High Pass Filter Design Considerations and Beginner Tips

July 19, 2019 Cadence PCB Solutions

Cars on a busy highway at night

 

I have to admit that I’m quite impatient when I’m on the road. One thing that pushes me over is when a slow-moving vehicle is hogging the fast lane. That’s the occasion when I summoned all my willpower to prevent impatience from turning into anger. 

I wish such drivers will learn the difference between fast and slow lanes and when to use one or the other. Similarly, you’ll want to keep low frequencies from creeping into circuitry meant for higher frequency signals. Failing to do so can produce very unpleasant results for the end user.

What Is A High Pass Filter 

There is nothing better to prevent low-frequency signals from affecting their higher frequency counterparts than the high pass filter. If you’ve forgotten what a high pass filter is, let’s get back to basic electronics.

A high pass filter consists of an RC circuit, where the capacitor is placed in series of the signal source and the resistor in parallel. The goal of the high pass filter is to block off low-frequency signals and allow high-frequency signals to pass through.

The functionality of a high pass filter is easily visualized when you recall the characteristics of the capacitor in low and high frequency. At low frequency, the capacitor acts like an open circuit, blocking signals off. However, the capacitor becomes conducive as frequency increases. 

In a high pass filter, the cut-off frequency is an important parameter. It is given by the following formula. 

fc  = 1/2𝛑RC

 

The cutoff frequency is the threshold where lower frequency signals are suppressed by the high pass filter.

 

Capacitors and resistors on an orange circuit board

Capacitor is modeled as conducive at high frequency.

 

You’ll find that the high pass filters are a commonly used circuit in audio applications. It helps to keep low-frequency noises, such as random buzzing sound from coupling into audio signals. Often, the signal is filtered before it is channeled to an amplifier.

Passive vs. Active High Pass Filter

There are two types of high pass filter that are commonly used in electronic applications, namely the passive and active high pass filter. The passive filter involves passive elements and is what has been described in the above section.

As for the active high pass filter, it involves an additional component, which is the operational amplifier. The operational amplifier magnifies the signal that has been filtered with the preceding RC circuit.

The limitation of an active high pass filter lies in its frequency response. While the passive RC filter has an infinite frequency response, making it a true high pass filter, the same cannot be said with the active high pass filter. 

The frequency response of the high pass filter is limited by the operational amplifier. Often there is an upper limit of the frequency before the signal starts being attenuated. 

Important Considerations When Designing A High Pass Filter 

It’s a mistake to be complacent just because a high pass filter consists of elementary components. You should remember that the role of a high pass filter is crucial in keeping low-frequency noises from high-frequency signals.

Also, the presence of a high pass filter implies that the particular circuit is an analog one, which means more care should be given in maintaining signal integrity

Here are some helpful tips to aid you in designing with high pass filters.

1. Proper Grounding 

A high pass filter is as effective as the grounding technique that you employ on the PCB. You’ll want to ensure analog, and digital grounds are properly separated and connected only via a single point.

 

Ground plane on a circuit board

Proper grounding is important in high pass filter design.

 

2. Return Path

Besides grounding, it also helps to have a good idea of the return path of the audio signals. It is important to remember that at high frequency, signals no longer take the path of less resistance, but rather of lesser impedance.

3. Components Placement

The overall component placement of the circuit on the PCB is crucial. You’ll want to keep low frequency and high-frequency components on their respective areas. The same applies to analog and digital components.

4. RC Components Value

Of course, you’ll want to establish the cut off frequency of the filter and choose the right value for the resistor and capacitor. It helps if you choose high precision components with lower tolerance values. 

Translating High Pass Filter Design Considerations to Simulation Successes

Building the circuit for a high pass filter might be the first step in moving forward with a design needing one, however, ensuring your high pass filter works as needed and is accurately modeled is just as beneficial. 

SPICE tools should make it simple to plug in the components you’re working with, customize the circuit in the capacity you want, and see the results. With accurate AC sweep capabilities and effective output voltage mapped with a bode plot, you’re able to easily see things like adding a capacitor to your high pass filter circuit and its effect on frequency passing. 

It will be wise to use a PCB design software with analysis tools to simulate the frequency response of the designed high pass filter. The OrCAD PSpice Simulator is also helpful in detecting potential noise coupling into the audio circuit. 

If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts

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