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PRL-95000 Series Coupling Capacitor/DC Block

Mini Modular Instruments > Accessories > Coupling Capacitors

Introduction

When AC coupling digital signals, it is the longest 1’s and 0’s that determinate the value of the coupling capacitor required, not the data rate or frequencies, as in the case of RF signals. Furthermore, the average duty factor of the signal should be approximately 50% if the receiver is level sensitive. Consequently, AC coupling of random logic signals should be avoided, unless the receiver can tolerate large DC level shift.

PRL currently offers two models--a single channel 0.1 µf capacitor (95100001-01), and a dual-channel 0.1 µf capacitor (95100002-01). Additional capacitor values will be available soon. Units are supplied in a 1 x 1.3 x 1.5-in extruded aluminum enclosure, with SMA connectors.

The Equivalent Circuit

Fig. 1 shows the basic circuit for AC coupling a signal to the load, where RT is the total resistance of the circuit, consisting mainly of the sum of the source and load resistance, and C is the coupling capacitor. The time constant, τ of the circuit is RTC, and it has the unit of µs if RT is in Ohms and C is in µF, or ns if RT is in kΩ and C is in pf.

Fig. 1 Basic AC-Coupling Circuit

For example

If RT = 50 Ω (0.05 kΩ) and C = 100 pf, then τ = 5 ns.
If RT = 100 Ω (0.1 kΩ) and C = 100 µf, then τ = 100 µs.

In most practical cases, the stray capacitance across the load can be neglected.

When a voltage step with amplitude E is applied to the input at τ0, the output immediately rises to E. The output starts to decay towards zero with a time constant τ. After 4τ, the output will have discharged 98% of E and is nearly at ground potential.

For coupling applications, it is clear that one needs a capacitor large enough so that the output DC level remains relatively unchanged during the duration of a rectangular pulse.