Designing Operational Amplifiers: PCB Layout Tips to Reduce the Noise

February 7, 2018 CircuitStudio

Stereo dials with a hand turning one

To celebrate his retirement, my father bought himself a set of “super cool” stereo amplifiers. Unfortunately, every time I tried to play some music, I jumped in shock, as the volume control defaults are pretty loud. I’d clearly forgotten that stereo amplifiers aren’t just another set of speakers.

I was, however, reminded of the inner workings of the stereo, specifically a unique component that you’ve probably come across more than once in design: operational amplifiers (op amps). Op amp PCB layout guidelines are particularly helpful devices within a Printed Circuit Board design; however, they can be difficult to use effectively and without causing undue errors within your device. Having a basic understanding of how they might fail will inhibit mistakes in your designing.

Basic Principles of Operational Amplifiers

Operational amplifiers are signal conditioning components capable of filtering and amplifying signals through basic arithmetic operation. An operational amplifier has three terminals: an inverting input, a non-inverting input, and an output which forms the basis of many configurations.

All operational amplifiers display these characteristics: high open loop gain, high input impedance, low output impedance, and a limited bandwidth. A typical amplifier can have a gain of 10,000 or more. The input resistance is less than 100 ohms and the output resistance is often in the range of 0.25 MOhm and above.

Chart of operational amplifier formula

Operational amplifier: a simple but versatile electronics component.

Understanding the basic working of an operational amplifier is easy enough. If you apply a positive input to the non-inverting input, the operational amplifier will produce a positive swing at the output. Similarly, if you apply a positive voltage to the inverting input, a negative swing will occur at the output. In short, the output is proportional to the difference between the voltages on both inputs.

By connecting the output and input of the operational amplifier with feedback components like resistors and capacitors, you can derive different functions and gain. The list of operational amplifier applications can go on forever, but the most ones include voltage comparators, active rectifiers, signal filters, and voltage followers.

PCB Layout Tips for Operational Amplifiers

You’ll often find operational amplifiers being the center point between analog and digital signals. One use of operational amplifiers is amplifying raw analog signals from sensors before being processed by a microcontroller. They are also commonly used in audio applications. When designing with operational amplifiers, it is important to be aware of the following PCB design guidelines:

1. Place a Bypass Capacitor:

The operational amplifier needs a stabilized voltage to produce the right output signal. To reduce the noise produced when switching the power supply, place a bypass capacitor close to the supply pin of the operational amplifier. This will also reduce the possibility of harmonic distortions.

2. Avoid Ground Plane Placement Close to Input Pins

It’s common knowledge that setting a ground plane enhances circuit stability. However, in the case of operational amplifier design, you should avoid placing a ground plane close to the operational amplifier’s input pins. This prevents the introduction of stray capacitance and ground noise that may significantly affect the output of the operational amplifier.

3. Maintain Thermal Stability

When using a power operational amplifier, thermal stability is crucial to maintaining a low-temperature junction for stable operations. Proper heat dissipation techniques, including heat dissipation vias or heat sinks, may help in dissipating excessive heat.

4. Ensure Analog and Digital Separation

Proper design techniques for analog devices apply to operational amplifiers as well. If you’re using an operational amplifier for the signal gain of a temperature sensor, then place it as close to the sensor as possible. Minimize the length of input signals and make sure the operational amplifier is far away from other high-frequency tracks.

Closeup of electronic circuit board that is blue

A bypass capacitor will ensure the operational amplifier produces the correct output.

Avoid Schematic Errors With Interactive Design Software

Operational amplifier design can be a challenging and lengthy process without the right PCB layout technology. The ability to access complete design history and efficiently customize component layout can mean the difference between a nonfunctional schematic and a high-performing design.

Incorporating operational amplifiers is much easier when you have the right PCB design software to help you follow complex design rules. Altium’s CircuitStudio® will help you develop a thermally stable design with accurate integration and adequate clearance between analog and digital components.

Need more tips on designing operational amplifiers? Talk to an expert at Altium.

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