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Input connections and noise suppression


There are two basic methods for connecting a voltage signal to the lock-in amplifier- the single-ended connection is more convenient while the differential connection eliminates spurious pick-up more effectively.

1. Exterior

 

信号连接1.jpg

 

Figure 1.  Signal input port


 

Figure 2.  BNC to BNC cable


 As shown in Figure 1 and Figure 2, the signal input port is Bayonet Nut Connector. With the matching BNC to BNC cable, you can set up the reliable access from the signal source to the coaxial cable and then to the lock-in amplifier input port.   

What's reliability? The most important thing is to minimize any noise that may appear in the measurement system. There may be some sources of external noise (such as engines, signal generators, etc.) in the environment whose influence can be minimized through the carefully designed coaxial cable and BNC connector. Similarly, the difference between the detection probe and the lock-in amplifier can also be solved.    


 2. Voltage input

 There are two basic methods for connecting a voltage signal to the lock-in amplifier- the single-ended connection is more convenient while the differential connection eliminates spurious pick-up more effectively.     

 

2.1 Mode1: single-ended voltage connecion (A) 

 The lock-in detects the signal as the voltage between the center and outer conductors of the A input only. The lock-in does not

force the shield of the A cable to ground, rather it is internally connected to the lock-in's ground via a resistor. The value of this resistor is selected by the user. Float uses 10 kΩ and Ground uses 10Ω. This avoids ground loop problems between the experiment and the lock-in due to differing ground potentials. The lock-in lets the shield 'quasi-float' in order to sense the experiment ground. However, noise pickup on the shield will appear as noise to the lock-in.      

 

2.2 Differential voltage connection (A-B)

 The lock-in measures the voltage difference between the center conductors of the A and B inputs. Both of the signal connections are shielded from spurious pick-up. Noise pickup on the shields does not translate into signal noise since the shields are ignored.

When using two cables, it is important that both cables travel the same path between the experiment and the lock-in. Specifically, there should not be a large loop area enclosed by the two cables. Large loop areas are susceptible to magnetic pickup.     

 

3. Current input

The current input on the OE series lock-in amplifiers uses the A input BNC. For high source impedances, greater than 1 MΩ, and small currents, use the current input. Its relatively low impedance greatly reduces the amplitude and phase errors caused by the cable capacitance-source impedance time constant. The cable capacitance should still be kept small to minimize the high frequency noise gain of the current preamplifier.    

 

4. AC vs DC coupling

 The signal input can be either AC or DC coupled. The AC coupling high pass filter passes signals above 200Hz and attenuates signals at lower frequencies. AC coupling should be used at frequencies above 200Hz whenever possible. At lower frequencies, DC coupling is required.