Introduction: A Frustrating Debugging Experience

Last year, in a project, a 16-bit ADC was acquiring sensor data. The measured noise was extremely high, with the SNR nearly 15dB lower than the theoretical value. After checking everything, the power supply ripple was fine, the reference voltage source was stable, and sufficient decoupling capacitors were added around the ADC. Finally, the problem was discovered in an inconspicuous place—a via was used for the analog input signal line, and it was moved to an inner layer.

At the time, that via was less than 3mm away from the digital clock trace's via. After redesigning, placing all analog signals on the top layer immediately solved the problem. This experience was quite painful and gave me a deeper understanding of the topic of "analog signal line vias."

In fact, this problem is quite common. Many engineers have polarized attitudes towards vias when designing PCBs: either they are afraid to use them, wanting to route all traces on the same layer; or they use them carelessly, completely disregarding vias. Both extremes can lead to problems.

What impact do vias have on analog signals?

To understand when to use vias and when not to, we must first understand what vias do to analog signals. A via is not simply a "wire connection"; it is essentially a structure with parasitic inductance and capacitance.

A 0.3mm diameter through-hole has a parasitic inductance of approximately 0.5~1.2nH and a parasitic capacitance of 0.3~0.8pF. These values ​​seem small, but their impact on analog signals can be much greater than you might imagine.

The Impact of Parasitic Inductance
Parasitic inductance interacts with capacitance in the signal path to create an LC filtering effect, leading to attenuation of high-frequency components. This effect is significant for high-frequency analog signals (such as RF front-ends). In my experience, at frequencies above 500MHz, the insertion loss of a single via can reach 0.2~0.5dB.

More problematic is that inductance slows down the rise and fall edges of the signal. For high-speed analog signals, this translates to bandwidth loss. For sampling clock signals, a slowed edge directly introduces jitter, affecting the ADC's SNR.

The Impact of Parasitic Capacitance

Parasitic capacitance is more insidious. Capacitance forms between the via pad and the reference plane, which is applied to the signal line, causing impedance drop. For high-impedance nodes (such as the op-amp input), this capacitance forms a voltage divider with the source impedance, leading to signal attenuation.

[Case Study] In a precision measurement circuit, the op-amp input impedance is 1MΩ, and the via parasitic capacitance is 0.5pF. At 100kHz, the capacitor impedance is approximately 3.2MΩ, and the effect is not significant. However, at 10MHz, the capacitor impedance drops to 32kΩ, and the signal is attenuated by 30 times!

Stub Effect: A Neglected Pitfall
If a via is not fully utilized (e.g., from L1 to L3, but the via runs through the entire board), the lower half of the via becomes a "stub." This stub acts like an antenna, resonating at a specific frequency.

The formula for calculating the resonant frequency is: f = c / (4 × L × √Dk_eff)

Where L is the stub length, and Dk_eff is the effective dielectric constant. Insertion loss increases dramatically when the stub length reaches a quarter wavelength. For a standard 1.6mm thick four-layer board, the stub resonant frequency is approximately 10~15GHz. However, if the board is thicker or the stub is longer, the resonant frequency will be lower, affecting higher frequency analog signals.

【Warning】The effect of the stub is not linear. Signal quality deteriorates drastically near the resonant frequency. If your analog signal frequency happens to fall near the resonant point, the consequences can be severe.

Return Path Disrupted

This is the biggest hidden danger of analog signal vias. When a signal changes layers, the return current also changes layers. If the signal changes from L1 to L3, the return current, which originally flowed on the ground plane of L2, now needs to find a path back to the corresponding ground plane of L3.

Without matching grounding vias, the return current has to take a longer route, forming a large current loop. This loop acts like an antenna, both transmitting and receiving interference. For weak analog signals, this is fatal.

When can you use vias?

Having discussed so many risks, does this mean analog signals can't use vias at all? Not necessarily. In some cases, using vias is reasonable, even necessary.

Low-frequency analog signals can use vias.

Analog signals with frequencies below 10MHz are not very sensitive to the parasitic parameters of vias. Ordinary audio signals, DC bias, and low-speed sensing signals can safely use vias for layer switching. Just be careful not to use too many.

Personally, I think the impact of vias on DC and low-frequency signals is negligible. Unless your signal is extremely weak (in the microvolt range), don't worry too much.

Power and ground lines must use vias.

Using vias for power and ground lines is necessary, and you should use many. Power Distribution Networks (PDNs) require low-impedance paths, and via inductance is a bottleneck. The equivalent inductance decreases with parallel connections.

【Recommendation】For power vias, at least 2-3 vias are recommended for 1A current. More vias are needed for high-current applications (e.g., power module inputs); don't skimp on space.

Visits can be used when a matching return path exists.

If a ground via is located next to a signal via, and the ground via is very close to the signal via (ideally less than 100mil), the return path is complete. In this case, the impact of vias on analog signals is greatly reduced.

Specifically, each time a signal via changes layer, place a ground via next to it to connect the ground planes of the old and new layers. For differential signals, it's better to place a ground via between two signal vias.

Blind vias/buried vias can be used.

Blind vias connect only an outer layer to an inner layer, and buried vias connect only an inner layer; their parasitic parameters are much smaller than those of through-hole vias. More importantly, blind and buried vias do not create long stubs, making them much more friendly to high-frequency signals.

If cost allows, blind or buried vias should be preferred for high-precision and high-frequency analog circuits. Especially for 24-bit and above ADCs and GHz-level RF circuits, blind and buried vias are almost standard.

When should you not use vias?

In some cases, it's best to avoid vias for analog signal lines, or be extremely cautious.

High-precision analog signals require caution.

For 16-bit and above ADCs/DACs, or systems with a signal-to-noise ratio requirement exceeding 80dB, the analog signal path should be as clean as possible. Parasitic parameters introduced by vias can lead to increased quantization errors and deterioration of INL/DNL.

[Example] A 24-bit data acquisition system was designed with a theoretical SNR of 112dB. Actual testing showed only 95dB. After investigation, it was found that the analog input lines had vias, and the stub resonant point fell right at the edge of the signal bandwidth. After changing the routing to the same layer, the SNR improved to 108dB.

Be cautious with high-frequency analog signals.

For analog signals exceeding 100MHz (RF, high-speed clock), the parasitic inductance of vias can become a bottleneck. Signal edges will be degraded, impedance discontinuities will appear, leading to reflections.

For RF signal layer switching, it's best to use specially designed via structures, combined with anti-pad optimization and ground via fencing. Simply placing ordinary vias directly will result in poor VSWR.

Do not place vias below sensitive analog areas.

Avoid placing unrelated vias near sensitive circuits such as crystal oscillators, phase-locked loops, reference voltage sources, and high-impedance input nodes. Vias can disrupt the integrity of the ground plane and "guide" noise from other layers.

【Note】Especially for digital signal vias, never pass through analog circuit areas. High-frequency noise from digital signals can couple to analog lines through the parasitic capacitance of the vias. In my experience, digital vias should be at least 10mm away from sensitive analog circuits.

Be cautious when the ground plane is interrupted.

If vias are densely packed, creating a large window (anti-pad) on the ground plane, the continuity of the ground plane is disrupted. Return current is forced to detour, forming a loop antenna.

This problem is particularly severe on mixed-signal PCBs. If the analog ground plane is interrupted by vias, digital noise can intrude into the analog area through coupling paths.

Practical Design Considerations

Having understood the principles and boundary conditions, how should we proceed in actual design? Here are a few personal tips:

Plan your routing strategy to minimize layer changes.

The best vias are those that are not drilled. During the placement phase, clearly define the routing path, and try to ensure that critical analog signals are completed on the same layer. If a layer change is absolutely necessary, prioritize changing it near the chip pins, and avoid suddenly drilling vias midway through the trace.

Optimize Via Parameters

If vias are necessary, optimize them to the extreme:

• Smallest via diameter:0.2mm or smaller, resulting in lower parasitic parameters.
• Appropriately enlarge anti-pads:Standard is 10mil, high-speed signals can be enlarged to 20~30mil.
• Packs should not be too large:Excessively large pads increase parasitic capacitance and take up space.

Matching Return Vias

For each signal via, consider the return path. If the signal changes from L1 to L3, and the ground plane is on L2, then a ground via should be placed next to the signal via to connect the grounds of L2 and L3.

The ground via should be as close to the signal via as possible; within 100mil is a safe range. Within 50mil is even better.

Analog-Digital Separation and Isolation

For mixed-signal PCBs, analog and digital areas must be physically isolated. Vias should also be separated, with analog vias in the analog area and digital vias in the digital area. Don't let digital vias "traverse" the analog area.

If mixed-signal devices like ADCs/DACs are present, place vias near the devices to prevent analog signals from traveling long distances through the digital area.

Simulation Verification:

For high-speed, high-precision designs, don't rely solely on experience. Use SI simulation tools to check the impedance, reflection, and insertion loss of vias. Especially the stub resonant point; simulation will reveal it immediately.

Common Misconceptions Clarified:

• "Fewer vias are better"

—Not entirely true. Signal vias should indeed be fewer, but power and ground vias should be more numerous. The key is to treat them differently.

• "Analog ground must be separated from digital ground"

—Not absolutely. Simple systems often benefit from a unified ground plane. Complex systems require separation, and even then, single-point connections are necessary.

• "Blind vias are too expensive and unnecessary"

—It depends on the application. For 24-bit ADCs and GHz RF, blind vias are a worthwhile investment. For ordinary applications, they are indeed unnecessary.

Summary:

Can analog signal lines use vias? The answer is: It depends. Low frequencies are not sensitive, so vias can be used; high precision requires caution, so avoid vias if possible; high frequencies require special handling, and if used, parameters should be optimized. The core principles are:

• Avoid vias if possible;

Plan your routing strategy well to reduce layer changes.

• If you must use vias, use them well;

Optimize via diameter, anti-pads, and use matching return vias.

• Detour sensitive signals;

Route high-precision, high-frequency analog signals to the top layer to avoid stubs.

• Divide analog and digital signals;

Do not cross zones with vias to avoid noise coupling.

• Simulate and verify;

Don't rely solely on experience for high-speed, high-precision designs.

Though vias are small, there's a lot to learn. Understand the principles, grasp the boundaries, and analog signal vias won't become pitfalls in your designs. I hope this experience is helpful.