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

 Lecture for measurement

2020/5/20 posted

Measurement of Small AC Signals and Low Noise Superimposed DC Signal

NF’s Amplifier provides both Input Protection Performance and Low Noise Characteristics

1. Original Current Limiting Protection Circuit

NF has created this low noise AC signal pre-amplifier “CMP61102” specifically for the measurement of small AC signals and small noise superimposed on large DC voltage. The amplifier provides both great input protection performance and low noise characteristics. (Figure 2.) And the amplifier protects the circuit by limiting the maximum current applied to the inputs, plus the noise characteristics of the amplifier is very low at 2 nV/√Hz. The protection circuit can support DC voltage in the order of few tens of V, thus there is no concern of damages even when the output of a power supply or power amplifier is directly connected/disconnected.


The electrical performance of the low noise AC signal pre-amplifier is as shown in the Table 2 below. As long as the input voltage (DC signal + AC signal) is within ±40 V, the amplifier will never fail. The maximum input voltage extension to ±80 V is also possible. This product achieves both high gain and wide bandwidth, which has a gain of 60 dB and upper cutoff frequency of 3 MHz. In addition, the cutoff frequency of Low Pass Filter (LPF) is switchable to 7 levels.

In addition, power transformers that transform commercial power inputs are generally large and heavy. And the series regulator needs a large heat sink for heat release.

Figure 2. Low noise AC signal pre-amplifier “CMP61102” Appearance (for reference)

Table 2. Specifications of low noise AC signal pre-amplifier “CMP61102” (for reference)

Description Electrical Performance
Maximum Input Voltage ±40 V
*Extension to ±80 V is possible
Input Impedance 100 kΩ
Voltage Gain 60 dB
Equivalent Input Noise Voltage Density 2 nV/√Hz or less
Upper Cutoff Frequency 3 MHz
LPF Switching Function
(Upper cutoff frequency setting values)
7 levels switchable via a rotary switch
10, 100, 1k, 10k, 100k, 1MHz, THRU
Lower Cutoff Frequency 1 Hz
*can be extended to 0.1 Hz or less
Maximum Output Voltage ±10 V

*Please contact us in advance for customized products.

Features of Low Noise AC signal pre-amplifier

  •  The most suitable for measurement of small AC signal superimposed on DC voltage up to ±40 V (Extension to ±80 V is possible)
  •  Low-noise characteristics, Equivalent Input Noise Voltage Density of 2 nV/√Hz.
  •  Equipped with LPF function, and 7 switchable levels of upper cutoff frequency
  •  Lower cutoff frequency of 1 Hz, low frequency range can also be measured
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