Capacitors are elementary circuit components that can provide a vast range of functionality. At its most basic level, capacitors store charge up to a maximum capacity that they can then use to cover or smooth power demands when total circuit draw is high. However, their role in network analysis is more complex, as the current in the circuit “sees” charged and uncharged capacitors differently.
Additionally, DC and AC frequencies propagate differently through capacitors depending on their intrinsic capacitive reactance. At high speeds, designers can leverage this difference in circuit response with DC-blocking capacitors to isolate DC frequencies while coupling AC frequencies at the resonant frequency.
| Circuit Applications for DC-Blocking Capacitors | |
|---|---|
| Isolation | By default, capacitors in series with a source block DC flow after reaching saturation. This attribute helps create isolation between different power nets. |
| Coupling | While blocking DC flow, capacitors can couple AC signals, allowing them to pass through the capacitor unchanged (if attenuation is reasonably low). |
| Noise Filtering | DC-blocking capacitors can remove noise generated by switched-mode power supplies (SMPS) or high-frequency signals that couple to the power net. |
Why Do Capacitors Block DC?
The name “DC-blocking capacitor” can be a misnomer as all capacitors can block DC when fully charged. As a brief electromagnetism refresher, recall that capacitors in series with a source will oppose a change in voltage (even sourcing current from their stored electric field to do so); DC flows only unidirectionally in steady state conditions, so capacitors eventually saturate based on their size and dielectric strength and block any further flow of DC (unless the current overwhelms the capacitor and causes dielectric breakdown). Since AC switches direction based on frequency, the capacitor can partially discharge its stored electric field by displacement current according to its time constant (smaller capacitance and less resistance mean greater immediate discharge capabilities). Therefore, the capacitor never reaches a saturation point with AC as it does with DC. In the real-valued world of resistance, a saturated capacitor becomes an open circuit with infinite resistance.
Impedance vs. Resistance
However, resistance is only one component of impedance: capacitive reactance, defined as XC = (2πfC)-1, increases as frequency or capacitance increases (or both). However, note that capacitance in the equation is in units of farads, which can be a significantly large scale for many circuit applications. On the other hand, frequency can often reach well into the MHz or GHz range; at this juncture, the capacitor with initial charged steady state conditions will block DC and couple AC signals.
DC-Blocking Capacitor Use Cases
If all capacitors exhibit DC-blocking behavior, why carve out a specific “DC-blocking capacitor” niche? In many electronic applications, the voltage varies across the board due to the input needs of the particular components; for example, 5V, 3.3V, and 1.8V are all common power nets for electronic devices that prioritize power consumption rates. Where these components have to connect or interface, DC-blocking capacitors can ensure that the biasing from one power net doesn’t affect another. Additionally, DC-blocking capacitors will pass signals between filters or tuned devices operating at high enough frequencies, like radio and telecom equipment. Therefore, the selection of DC-blocking capacitors must be such that attenuation for the desired bandwidth is minimal (ideally zero).
Optimizing DC-Blocking Capacitor Selection and Implementation
Designers will likely determine a maximum acceptable attenuation rate and check components against this value using the reciprocal S21 transmission coefficient (also known as insertion loss). The insertion loss indicates power or voltage lost to transmission on a logarithmic scale (i.e., -3 dB is half-power loss, -6 dB is half-voltage loss, etc.). By selecting the maximum acceptable loss in dB and knowing the input power/voltage level, it’s trivial to calculate the output power/voltage level to see if it meets the circuit requirements based on the DC-blocking capacitors’ voltage rating. The voltage rating, at minimum, should be equal to the voltage difference between input and output. Finally, recall that XC is proportional to frequency and capacitance: circuit designers can further minimize insertion loss and impedance by selecting a DC-blocking capacitor with the minimum capacitance necessary for the desired frequency band.
Frequency Range and Voltage Rating
More practically, designers can select between DC-blocking capacitors using an iterative testing method to check in-circuit performance. Even with circuit modeling software saving time and money over a physical implementation, this can be a gradual (at best) process. Alternatively, selecting or purchasing wider frequency band capacitors is more likely to meet the circuit requirements but comes at an increased per-unit cost. Even then, these wider frequency band capacitors may not possess the requisite voltage rating for the application. The prevailing selection criteria should be (in order):
- Find the matching frequency band for the minimum and maximum application frequency.
- Use the lowest capacitor voltage rating possible to meet the voltage difference between the input and output sides of the capacitor.
Using Multiple DC-Blocking Capacitors
An even more comprehensive solution to selecting DC-blocking capacitors is to layer the effect of multiple distinct resonant frequencies. As capacitance increases, the resonant frequency of a capacitor increases. Recall that the resonant frequency is where the capacitive and inductive reactance are equal (and therefore cancel out): below this frequency, the capacitive reactance dominates, and above it, the inductive reactance dominates. If placement space allows, circuit designers combining multiple DC-blocking capacitors of different capacitances can cascade multiple AC shorts (i.e., zero idealized reactance) that more thoroughly couple input to output and minimize attenuation.
Additional Selection Considerations
It’s worthwhile to more explicitly state that driving a DC-blocking capacitor above its resonant frequency causes it to act as an inductor owing to the parasitic effects of the component dimensions. Consider a standard chip capacitor: the inductance is inversely proportional to the pad dimensions. A component manufacturer that can expand the pad dimensions without affecting the electrical characteristics or the solderability reduces the parasitic inductance, slowing the growth of inductive reactance as a function of frequency after the resonant frequency. Space permitting, larger capacitors (pads and footprints) will provide more robust performance at high frequencies.
Your Contract Manufacturer Prioritizes High-Speed Performance
Selecting DC-blocking capacitors will depend on the application, but designers should understand the cost and performance optimization available. While simulations will be crucial to analyze the circuit response of individual capacitors, more thorough methods of DC isolation, like combining the resonant frequencies of multiple DC-blocking capacitors, can coax out even better isolation. Here at VSE, our engineering team members are experts on high-speed performance and are committed to building our customers’ electronics. We’ve been realizing life-saving and life-changing devices with our manufacturing partners for over forty years.
