Op-Amp Layout Guidelines for Mixed Signal Designs

Key Takeaways

• Op-amp layout needs to focus on a compact placement with isolation between input/output and digital/analog.

• A guard ring around the input nets can prevent leakage current to or from the input region.

• Designers will also want to account for the analog/digital grounds to prevent noise on the more sensitive analog components.

Op-amp layout guidelines often utilize the standard 8-pin DIP package.

Operational amplifiers, or op-amps, are a cornerstone of circuit design. They are indispensable for their ability to amplify low-voltage/current signals according to the feedback network that links the output to the input. The feedback is essential not only in terms of operation but also for the layout; as these circuits fold the output back into the input, the overall footprint of the circuit can occupy a relatively tiny area. While net advantageous, there are some pointers designers will want to keep in mind to maximize performance. These op-amp layout guidelines touch on general and specific best practices worth monitoring during the layout and preceding phases of board design.

Op-Amp Layout Guidelines by Design Phase

Providing Low-Impedance Signal Pathways

Op-amps are fundamental circuit elements that provide active (i.e., power pins) amplification of input signals based on their differential input and the feedback circuit. While the ideal performance of the op-amp and any interfacing components is relatively straightforward, additional, non-idealized complications can lead to considerable error between simulation and the physical implementation. Moreover, designers must be aware of a poor circuit layout's impact on performance. Most of these layout techniques are generally applicable, but some additional items are worth heeding due to the unique operation of the device.

Low-capacitance bypass caps will be necessary for the power pins to form a low-impedance pathway between the appropriate pin/plane combination. This configuration will reduce the chance and severity of noise coupling to the line, which distorts the amplified signal. Consider running a trace directly from the op-amp pin in question to the appropriate pad of the capacitor before terminating in a via (if the project allows for via-in-pad fabrication, place the via directly on the pad itself). Without a via-in-pad, simply run the trace past the pad of the capacitor before terminating; note that a via placed between the capacitor pad and op-amp pin is substandard due to the possibility of line reflections. Additionally, increasing the width of the power traces above the standard trace width can also improve impedance; however, ensure that the thickness of any trace is less than that of the pad dimensions (a polygon or fill that expands after the extent of the amplifier pins may be a suitable option).

Along the same lines, the closer feedback network components placement is to the op-amp, the better; provide only enough room for component placement to enable trace breakout. Keeping short trace lengths is critical for a low-impedance pathway from these supporting components to the op-amp. A tight placement also synergizes with a guard ring, which can protect against stray leakage currents flowing into or out of the differential inputs. By encircling the inputs and any associated pads with a trace tied to one of the input nets at some voltage, currents cannot flow into or out of the differential inputs because there is no electropotential difference within the enclosed region.

Keeping Op-Amp Layout Guidelines Grounded

The issue of ground is an important factor in any PCB operation, and arguably doubly so for op-amps due to their analog nature. Mixed signal boards containing both digital and analog components must balance the ground planes carefully against isolation concerns: the high-frequency nature of digital electronics can produce noise that can easily couple to analog components through the ground plane. This situation can cause misconceptions regarding the ground plane. Essentially, two discrete ground planes are necessary – analog ground and digital ground – yet the planes still need a single connection point to ensure the “distinct” ground planes do not differ as a reference potential (remember: ground is ground is ground!)

This description may sound like double talk, but the detail is essential. The primary goal with the respective ground planes is to isolate any high-frequency noise to the digital section of the board, where its impact is far less detrimental than it would be coupling to the analog components. Effectively, the relative isolation of the analog/digital components and their traces is the most important outcome. Separation of input traces from supply/output traces (excluding the feedback line) will also help limit noise on the input; where these traces are nearby, keep parallel track lengths to a minimum and use perpendicular crossings on signal layers to minimize coupling opportunity. 

A good ground layout will also rely heavily on the stackup of the board. This treatment will begin by assessing the layer count necessary to meet the routing goals of the design; while an increase in layers may be outside of the designer’s control, consider the performance benefits of a multi-layer board over single- or double-layer boards if cost allows. The primary advantage is improved EMI rejection due to the plane effect, which offers excellent noise-canceling properties due to the equal-magnitude, opposite-direction current flows. The referenced ground/power planes must be continuous for this effect, i.e., a trace passing over a gap, discontinuity, or referencing multiple ground/power planes in its travel path will instead become a significant source of EMI radiation that will require correction before laboratory testing.

Cadence Simulations Beget Signal Integrity

Op-amp layout guidelines are similar to general best practices. Still, mixed-signal designs (which are increasingly more likely with the overall trend of component digitization) mean designers will want to carefully account for signal integrity to prevent noise, distortion, and other issues.  The first step should always be thoroughly modeling circuit characteristics and a stackup founded on low-EMI principles. Fortunately, Cadence’s PCB Design and Analysis Software suite grants design teams a robust and expansive simulation environment with exceptionally accurate real-world results. Designers can then easily transfer simulation data to OrCAD PCB Designer for rapid design prototyping and revisions that reduce time-to-market.

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