2.1 HCSL Application

The VC-709 incorporates a standard High Speed Current Logic, HCSL ,output scheme which is a 15mA current source switched between Out and Complementary Out. Being unterminated drains, as shown in Figure 2-1, they require external 50Ω resistors to ground as shown in Figure 2-2. HCSL is a high impedance output with quick switching times, in can be advantageous to use a 10Ω to 30Ω series resistor as shown in Figure 2-3, to help reduce overshoot/ ringing.

Figure 2-1. Standard HCSL Output Configuration

Figure 2-2. HCSL Single Resistor Termination Scheme

Figure 2-3. HCSL 10Ω–30Ω Series Resistor Occasionally Used to Help Reduce Overshoot

One of the most important considerations is terminating the Output and Complementary Outputs equally. An unused output should not be left unterminated, and if it one of the two outputs is left open it will result in excessive jitter on both. PC board layout must take this and 50Ω impedance matching into account. Load matching and power supply noise are the main contributors to jitter related problems.