FEMTO are pleased to introduce their new product: CAB-SC1-BB, low-capacitance coaxial cable assemblies with BNC connectors. These cables complement their portfolio of cable accessories. The low-capacitance CAB-SC1 series cables are high-quality coaxial cables designed for excellent signal quality. They are particularly well-suited for measuring small signals by minimising noise caused by cable capacitance. With a characteristic impedance of 150 Ω, these cables have a capacitance of only 30 pF/m – less than a third of the capacitance of conventional 50 Ω coaxial cables.

Key Features:

• Capacitance 30 pF/m,
• recommended frequency range DC to 20 MHz,
• ideally suited for input of LCA, HCA, DLPCA, DHPCA

Please note, these cables are only for quiet environments. For applications involving mechanical disturbances, we recommend our low-noise CAB-LN1 cables. For environments with strong electromagnetic interference, we recommend our CAB-HF1 RF cables. 

Download datasheet here. 

Brief description
The CAB-SC1 low-capacitance cables are high-quality, high-impedance coaxial cables designed to deliver the highest signal quality. They are recommended for measuring small signals while minimising noise caused by cable capacitance.

Features

• 150 Ω characteristic impedance
• 30 pF/m capacitance
• Pre-assembled with high-quality BNC connectors
• Available in various lengths from 50 cm to 10 m
• Additional lengths available on request (minimum order quantity 5 items)

Advantage
When measuring small signals – especially small currents using transimpedance amplifiers – the cable between the signal source and the amplifier should be as short as possible to minimise the effect of cable capacitance. A 50 Ω cable has a capacitance of about 100 pF/m. FEMTO’s low-capacitance cables have a capacitance of approximately 30 pF/m, so they generate much less noise for the same cable length.

Note
Low-capacitance cables offer no advantages in terms of triboelectric noise or electromagnetic shielding; in fact, they are even slightly disadvantageous. We recommend FEMTO’s low-noise cables in case of vibrations and FEMTO’s RF cables in case of EM-affected environment.

Why are low-capacitance cables needed?
Amplifier inputs are always sources of noise voltage. According to Ohm's law, this noise voltage generates a noise current across the input impedance. In the case of transimpedance or current amplifiers, this is particularly critical, since the noise current superimposes on the measurement current and is also amplified. It is part of the total noise and can easily even become the dominant noise.

If the total impedance at the amplifier’s input is high, the noise current is low and negligible. However, if the impedance is low, the noise current is high. The impedance is particularly low when the capacitance is high; in this case, the capacitance at the input consists of the capacitance of the actual signal source – for example, a photodiode – and the capacitance of the cable. A 50-ohm coaxial cable has 100 pF/m and can therefore easily account for most of the capacitance at the amplifier input.

The solution to this problem is to use cables that are as short as possible and/or cables with lower capacitance. FEMTO's low-capacitance cables have a capacitance of only 30 pF/m, so the current noise caused by impedance is significantly lower.

Figure: Comparison of noise spectra with Low-Capacitance Cable Vs 50 Ohm Cable (RG 58). Integrated noise (legend, values in lower brackets) with Low-Capacitance Cable is more than a factor of 2 lower compared to a standard RG58 cable.

Applications:

• Measurements of small signals, particularly currents from a remote signal source, where long 50 Ω coaxial cables would introduce excessive noise due to their capacitance.
• Measurements of currents down to the picoampere range
• Photodetectors and Ionisation Detectors
• Scanning Tunnel Microscopy (STM)
• Scanning Probe Microscopy (SPM)
• Spectroscopy
• Piezoelectric and pyroelectric detectors

Brief introduction to low-capacitance cables:

These cables are ideal when significant distances between the signal source and the amplifier are unavoidable—particularly when using transimpedance amplifiers. The noise in transimpedance amplifiers can be categorized into current noise and voltage noise. While the amplifier's current noise is fixed and adds in quadrature to the signal source noise, the amplifier's voltage noise interacts with the source and cable impedance, generating additional current noise. With high-impedance signal sources, cable capacitance is a key factor determining the level of noise generated; consequently, using a low-capacitance cable can significantly reduce noise. This applies only when the cable is used in a mechanically and electromagnetically quiet environment (free from vibrations, cable movement, and significant electromagnetic interference/EMI). For applications involving mechanical disturbances, we recommend our low-noise CAB-LN1 cables. For environments with strong electromagnetic interference, we recommend our CAB-HF1 RF cables.

We recommend the low-capacitance CAB-SC-1 cable for all our current transimpedance amplifiers, with the exception of HCA amplifiers with model names ending in "-C". These "-C" amplifiers are optimized to have extremely low voltage noise; therefore, using low-capacitance cables with them does not result in a significant reduction in noise levels.

Using CAB-SC-1 cables can also be advantageous when operating voltage amplifiers with FET input stages (DLPVA-101-F-S, DLPVA-101-F-D, HVA-10M-60-F, HVA-200M-40-F) with high-impedance signal sources. The source resistance, combined with the cable capacitance and the amplifier's input capacitance, forms a low-pass filter. Using CAB-SC-1 cables instead of standard coaxial cables allows for increased usable bandwidth. However, a disadvantage is the impedance mismatch between the cable and the amplifier, which causes signal reflections that have a disruptive effect, particularly in the MHz range.