The Customer's Problem

Their stage, a parallelogram flexure – driven by a voice coil actuator with interferometric feedback and using our PMAC technology, was already a high-performance design. Modelling and testing pointed to the same bottleneck: the amplifier was too noisy.

Why Amplifier Noise Is So Hard to Eliminate

At this level of precision, noise sources that are invisible in ordinary systems become dominant. Through analysis of the full signal chain, we identified two categories of noise that had to be addressed simultaneously.

External to the amplifier:  Analogue cabling between a DAC and amplifier picks up electromagnetic interference. Even specialist microphone-grade cable exhibits measurable microphonic effects. Power supply ripple and switching noise couple directly into the signal path.

Inside the amplifier: Ceramic capacitors convert mechanical vibration into voltage. Resistors generate thermal noise. Driver transistors couple switching energy capacitively to the heatsink. The internal feedback loop and cooling fans each contribute their own noise floor.

Our standard linear amplifier, even when paired with a high-resolution 20-bit DAC that cleanly resolved single-bit steps, although precise, fell short of sub-nanometre work.

Our Solution: An Integrated, Purpose-Built Low-Noise Amplifier

Rather than trying to patch an existing design, Faraday Motion Controls designed and built a new amplifier from the ground up with one goal: make the amplifier quieter than the DAC.

Eliminating External Noise at the Architecture Level

We combined the DAC, amplifier, and power supply into a single sealed package – removing the vulnerable analogue cabling entirely. Digital control signals from the motion controller enter through galvanic isolation, preventing ground loops and conducted interference from reaching the analogue domain.

Minimising Internal Noise Through Disciplined Design

We applied proven techniques from high-end audio engineering to every aspect of the internal design:

• Component selection that avoids microphonic elements in critical signal paths
• Complementary-pair output transistors controlled by modern ultra-low-noise op-amps for feedback
• Physical screening between output transistors and the case/heatsink to block capacitive coupling
• Internal DC-DC conversion with dedicated filtering and regulation, isolating the amplifier from external mains-borne noise

The result: an amplifier with a noise floor lower than our previous 20-bit DAC product.

Real-World Validation

First Test – Revealing the Next Bottleneck

Initial testing on the customer's stage achieved only ±2–3 nm stability – no better than the previous amplifier. Investigation with the stage locked revealed that the position sensor (interferometer) noise alone exceeded the sub-nanometre target. The servo was faithfully "correcting" for sensor noise, not real motion. The amplifier was no longer the problem.

Second Test – Sub-Nanometre Achieved

After switching to a lower-noise interferometer, the results were transformative:

The amplifier output required to hold position was lower than the noise floor of the original amplifier – confirming the customer's modelling that amplifier noise had been the limiting factor all along.

What This Means for Your Application

This project demonstrated that with the right amplifier design, sub-nanometre positional stability is achievable in real-world conditions. The remaining performance limit is now the position feedback sensor, not the drive electronics.

If your precision positioning system is hitting a noise floor you can't explain, or if you're designing a new platform that demands sub-nanometre or picometre-level stability, the amplifier may be the component worth examining first.

We can design and build custom low-noise amplifiers and drive electronics for precision motion applications. Whether you need a complete integrated solution or want to explore whether amplifier noise is limiting your system, we'd welcome the conversation.

Contact us to discuss your Motion Control requirements.