By richard wilson 2nd February 2015
There are a number of reasons why an engineer would want to “split” a voltage rail in their design. Sometimes parts of the circuit, like a sensor or an IC, require a bipolar supply.
Other reasons are to make the best use of the dynamic range of your ADC if it has a bipolar input, or to generate a mid-rail bias voltage in a single supply rail system. Whatever the reason, this article collects together a variety of different techniques that can be used, including some more unusual ones.
Some of the techniques given have a small footprint with low number of external parts, or a low quiescent current.
There are several different meanings of the phrase “rail splitter.” The usual meaning is creating a new “0V” reference point, usually the mid-point Vdd/2 of a single supply rail Vdd.
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| Figure 1 – TLE2426 Classic Rail Splitter IC Equivalent Circuit |
The available voltage remains the same as before, only you can view it as being distributed as a bipolar supply ±Vdd/2 about the new “0V” reference. A linear conversion is often used to generate Vdd/2, but a switched-mode conversion can also be used for high output power.
The new “0V” can be called a “virtual ground.” Figure 1 illustrates the concept, where an input voltage is divided by two and buffered to give Vout.
Another type of rail splitter takes a single rail Vin and splits it into bipolar outputs, ±Vout. For example, 3.3Vin might be “split” into ±5Vout. Such a process is usually done using two switched-mode conversions, often a boost plus a buck-boost regulator.
A third type produces a negative output from a positive input, for example creating –5Vout from 5Vin, to give a ±5Vout bipolar supply. (It could be argued that a different terminology should be used, as these circuits don’t create a new “0V,” so they aren’t virtual ground circuits.)
