Why High-Impedance Sensors Need More Than Just Low Noise

When engineers design front-end circuits for microphones, hydrophones, piezoelectric sensors, MEMS devices, and other high-impedance signal sources, noise is often the first specification they look at.

And for good reason.

Weak signals can easily be overwhelmed by amplifier noise, making low-noise devices essential for preserving signal integrity.

But noise is only part of the equation.

In many high-impedance applications, input capacitance can have an equally significant impact on overall performance.

The Challenge with High-Impedance Sources

High-impedance sensors behave differently than low-impedance signal sources.

As frequency increases, input capacitance interacts with source impedance to influence bandwidth, phase response, and distortion performance. Junction capacitances within active devices are inherently nonlinear, which can introduce unwanted effects as signals become larger or frequencies increase.

For applications such as:

• Condenser microphones

• Electret microphones

• Piezoelectric sensors

• Hydrophones

• MEMS sensors

• Electrometer inputs

• Precision instrumentation

  
  

both low noise and low capacitance become critical design requirements.

Looking Beyond Noise

Many designers focus exclusively on achieving the lowest possible noise figure.

However, reducing capacitance can often simplify circuit design while improving real-world performance.

Lower capacitance helps maintain high input impedance across a wider frequency range and can reduce distortion mechanisms associated with nonlinear device capacitances. In some applications, it may also reduce the need for additional circuit techniques used to manage capacitance-related effects.

The result can be cleaner signal acquisition, wider bandwidth, and more straightforward circuit implementation.

The LSK489 Approach

The Linear Systems LSK489 was developed specifically for applications where designers need an exceptional combination of low noise and ultra-low input capacitance.

This monolithic dual N-channel JFET combines:

• 1.8 nV/√Hz typical noise voltage

• 4 pF typical input capacitance

• Excellent device matching and thermal tracking

• High common-mode rejection performance

• Extremely low leakage current

• Available in TO-71, SOIC-8, SOT-23, and DFN packages

  
  

The combination makes it particularly well suited for demanding sensor and instrumentation front ends where both signal integrity and source loading are important considerations.

Applications That Benefit

Microphone and acoustic sensing systems are excellent examples.

Whether working with condenser microphones, electret elements, underwater acoustic sensors, precision vibration transducers, or MEMS devices, designers often face the challenge of extracting extremely small signals without compromising bandwidth or introducing distortion.

The same considerations apply to modern instrumentation and electrometer circuits, where preserving the original signal is often just as important as amplifying it.

The Takeaway

Low noise will always be important.

But when working with high-impedance sources, capacitance deserves equal attention.

The best front-end performance often comes from balancing both characteristics rather than optimizing only one.

For designers building the next generation of microphone preamps, acoustic sensing systems, hydrophones, MEMS interfaces, and precision instrumentation, understanding the role of input capacitance can be the difference between a good design and a great one.

Interested in learning more about the LSK489?

Visit the LSK489 product page HERE!