Thu 22 May 2008
One of the circuits that I am working on has an optically-isolated sub-circuit which runs on batteries. This is an analog device where the non-isolated input uses LOC110 optocouplers to send some program voltages to the isolated side which then drives a load accordingly and sends back the applied voltage and current back to the non-isolated side. One of the problems with the initial design is maintaining proper signal gain and offset across the optocoupler while the batteries on the isolated side drain.
The LOC110 is a nice device that has an infra-red LED that is driven and a matched pair of photo-resistive elements so that one can be used to control the LED and thereby set the current through the second device. One solution to the problem above was to use a pair of matched voltage references and track the current through the photo-resistive elements while driven at the reference voltage. This proved to be mostly successful, however, it did entail some research regarding voltage reference design.
The most simple voltage reference design involves only one diode operating in a controlled reverse breakdown mode. The exact reverse breakdown voltage depends on the device design (see this post about breakdown mechanisms), however, the general idea is that the current through the device increases very quickly when a certain reverse voltage bias is achieved. One only has to put a resistor in series with the reverse-biased diode that will ensure that the reverse-breakdown criteria are met given the power supply and a voltage reference is born. The devoted fan of μblog will surely exclaim: But Nick! Didn’t you use a reverse-biased diode as a temperature sensor once? What good is a voltage reference that varies with temperature? I would agree completely and exclaim that datasheets for voltage references display the equivalent circuit model of a diode in reverse breakdown, however, the actual designs are a bit more complicated.
This application note from Analog Devices provides some good insights into designing single-technology voltage references. The basic idea in many cases is to create a differential voltage by building mismatched devices or by introducing resistances, such as R3 above, and then using an amplifier to generate a reference voltage. To maintain good stability over a temperature range, the amplifier can be designed to increase gain in order to compensate for the beta degradation at higher temperatures. It is still possible to do all of this using a bi-polar process by designing biasing current sources that change appropriately with temperature. It is also possible, although more difficult, to introduce FET devices which can have opposite thermal effects as compared to BJTs. In this case, it would be fairly straight forward to increase the gain of the difference amplifier through increased FET trans-conductances with increased temperatures.
In the end, I still have some drift and offset problems that are not associated with temperature, however, the design process has been a good introduction to and has developed an appreciation for voltage reference designs.
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