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Timothy Claycomb

Jan 8, 2016

opampportal

Precision
amplifier blog

pampportal

OPA191

Precision
Amplifier

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The basics: How to layout a PCB for an op amp

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Applications engineers tend to overlook printed circuit board (PCB) layout
during circuit design. It is often the case that a circuit’s schematic is
correct, but does not work, or perhaps works with reduced performance. In this
post, I will show you how to properly lay out an operational amplifier (op amp)
circuit PCB to ensure functionality, performance, and robustness.

An intern and I were recently working on a design together using the OPA191 op amp in a noninverting configuration with a gain of
2V/V, a 10kΩ load, and a supply voltage of +/-15V. Figure 1 shows the design
schematic.

Figure 1: Schematic of the OPA191 in a noninverted configuration

I tasked the intern with laying out the PCB for this design. I gave him some
general guidelines for laying out PCBs – keep traces as short as possible and
keep components close together to minimize board space – and sent him on his
way. How hard could it be? It’s just a few resistors and a couple of capacitors,
right? Figure 2 shows the first attempt at the layout. The red lines are traces
routed on the top layer and the blue lines are traces routed on the bottom
layer.

Figure
2: First layout attempt

That’s when I realized that PCB layout isn’t as intuitive as I thought; I
should have been a little bit more detailed with the guidelines. He did
everything I recommended, keeping the traces relatively short and the components
close together. The layout could be improved, however, to reduce PCB parasitic
impedances and optimize performance.

The first improvement we made is moving R1 and R2 next to the inverting pin
(pin 2) of the OPA191; this will
help reduce stray capacitance on the inverting pin. The inverting pin of an op
amp is a high-impedance node and is therefore “sensitive.” Long traces can act
as antennas, which allow high-frequency noise to couple into the signal chain.
PCB capacitance on the inverting pin can cause stability issues. Therefore, the
connections on the inverting pin should be kept as small as possible.

Moving R1 and R2 next to pin 2 allows R3, the load resistor, to rotate 180
degrees, which then allows the decoupling capacitor, C1, to move even closer to
the positive supply pin (pin 7) of the OPA191. It is
extremely important to place decoupling capacitors as close to the supply pins
as possible. Having long traces between the decoupling capacitor and the supply
pin adds inductance on the supply pin, which can degrade performance.

Another improvement we made concerns the second decoupling capacitor, C2. The
via connecting –VCC and C2 should never be put between the capacitor and the
supply pin; it should be in a place where the supply voltage must pass through
the capacitor before entering the supply pin of the device.

Figure 3 shows how to move each component and via to improve the layout.

Figure 3: Location of components for improved
layout

Even after moving the components to new locations, it’s still possible to
make additional improvements. You could make the traces as wide as possible to
reduce the inductance of the trace – about the size of the pad to which the
trace will connect. Another is to pour a ground plane on the top and bottom
layer. This will create a solid, low-impedance path for return currents.

Figure 4 shows our final layout.

Figure 4: Final layout

The next time you are laying out a PCB, make sure to follow all of these
layout practices:

Make the connections to the inverting pin as short as possible.

Place decoupling capacitors as close to the supply pins as possible.

If using multiple decoupling capacitors, place the smallest decoupling
capacitor closest to the supply pin.

Do not place vias between decoupling capacitors and supply pins.

Make traces as wide as possible.

Do not route traces with 90-degree angles.

Pour at least one solid ground plane.

Do not sacrifice good layout to label a component with silkscreen.

Additional resources

Learn more about stability in our TI Precision Labs – Op Amps training series on stability.

Read about issues related to decoupling capacitors in amplifier expert Art
Kay’s blog, “The decoupling capacitor … is it really necessary?”

Find commonly used analog design formulas in our wildly popular and free
Analog Engineer’s Pocket Reference e-book (myTI login
required).

Read more blogs about precision amplifiers.

Georgy

I think no satisfaction.

1 negative sourse line will going around left via "Uin",  so ground
plane will not ruptured.

2 make traces VCC as wide as possible? There not are...

3 groundplane  will not be placed ander Vin line, it decrease
parasitic capacitance.

4 you will read Figure 65. Operational Amplifier Board Layout for
Noninverting Configuration DATASHEET from.

Jan 18, 2016

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