero in less than 100 ms. Spike height can be reduced by a factor of four with output filtering just beyond the isolator’s bandwidth. AD204 + – 5V M 5m 20A + 200V dc – 100mV Figure 20. Motor Control Current Sensing Floating Current Source/Ohmmeter. When a small floating current is needed with a compliance range of up to ±1000 V dc, the AD204 can be used to both create and regulate the current. This can save considerable power, since the controlled current does not have to return to ground. In Figure 21, an AD589 reference is used to force a small fixed voltage across R. That sets the current that the input op amp will have to return through the load to zero its input. Note that the isolator’s out- put isn’t needed at all in this application; the whole job is done by the input section. However, the signal at the output could be useful as it’s the voltage across the load, referenced to ground. Since the load current is known, the output voltage is propor- tional to load resistance. AD204 LOAD R 7.5V 1 F + 30k AD589 + – VO = RL VR R ILOAD = (2mA MAX) VLOAD 4V 1.23V R Figure 21. Floating Current Source Photodiode Amplifier. Figure 22 shows a transresistance connection used to isolate and amplify the output of a photo- diode. The photodiode operates at zero bias, and its output current is scaled by RF to give a 5 V full-scale output. AD202 OR AD204 0V TO 5V 500k PHOTO DIODE 10 A FS Figure 22. Photodiode Amplifier (NOTE: Circuit figures shown on this page are for SIP-style packages. Refer to Page 3 for proper DIP package pinout.) |
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