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

Posted 2019/9/23

Current amplifier (Transimpedance amplifier)

To measure a very small current signal

In order to measure a very small current signal, it is important to use a low-noise I/V amplifier. In addition, it is important to set up appropriate measurement equipment under optimum measurement conditions. Be aware of the following points when measuring very small current signals.

1. Noise reduction method before amplification

It is necessary to pay attention to the following two points.

•  Reduce the effects of external noise
•  Reduce the input additional capacitance Cs of the current amplifier

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Reduce the effects of external noise

In order to detect a very small signal, it is important to minimize the effect of external noise. The following measures are recommended to reduce the effect of external noise.

• (i)  Make the wiring from the sensor to the input of the current amplifier as short as possible.
• (ii) Use shielded cable (such as coaxial cable) to connect. If shielded cables cannot be used, cover cables with copper or aluminum foil or put them in a shielding case.
• (iii) The sensor and I/V amplifier should be kept as far as possible (1m or more is desirable) from instruments with a built-in transformer (such as a DC power supply, i.e., a product that generates magnetic noise).
• (iv) Install and fix the current amplifier and connection cable in a location with low vibration. If the cable vibrates, microphonic noise may occur. In the case of current amplifiers with high gain, vibration due to sound (voices), fans, etc. may also have an effect.
• (v) If the place in which to install the current amplifier is a conductor (such as a metal), the I/V amplifier should be insulated from the installation point to prevent the noise due to the GND loop.Furthermore, by using "bottom plate" included with the SA-600 series, the amplifier can be easily insulated.

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Reduce the input additional capacitance Cs of the current amplifier

Generally, as Cs of the current amplifier increases, the following effects occur.

• (a) Operation of the current amplifier becomes unstable
• (b) Noise increases

Regarding the (a)
Please refer to "Stable against sensor capacitance".NF SA-600 series achieves stable current amplification regardless of Cs.

Regarding the (b)
Increase noise is explained by using the circuit diagram (shown in Fig.8) of a general current amplifier and a graph of input-referred noise current (shown in Figure 9). As shown in the Fig.9, input equivalent noise current increases as frequency increases, and it attenuates when the upper cutoff frequency is exceeded. If Cs is large, the noise level increases because the input equivalent noise current starts to increase from lower frequency.
The output-noise waveform of SA-607F2 (LPF setting: THRU) when Cs is 10 pF and 1000 pF is shown in Figure 10. This shows that output noise increases more than 10 times when Cs becomes 100 times larger. Thus to measure very small signals, it is important to reduce noise by reducing Cs.
The following is effective for reducing Cs.To select a sensor with a small capacitance. To be shorten the wiring length from the sensor to the current amplifier as much as possible. To select a cable with a small capacitance (cable with a characteristic of high impedance). As reference, table 5 shows relationship between BNC coaxial cables and their capacitance. Typical commercial coaxial cable has a characteristic impedance of 50 Ω or 75 Ω. Using the short cable is the most simple method to reduce Cs.

 

Table 5: Types of BNC coaxial cables and their capacitance

Characteristic impedance	Coaxial cable	Capacitance
50Ω	3D-2V etc., RG-8/u, etc.	About 100 pF/m
75Ω	3D-2V etc., RG-11/u, etc.	About 67 pF/m
95Ω	RG-22/u, etc.	About 53 pF/m
125Ω	RG-63/u, etc.	About 33 pF/m

Related keywords :  Ultra low noise amplifier, LNA