Current monitor compensates for errors
Measure current up to 5A in the presence of 500V common-mode voltage.
Chau Tran and Paul Mullins, Analog Devices, Wilmington, MA; Edited by Martin Rowe and Fran Granville -- EDN, September 9, 2010
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You sometimes need to measure load currents as large as 5A in the presence of a common-mode voltage as high as 500V. To do so, you can use Analog Devices’ AD8212 high-voltage current-shunt monitor to measure the voltage across a shunt resistor. You can use this circuit in high-current solenoid or motor-control applications. Figure 1 shows the circuit, which uses an external resistor and a PNP transistor to convert the AD8212’s output current into a ground-referenced output voltage proportional to the IC’s differential input voltage. The PNP transistor handles most of the supply voltage, extending the common-mode-voltage range to several hundred volts.
An external resistor, RBIAS, safely limits the circuit voltage to a small fraction of the supply voltage. The internal bias circuit and 5V regulator provide an output voltage that’s stable over the operating temperature range, yet it minimizes the required number of external components. Base-current compensation lets you use a low-cost PNP pass transistor, recycling its base current, IB, and mirroring it back into the signal path to maintain system precision. The common-emitter breakdown voltage of this PNP transistor becomes the operating common-mode range of the circuit.
The internal regulator sets the voltage on COM to 5V below the power-supply voltage, so the supply voltage for the measurement circuit is also 5V. Choose a value for the bias resistor, RBIAS, to allow enough current to flow to turn on and continue the operation of the regulator. For high-voltage operation, set IBIAS at 200 μA to 1 mA. The low end ensures the turn-on of the bias circuit; the high end is limited, depending on the device you use.
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IBIAS=(V+–5V)/ RBIAS=495V/1000 kΩ=495 μA.
The circuit creates a voltage on the output current approximately equal to the voltage on COM plus two times the VBE (base-to-emitter voltage), or V+– 5V+2VBE. The external PNP transistor withstands two times the base-to-emitter voltage of more than 495V, and all the internal transistors withstand voltages of less than 5V, well below their breakdown capability.
Current loss through the base of the PNP transistor reduces the output current of the AD8212 to form the collector current, IC. This reduction leads to an error in the output voltage. You can use a FET in place of the PNP transistor, eliminating the base-current error but increasing the cost. This circuit uses base-current compensation, allowing use of a low-cost PNP transistor and maintaining circuit accuracy. In this case, current-mirror transistors, the AD8212’s internal resistors, and amplifier A1 combine to recycle the base current.
Figure 2 shows a plot of output-current
error versus load current with and
without the base-current-compensation
circuit. Using the compensation circuit
reduces the total error from 1 to 0.4%.
You should choose the gain of the load
resistor, RL, to match the input voltage
range of an ADC. With a 500-mV
maximum differential-input voltage, the
maximum output current would be 500
μA. With a load resistance of 10 kΩ,
the ADC would see a maximum output
voltage of 5V.Talkback
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1. The circuitry is drafted wrong regarding polarity of inputs of Op Amp inside of AD8212. So it could not work at all.
2. Assuming that real circuitry is the same as in datasheet, so the proposed idea is to fix the issue with limited voltage range for built in PNP transistor.
If ADI could include HV PNP transistor inside of chip the whole idea of compensation is not required.
3. The proposed compensation does not look right also because changes of base current of external PNP transistor through current mirror create the positive feedback loop, so it will hard to talk about error compensation.
Vladimir Doubovis - 2010-14-12 09:59:31 PST -
years ago our designers found a high voltage op amp .. they used it in a production circuit.. the app sheet was designed to sell expensive op amps... and we bought a ton of them .. i find that almost always you can design around common parts .. i did that using one low cost high voltage fet and a standard dual amp. my circuit had a much greater MTBF ..and cost hundreds of times less. it was more stable (including thermal and much better protected ... i had a huge cost reduction going on until someone tipped off the expensive op amp supplier and they dropped their price if we bought a lifetime supply ...someone in purchasing accepted that deal and the rest is history .. we likely sitll have a few thousand of those in stock .. yikes ...factory app sheets are designed to show you the possibilities a product can do ... not that it should do ...
harry - 2010-13-9 00:36:29 PDT -
I don't think that there is anything wrong in publishing design ideas submitted by designers from the manufacturerers of the devices as long as they are useful ideas. Generally, these designs tend to be more valid then many I have seen. We don't always have time to look through data sheets and application notes to pick up good ideas that are generally relevant to many. You may choose to have rules regarding awards but these guys also need recognition for their contribution to the state of the art.
Madhu - 2010-12-9 13:53:35 PDT -
First of all, this is an interesting and potentially useful idea.
If the authors of the design idea are also the authors of the application note or data sheet, then it would be totally acceptable for this idea to be published. It is certainly not the first time I have seen such. The big question should be the validity of the design and the potential value to others. This one seems to pass both of those tests, so why complain, except that possibly it would be helpful to ones career to publish some good design ideas.
William Ketel - 2010-10-9 16:35:38 PDT -
I didn't know that EDN accepts excerpts from a data sheet as a Design Idea with little modification.
Stuart Michaels - 2010-10-9 14:48:39 PDT
