Guide to Cable Selection for Low-Noise Measurements with FEMTO Amplifiers


This guide helps with selecting the correct cables for specific measurement tasks. Cable selection depends on both the parameters of the measurement, measured variable and bandwidth, as well as on the amplifier type used and external environmental conditions, such as electromagnetic interference.

Introduction


In precision measurement technology, the amplifier is not the only thing that counts. The cable from the signal source to the amplifier also crucially influences the measurement result. Particularly with extremely low-noise current amplifiers (transimpedance amplifiers - TIAs), unsuitable cables can massively worsen the signal-to-noise ratio (SNR).

The physical challenges


Cable capacitance and noise characteristics

Every cable has a capacitance CCables, which runs parallel to the amplifier's input. The input noise voltage of a current amplifier leads to noise currents via the capacitances of the signal source and the cable. The larger the capacitances, the larger the noise currents. Furthermore, the effective bandwidth of the system decreases, as the cable capacitance and the amplifier's input impedance form a low-pass filter. The lower the cable capacitance, the lower the noise. Typical 50 Ω coaxial cables have a capacitance per unit length of 100 pF/m.

Figure 1: Schematic diagram of a TIA measurement setup with a current source (ideal current source I, internal resistance Ri and internal capacitance Ci), Cable (cable capacitance CCable) and transimpedance amplifier (operational amplifier (opamp), input resistance RInput and feedback resistor Rf)
Figure 1: Schematic diagram of a TIA measurement setup with a current source (ideal current source I, internal resistance Ri and internal capacitance Ci), Cable (cable capacitance CCable) and transimpedance amplifier (operational amplifier (opamp), input resistance RInput and feedback resistor Rf)

Triboelectric Effects (Microphony)

When a standard coaxial cable is moved, bent, or subjected to vibrations, friction occurs between the dielectric and the shield. This generates charge separations that can be measured as current pulses (pA to nA). In applications with high gains (105 V/A or larger) the slightest mechanical disturbance leads to massive signal noise.

Shielding and EMI immunity

Electromagnetic radiation (50 Hz hum, RF signals) couples capacitively or inductively into the cable between the source and amplifier. Currents in the shielding braid, e.g. from EMI, earth loops or leakage currents, couple inductively with the inner conductor of the cable, thus causing interference to the useful signal. A high coverage of the shielding braid is essential to protect the sensitive useful signal from the „polluted“ laboratory environment. A well-thought-out earthing concept is a prerequisite for the interference-free detection of the smallest currents.

Use cases


The cable requirements differ fundamentally depending on the frequency range and signal strength:

Cable type Low-noise cable Low-Capacitance Cable RF Cable
Frequency range Low frequency (DC to 1 MHz) Broadband / fast pulses (up to ~20 MHz) High frequency (1 MHz to 3 GHz)
Typical signal strength very small Small to medium Moderate to high
Challenge Mechanical influences (vibrations) capacitive load EMI / Reflections
Primary focus Minimising triboelectric noise Minimising cable capacitance and the noise caused by it Improved shielding and impedance matching (50 Ω)

Generally, the connection cable from the source to the amplifier should be as short as possible. If longer cable lengths cannot be avoided, low-capacitance cables are also recommended in mechanically undisturbed environments for low bandwidths and high gains.

FEMTO's specialised cable solutions


FEMTOs Low-Noise Cables: Maximum Stability Against Vibrations

These cables are the first choice when measuring the smallest currents.

  • An additional layer of conductive material (graphite) between the dielectric and the shield ensures that charges generated by friction, deformation, and/or vibration are immediately dissipated.
  • Enables signal acquisition in the presence of mechanical or acoustic interference such as vibrations from pumps or motors.
Figure 2: Comparison of the noise spectra with FEMTO's low-noise cable and a typical RG58 cable, both subjected to identical vibrations commonly found in laboratory and industrial environments. The low-noise cable proves to be virtually unaffected by the mechanical disturbances.
Figure 2: Comparison of the noise spectra with FEMTO's low-noise cable and a typical RG58 cable, both subjected to identical vibrations commonly found in laboratory and industrial environments. The low-noise cable proves to be virtually unaffected by the mechanical disturbances.

FEMTO Low-Capacitance Cables: Minimising Broadband Noise

If the amplifier's bandwidth is to be fully utilised without noise at high frequencies dominating the signal, capacitance is crucial. Transimpedance amplifiers exhibit voltage noise at the signal input, which leads to noise currents through the impedance of the source and cable. At higher frequencies, the impedance reduction due to the capacitance of the source and cable dominates here. Therefore, the cable between the source and amplifier should be as short as possible. However, a longer cable is often unavoidable.

  • The use of special dielectrics (e.g., cellular PE) drastically reduces the capacitance per metre. This dampens the rise in input noise and extends the usable frequency range.
  • Allows reading high-speed photodiodes and broadband TIAs (e.g., HCA series) with long signal lines in a mechanically undisturbed environment.

Figure 3: Comparison of noise spectra with FEMTO's low-capacitance cable and a standard RG58 cable up to 1 MHz. The low-capacitance cable shows a significant reduction in noise levels at frequencies between 10 kHz and 1 MHz. The noise reduction at approximately 50 Hz is attributable to the additional improved shielding of the low-capacitance cable.
Figure 3: Comparison of noise spectra with FEMTO's low-capacitance cable and a standard RG58 cable up to 1 MHz. The low-capacitance cable shows a significant reduction in noise levels at frequencies between 10 kHz and 1 MHz. The noise reduction at approximately 50 Hz is attributable to the additional improved shielding of the low-capacitance cable.

FEMTO RF Cables: Superior Shielding in EMI-Critical Environments

In modern laboratory environments, high-frequency interference is ubiquitous. A simple braid screen often acts like an antenna in these situations.

  • The double shielding guarantees extremely high shielding attenuation up into the GHz range. Earth loops and leakage currents in the outer shield braid have very little influence on the measurement signal.
  • Enables measurements and RF signal analysis in environments with high radio frequency interference.

Figure 4: Comparison of noise spectra with FEMTO's RF cable and typical RG58 cable up to 1 MHz under the influence of an identical EM interference source. While the RG58 cable picks up both low and high-frequency interference, the RF cable is almost unaffected.
Figure 4: Comparison of noise spectra between FEMTO’s RF cable and a typical RG58 cable up to 1 MHz when exposed to an identical source of electromagnetic interference. Whilst the RG58 cable picks up both low- and high-frequency interference, the RF cable remains virtually unaffected.

Decision Matrix: Which Cable for Which Amplifier?

To simplify the selection of a cable for connecting the source, please use the following matrix based on your FEMTO amplifier or photoreceiver model:

Amplifier series Typical application Recommended
FEMTO cable		 Why?
LCA
DLPCA-200
DDPCA-300		 Current measurement, high gains (>105) with bandwidths below 1 MHz Low-Noise Eliminates vibration artefacts at high gains
Low-Capacitance For cable lengths over 1 m in a mechanically undisturbed environment, to minimise capacitively induced noise
HCA
DHPCA-100		 Current measurement, bandwidths of 1 MHz to 100 MHz Low-Capacitance Keeps the capacitively conditioned noise floor flat at high frequencies
RF cable For 50 Ω Impedance matching and/or in an electromagnetically disturbed environment
DLPVA
HVA
DHPVA
HSA
DUPVA		 Voltage measurement, bandwidths from DC to GHz RF cable The source capacitance plays a subordinate role, focus on maximum EM shielding and 50 Ω impedance matching
Lock-In-Amplifiers Lock-In Detection Low-Noise In a mechanically disturbed environment (vibrations)
RF cable In an electromagnetically stressed environment

For connecting our amplifiers and photodetectors to the signal receiver (e.g. oscilloscope or ADC card), we recommend our RF cable without exception, in order to minimise interference.

Conclusion


The performance of low-noise amplifiers can only be fully exploited if the cables do not become the limiting factor. While standard coaxial cables are sufficient for general measurement tasks, high-end applications require a targeted selection based on capacitance, mechanical decoupling, and shielding efficiency. FEMTO offers precisely matched components for this purpose, to systematically minimise noise, microphony, and interference.

You can find a selection of the cables offered by FEMTO at www.femto.de/en/cable.