Datasheet AD202, AD204 (Analog Devices) - 10

AD202/AD204 High Compliance Current Source.
In Figure 19, an isolator
Floating Current Source/Ohmmeter.
When a small floating is used to sense the voltage across current-sensing resistor RS to current is needed with a compliance range of up to ± 1000 V dc, allow direct feedback control of a high voltage transistor or FET the AD204 can be used to both create and regulate the current. used as a high compliance current source. Since the isolator has This can save considerable power, since the controlled current virtually no response to dc common-mode voltage, the closed- does not have to return to ground. In Figure 21, an AD589 loop current source has a static output resistance greater than reference is used to force a small fixed voltage across R. That 1014 W

even for output currents of several mA. The output sets the current that the input op amp will have to return current capability of the circuit is limited only by power dissipa- through the load to zero its input. Note that the isolator’s out- tion in the source transistor. 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
–10V TO +250V
useful as it’s the voltage across the load, referenced to ground.
AD202
Since the load current is known, the output voltage is propor-
V OR I C L = LOAD R AD204
tional to load resistance.
S 7.5V AD204 RS 1k 30k LOAD + 470pF 1 F 100k + +15V V +5V REF R R VO = RL 10k R MPS AD589 U10 20k – 1k + VC – I 1.23V LOAD = (2mA MAX) –15V R VLOAD 4V
Figure 19. High Compliance Current Source
Motor Control Isolator.
The AD202 and AD204 perform Figure 21. Floating Current Source very well in applications where rejection of fast common-mode
Photodiode Amplifier.
Figure 22 shows a transresistance steps is important but bandwidth must not be compromised. connection used to isolate and amplify the output of a photo- Current sensing in a fill-wave bridge motor driver (Figure 20) is diode. The photodiode operates at zero bias, and its output one example of this class of application. For 200 V common-mode current is scaled by R steps (1 ms rise time) and a gain of 50 as shown, the typical F to give a 5 V full-scale output. 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
AD202
reduced by a factor of four with output filtering just beyond the
OR AD204
isolator’s bandwidth.
10 A 500k FS PHOTO DIODE 20A 5m 0V TO 5V M + + 200V dc – AD204 5V
Figure 22. Photodiode Amplifier
100mV –
Figure 20. Motor Control Current Sensing (NOTE: Circuit figures shown on this page are for SIP-style packages. Refer to Page 3 for proper DIP package pinout.) –10– REV. D Document Outline FEATURES APPLICATIONS GENERAL DESCRIPTION PRODUCT HIGHLIGHTS SPECIFICATIONS AD246–SPECIFICATIONS AD246 Pin Designations PIN DESIGNATIONS ORDERING GUIDE DIFFERENCES BETWEEN THE AD202 AND AD204 INSIDE THE AD202 AND AD204 USING THE AD202 AND AD204 Powering the AD204 AD246 Clock Drive Input Configurations Adjustments Common-Mode Performance Dynamics and Noise Using Isolated Power Operation at Reduced Signal Swing PCB Layout for Multichannel Applications Synchronization APPLICATIONS EXAMPLES Low Level Sensor Inputs Process Current Input with Offset High Compliance Current Source Motor Control Isolator. Floating Current Source/Ohmmeter Photodiode Amplifier OUTLINE DIMENSIONS Revision History