Apparatus including a reference circuit configured to provide a particular impedance and having a first plurality of switching devices and a resistive device coupled to each other in parallel; a second plurality of switching devices coupled to each other in parallel and coupled in series with the reference circuit between a supply node and a supply return node; and processing logic coupled to the second plurality of switching devices and configured to selectively enable and disable a combination of switching devices of the second plurality of switching devices that results in an impedance of the enabled switching devices more closely matching the particular impedance of the reference circuit than at least one other combination of enabled and disabled switching devices of the second plurality of switching devices.