Microphone PCB: Design for Low Noise Audio

What Makes an Audio Board Different

An audio board handles signals that are small, slow and analogue. A microphone capsule produces millivolts or fractions of a millivolt, the preamplifier works at a level where a stray microampere of injected current is audible, and the whole chain has to stay linear and quiet in a product that also contains a processor, a display and a switching power supply.

The design problem is therefore not gain but noise. Every decision about the ground return, the trace length, the component placement and the shielding is a decision about how much noise is added to the signal before it is amplified.

The Capsule

Electret condenser capsules. A permanent polarised diaphragm and a built-in junction field effect transistor, needing a supply through a load resistor, usually in the range of 2 to 10 kilohms. They are available in a wide range of sensitivities and sizes and are still the standard in measurement and voice applications. The load resistor value trades gain against the noise contribution of the capsule itself.

MEMS microphones. A silicon diaphragm with an integrated amplifier, supplied as a surface mount package with an analogue or a digital output. They are consistent, small, reflow compatible and inexpensive, and they have made the loaded PCB microphone practical. The digital output versions carry a pulse density or I squared S signal, which removes the analogue routing problem entirely.

Dynamic capsules. A moving coil, needing a transformer or a low noise preamplifier, used in some instrument and studio applications for their acoustic character.

The choice already fixes much of the design. A digital MEMS part removes the analogue chain; an electret capsule forces the designer to manage a high impedance node.

The Preamp and the High Impedance Node

An electret capsule with its load resistor presents a high impedance at the node where the signal is taken. That node is the most sensitive point on the board, and three rules protect it.

Keep the trace as short as physically possible. Place the load resistor and the first amplifier stage immediately next to the capsule. A few centimetres of trace at that impedance is an antenna for capacitive pickup and a path for leakage current.

Guard the node. A guard ring driven at the same potential as the signal reduces the effect of both leakage and capacitance. At the very least, surround the node with ground copper connected to the analogue ground.

Keep the board clean. Flux residue and surface contamination create leakage paths that add to the signal and, in humid conditions, produce a drift that looks like a microphone fault. A coated or carefully cleaned board is part of the design.

The preamplifier itself is chosen for its input voltage noise and its bias current. A low noise bipolar input suits a low impedance source such as a dynamic capsule or a MEMS analogue output; a junction field effect or CMOS input suits the high impedance of an electret load, where the bias current matters more than the voltage noise.

Grounding

The reference for an audio signal is the ground, and the current in that ground is the noise the signal sees. Three practices reduce it.

Separate the analogue and digital returns and join them at one point. The analogue return carries the small signal currents and the digital return carries the switching currents of the processor. They meet at a single reference point, usually under the analogue to digital converter, so that no digital current flows through the analogue reference.

Keep the return path under the signal. A trace over a continuous ground plane has a small loop area and is hard to inductively couple into. A trace that wanders away from its return is both an antenna and a victim.

Do not split the ground plane unless there is a reason. A split plane forces every signal that crosses it to find a return path around the boundary, which is worse than a single plane with a carefully placed analogue region. Our notes on PCB design and layout describe the layout techniques.

Supplies and Gain Staging

A microphone board usually needs at least two rails: the analogue supply for the preamplifier and the converter, and the digital supply for the processor. They are taken from a common source and filtered separately, with the analogue rail filtered again close to the preamplifier. A ferrite bead with a capacitor forms a simple filter; where the noise requirement is strict, a linear regulator is used for the analogue rail and a switching regulator for the digital one, because a switching converter on the analogue rail injects its ripple directly into the signal.

Gain staging is chosen so that the loudest expected signal does not clip and the quietest is above the noise floor of the converter by a useful margin. On a microphone that has to work in both a quiet room and in front of a speaker, the analogue gain is kept modest and the rest of the gain is applied in the digital domain, where it can be changed without changing the analogue design. A pad or a switched attenuation at the input handles the loudest sources.

Acoustic Design of the Board

The microphone is a mechanical device before it is an electrical one, and the board is part of its acoustic environment.

The port. A MEMS microphone has a hole in its package, and the board has an opening behind or in front of it. The size and the depth of that opening, and the volume of the cavity behind it, form a resonator that changes the frequency response and the sensitivity. The port is designed with the microphone manufacturer data rather than cut to suit the housing.

The gasket. A seal between the microphone and the housing prevents the sound from leaking around the package and prevents dust from reaching the diaphragm. The board has to provide a flat surface and a defined area for it.

The array. Where several microphones are used for beam forming or for noise cancellation, their spacing sets the frequencies where the array has directivity, and their phase matching matters more than their individual accuracy. The board layout fixes the spacing, so it is a design decision rather than an assembly one.

Vibration and handling. A board that flexes transmits mechanical vibration into the microphone, and a hand holding the product produces a low frequency rumble. The mounting and the stiffening of the microphone area are part of the acoustic design.

Shielding and Interference

A microphone board is usually inside a product with radios, displays and switching supplies. The analogue front end is protected by distance, by a ground plane and, where necessary, by a metal can over the preamplifier. The digital interface to the codec or the processor is routed away from the analogue input, and where the two must cross, they cross at right angles.

Radio frequency pickup deserves a specific mention, because a nearby transmitter can be demodulated by a non linear junction in the analogue path and appear as an audio tone. A series resistor and a capacitor at the input, placed close to the device, and a clean ground under the preamplifier are the usual remedies.

Testing

Audio boards are tested for sensitivity, frequency response, total harmonic distortion and noise. The measurement is made in a defined acoustic environment, usually with a calibrated sound source and, for accurate work, in an anechoic chamber or with a head and torso simulator.

The production test is usually electrical: a signal is injected at the preamplifier input, or the microphone is excited by a small speaker in a fixed jig, and the output level and the distortion are checked against limits. Where the product has a beam forming array, the phase relationship between the channels is verified as well as the individual levels, because a phase error that passes an individual test destroys the array performance. Our notes on PCBA testing describe the test methods, and our notes on quality management describe the process control.

Design Practice

• Put the capsule and the first stage together. Nothing else on the board is as sensitive as that node.
• Use one analogue ground and keep the digital currents out of it. Join the two returns at a single point under the converter.
• Regulate the analogue supply separately. A switching converter on the analogue rail puts its ripple straight into the signal.
• Design the acoustic port with the microphone data. The port and the cavity change the response before any electronic decision does.
• Plan the array spacing early. It is fixed by the layout and it decides the directivity.
• Specify cleanliness and coating. Leakage across a contaminated board is a real microphone fault. Our notes on PCB assembly describe the handling and the cleaning, and our notes on PCB manufacturing cover the board itself.

FAQ

Should I use a MEMS or an electret microphone? MEMS for a compact, reflow assembled product, especially for arrays where consistency matters. Electret where the sensitivity, the size or the cost favours it.

Why is my microphone noisy? Usually the high impedance node at the capsule, the analogue ground carrying digital current, or a switching supply sharing the analogue rail.

Does the board layout affect the audio quality? Yes, and more than most designers expect. The acoustic port, the gasket, the ground return and the input trace all change the measured result.

How are microphone boards tested in production? With an electrical or acoustic jig that measures the output level and the distortion against limits, and, for arrays, the phase matching between channels.

Conclusion

A microphone board is a low level analogue circuit inside a digital product. Put the capsule and the first stage together, guard the high impedance node, keep the analogue return free of digital current, regulate the analogue supply separately, and design the acoustic port and the array spacing as part of the layout rather than after it. The gain is the easy part; the noise floor is where the design is won.