# Fluctuation-Enhanced Sensing (FES): A Promising Sensing Technique
## 6 Reducing Errors and Enhancing the Sensory Information Channel Capacity
### Dedicated Instrumentation
In a situation in which a sufficiently large \(R_{R}\) can be used for reducing the contribution of its thermal noise to a negligible level, the noise introduced by the equivalent input voltage noise of the amplifier and from the bias system may become relevant. If the impedance of the DUT is large or in the same order of the feedback resistance and the noise introduced by the bias system is comparable to that due to the equivalent input voltage noise of the amplifier, the argument leading to Equation (13) can be extended to Equation (17) and, at least at low frequencies, the contributions by \(S_{VV}\) and \(S_{VB}\) can be assumed negligible. However, if the DUT has a significant parallel capacitive component, that can also be the result of the parasitic capacitances in the cables connecting the DUT to the system, the impedance of the DUT decreases with frequency and the noise introduced by \(S_{VV}\) (and possibly by \(S_{VB}\)) can soon mask the noise coming from the DUT [107]. Note, moreover, that methods have been proposed that allow to reduce the noise introduced by the feedback resistance either by cross correlation approaches [108; 109] or by exploiting configurations in which the feedback impedance is replaced by a capacitor that, intrinsically, does not introduce noise (noise however, is still introduced by the DC path required to close the DC feedback path from the output of the OA to the inverting input) [110; 111; 112]. In all these cases, the ultimate level of background noise is set by the equivalent input voltage noise source of the operational amplifier, assuming that the noise of the bias system can be made negligible. As far as the bias system is concerned, in the configuration in Figure 13 it is the voltage source \(V_{B}\) that supplies the DC current to the DUT. A different configuration can be used, however, in which the voltage source providing the voltage bias does only supply the extremely low current required to bias the non-inverting input of the OA. This circuit configuration is shown in Figure 14.  
Due to the virtual short between the inputs of the operational amplifier, the voltage bias across the DUT in Figure 14 is the same as in Figure 13 for the same value of \(V_{B}\). The voltage source \(V_{B}\) must only supply the very low bias current at the input of the JFET (or MOSFET) input operational amplifier. Moreover, in this configuration the DUT has got one terminal connected to system ground. The price to be paid is the fact that in the circuit in Figure 14, the DC output voltage \(V_{ODC2}\) is:  
\[V_{ODC2}=-V_{B}-R_{R}I_{B} \tag{18}\]  
According to Equation (18), \(|V_{ODC2}|\) is larger than \(|V_{ODC1}|\) in Equation (14) for the same bias condition for the DUT. This results in a lower maximum bias current for remaining in linearity or, to maintain the same maximum bias current as in Figure 13, a lower value for \(R_{R}\) and, hence, a higher level of background noise. Figure 12:0 Alternative circuit for current noise measurements. The circuit topologies and design guidelines summarized above can be used for the development of dedicated instrumentation tailored to the specific sensing application at hand. System integration, that is the ability to combine and optimize the electronics sections together with the mechanical structure (for support and for electromagnetic shielding) and dedicated control and data elaboration software can play a significant role in the realization of effective and user friendly noise measurement systems [70; 71; 113; 114; 115; 116].