Rail Splitter Notes
Many power supplies are based on having just two outputs (positive and negative) to power devices. But what do you do to power, say, an op amp circuit where you want not just positive and negative outputs but also a ground to “split” the power rails?
A simple method is to use a “rail splitter” that takes the two inputs and converts them to three outputs. Now we have our positive and negative rails with a ground electrically between them.
There are a number of ICs that may be used for this purpose, but many of them are for powering low current devices of less than 40ma or so; what about powering devices that may draw as much as 100ma at 12 volts or 75ma at 5 volts? In addition, there is also the problem of unbalanced loads that must be taken into consideration.
Let us look at a typical case of a circuit that has a load of 52ma on the positive rail and 72ma on the negative rail but the voltage, ideally, must have +12v on the one rail and -12v on the other rail.
With the rail splitter described here, bench tests with resistive loads under the above conditions, we find that the output voltage (ideally at +/-12vdc) is +12.030 and -11.800. Now the load we're talking about here is a relatively heavy load with a fairly large load imbalance and may consist of a number of ICs in a logic or low level audio circuit.
Question: Is this voltage differential too high and can it be reduced?
Most likely, your IC doesn't care if it’s working at +12.130v and -11.900v, but it may not like +12.030v and -11.800v. The voltage differential here is the same for both voltage levels, but the first example is now a little more balanced with regard to the +/-12v that we want.
The solution to this problem, of course, is to adjust the power supply output voltage to better center the voltage around the +/-12v that we want; it won’t be perfect but it should be good enough for most applications.
But what if you can't adjust your power supply? Another (perhaps less desirable) solution would be to place a dummy resistor to better balance the load between positive and negative. This can be viable with a load on a rail that is relatively steady, but will be less successful with a dynamic load that changes.