In the process of circuit design, application engineers often ignore the layout of printed circuit board (PCB). The common problem is that the schematic of the circuit is correct, but it doesn’t work, or it only runs with low performance. In this blog, I will show you how to correctly lay out the circuit board of operational amplifier to ensure its function, performance and robustness.
Recently, an intern and I were designing opa191 operational amplifier with a gain of 2V / V, a load of 10K Ω and a supply voltage of + / – 15V. Figure 1 shows the schematic diagram of the design.
Figure 1: schematic diagram of opa191 with non inverting configuration
I assigned an intern to lay out the circuit board for the design, and gave him general guidance on PCB layout (i.e. shorten the routing path of the circuit board as much as possible, and keep the components closely arranged to reduce the circuit board space), and then let him design by himself. How difficult is the design process? It’s just a few resistors and capacitors, isn’t it? Figure 2 shows the layout he tried to design for the first time. The red line is the path of the top layer of the circuit board, while the blue line is the path of the bottom layer.
Figure 2: first layout trial scheme
At that time, I realized that the layout of the circuit board was not as intuitive as I thought; I should give him some more detailed guidance. In the design, he fully complied with our suggestions, shortened the routing path, and closely arranged the components together. However, this layout can be further improved to reduce the parasitic impedance and optimize its performance.
The first improvement we have made is to move the resistors R1 and R2 to the inverting pin (pin 2) of opa191; This helps to reduce the stray capacitance of the inverter pin. The inverting pin of operational amplifier is a high impedance node, so it has high sensitivity. Longer routing paths can be used as wires to couple high-frequency noise into the signal chain. The PCB capacitance on the inverter pin will cause stability problems. Therefore, the smaller the contact on the inverter pin, the better.
By moving R1 and R2 to pin 2, the load resistor R3 can be rotated 180 degrees to make the decoupling capacitor C1 closer to the positive power supply pin (pin 7) of opa191. It is extremely important to keep the decoupling capacitor as close to the power supply pin as possible. If the path between the decoupling capacitor and the power supply pin is long, the inductance of the power supply pin will be increased and the performance will be reduced.
Another improvement we have made is the second decoupling capacitor C2. The connection between VCC and C2 should not be placed between the capacitor and the power supply pin, but should be placed at the position where the power supply voltage must enter the power supply pin of the device through the capacitor.
Figure 3 shows how to move each part and guide hole to improve the layout.
Figure 3: location of components in improved layout
After the components are moved to the new position, some other improvements can still be made. You can widen the routing path to reduce the inductance, which is equivalent to the size of the pad connected by the routing path. It can also irrigate the top and bottom ground layers of the circuit board, creating a solid low impedance path for the return current.
Figure 4 shows our final layout.
Figure 4: final layout
The next time you layout a printed circuit board, be sure to follow the following layout conventions:
Shorten the connection of inverting pin as much as possible.
Keep the decoupling capacitor as close to the power supply pin as possible.
If multiple decoupling capacitors are used, place the smallest one closest to the power supply pin.
Do not place the pilot between the decoupling capacitor and the power supply pin.
Widen the routing path as much as possible.
Don’t make a 90 degree angle in the routing path.
Perfusion at least one solid ground layer.
Don’t abandon good layout in order to use silk screen layer to mark parts.