Complex photonic devices require many electrical connections. A quick way to save a lot of connections is to use a common ground wire for several electro-optic devices (e.g. heaters, or photodiodes). For N devices, this saves N – 1 connections, which can translate to significant reductions in wire-bonding or multi-point probe size, and ultimately to savings in time and money.
Crosstalk is a well-known problem with common-ground connections, even for DC signals. It arises from small resistances between different parts of the on- and off-chip ground network. In its simplest form, this is a resistance between the on-chip common ground and the off-chip true ground, labeled RGND in the above diagram. As the input voltages (V0..3) change, the current flowing through RGND also changes, and so the voltage of the on-chip common ground changes depending on the specific configuration of the input voltages. In this way, the voltage dropped across any given load resistor (R0..3) depends on all the other voltages—the definition of crosstalk. This problem manifests itself most strongly when the load resistances are small, though is always present.
To counter this problem, all Qontrol drivers have the ability to drive a specified current through each output. Such a setup is shown on the right side of the above diagram. Since the current flowing though each load is chosen by us (I0..3) we directly control the power dissipated in each load, independent of how much current flows through RGND.
Caution: When driving in current mode, be sure to set a maximum voltage for each port. This will protect your device in the event of a larger-than-expected load resistance, or an open circuit.