Driving the future of technology : Original solution in measurement and control

 Technical notes

03/11/2020 posted


Low noise Preamplifiers for measuring small signals.


What is a preamplifier?

The Improvements of signal-to-noise ratio (SNR) is very important in small signals measurements. And preamplifiers are used to amplify signals when they are affected by noise, generally in sensor-based measurements, where the purpose is to improve the SNR. Yet, on the other hand, preamplifiers are sometimes used to change the output impedance of sensors as well.
Choosing an appropriate preamplifier based on the purpose of application is the key to measure signals from sensors with better quality.

Why are there so many different types of preamplifiers?

Because there are various requirements for different measurements, it is necessary to choose a correct preamplifier based on the type of sensor used.

In the case of thermal sensors, we normally choose a voltage-input type preamplifier, because the sensors typically provide voltage outputs. In the case of photodiodes, a current-input type preamplifier would be suitable because the photodiodes for photodetection provides current outputs.
Furthermore, the requirements for preamplifiers' output signals may vary depending on the purpose of application. At the same time, high-speed response is necessary for image processing, low noise is required for spectroscopic measurements of semiconductor properties and so on.

What are the features of NF preamplifiers?

Wide Bandwidth and Low Noise.

Other manufacturers often offer preamplifiers with narrow bandwidth which is specified in a particular frequency range. They are only effective in the specified frequency range, thus a different preamplifier for each particular frequency range is required if you wish to measure in many different frequencies.


Figure.1 Equivalent Input Noise Voltage Density against Cut-off Frequency



Figure.2 Equivalent Input Noise Voltage Density against Input Impedance

NF's preamplifiers have a wide bandwidth and low noise, so they can be used to measure a wide range of frequencies. NF also offers FRA (Frequency Response Analyzer) and lock-in amplifiers that can measure in a wide frequency range. These wide bandwidth and low noise preamplifiers are suitable to use in combination with wide frequency measuring instruments.

Conversely, if the preamplifier bandwidth is wide and noise increases, it can be combined with a filter that limits the frequency band to reduce noise. NF offers multiple types of filters that can fit various applications as well. Not only the preamplifier, but the combination of other devices can also fulfill the requirements.

Which preamplifier should you choose?

There are so many different factors need to be considered when it comes to selecting the perfect product, but the main ones you absolutely need to know are right here:

Voltage input type or Current input type?

Selection depending on the sensor outputs, whether voltage output or current output.

AC coupling or DC coupling?

If only the AC signal is needed, DC signals need to be cut by an AC coupled preamplifier. In low frequency measurements, a DC coupled preamplifier is required because AC coupled preamplifiers cannot amplify low frequency signals.

Differential input or unbalanced input?

The differential output type sensor can reduce the influence of noise caused by the GND line, but the maximum effect can be obtained by combining it with the differential input type preamplifier.

Input impedance, larger or small?

Even with a voltage output type sensor, the impedance of the sensor may be large or small. The input impedance of the preamplifier depends on whether the input element is a FET or a bipolar transistor, a suitable type needs to be selected for the sensor.

Frequency bandwidth, too wide or too narrow?

If fast response is important, use a wide frequency band preamplifier. On the other hand, in the case of low frequency measurement, noise may be reduced by using a preamplifier in the corresponding band.

Preamplifier Lineup

Voltage Input Type Preamplifier

Selection Guide (PDF)

Model Description Frequency Voltage Noise Voltage Gain Product Detail
SA series
Low Noise Preamplifier
Super low noise,
9 models line-up
:DC to 20 MHz
:1 kHz to 100 MHz
:0.25 nV/√Hz
Fixed detail
Low Noise Preamplifier
Comply with lock-in amplifiers DC to 1 MHz 5 nV/√Hz Fixed: x100 (40 dB) detail
Low Noise Preamplifier
One of the best-selling items over the years. DC to 1 MHz 2.5 nV/√Hz Fixed: x100 (40 dB) detail
Differential Amplifier
Stand-alone type DC to 10 MHz 4 nV/√Hz Switchable
x10 to x1000
(1, 2, 5 step)

Current Input Type Preamplifier

Selection Guide (PDF)

Model Description Frequency Current Noise Gain (V/A) Product Detail
SA-600 series
Wideband Current Amplifier
High gain and wide bandwidth,
4 models line-up
:DC to 500 kHz
:2.5 fA/√H
10 M/100 M/1 G/10 G
Depends on the model
Current Input Preamplifier
Switchable gains,
The model can be powered with batteries or external power sources.
DC to 2 kHz
(at Gain 100M)
2 pA/√Hz
(at Gain 10k)
10 k/10 M/100 M
Programmable Current Amplifier
High gain and wide bandwidth,
GPIB/USB interfaces
DC to
14 kHz to 500 kHz
(Via gain setting)
2.5 fA to 6 pA/√Hz
(Via gain setting)
10 k to 10 G
and x1 or x10
Programmable Current Amplifier
High gain and wide bandwidth
LAN/GPIB/USB interfaces
1 k to 10 G

DC Power Supply for Preamplifier

Model Description Output Voltage Output Noise Output Voltage Stability Product Detail
Low Noise DC Power Supply
Comply with SA series ±12 V to ±15 V 10 µVrms or lower ±10 ppm/°C detail
Low Noise DC Power Supply
Comply with SA series and General purpose 0 V to ±15 V
DC Power Supply
Comply with LI-75A and LI-76 ±20V - - -

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