## 3 Results and Discussion  
### Hardware System Characteristics  
The main hardware system consists of three fabricated printed circuit boards (PCBs), shown in Figure 5. The first PCB (Figure 5a) is a four-layer potentiostat circuit that controls light excitation using a 9-volt battery for power. The current source circuit is connected via a 4-wire bus to control the DAC (Figure 5b), which in turn regulates the output current supplied to the monochrome LED array in the third PCB (Figure 5c).  
Figure 6 illustrates the current measurement range and accuracy of the measurement system described in this report. The black data points represent the correlation between the  
Figure 5: The fabricated PCB circuits: (**a**) potentiostat circuit, (**b**) current source controlled by DAC, and (**c**) monochrome LED array.  
theoretical current values calculated using [PERSON]'s law (\(I_{\text{ideal}}\)) and the average of measured current values (\(I_{\text{mean}}\)) presented on a logarithmic scale. The system exhibits a wide measurement range, spanning from 1 nA to 18 mA, with a relative standard deviation (RSD) shown by the red points consistently below 1%. However, for current values below 1 nA the RSD increases above 1%. In particular, the RSD for a current of 0.1 nA exceeds 10%. In simpler terms, the designed system offers high current measurement precision in the range of 1 nA to 18 mA, with a precision of 99%. This relationship is further corroborated by the fitted line (blue line) in Figure 6a, which has a slope of 0.99917 and an intercept of \(-0.00369\), with an R-square value close to 1, indicating a strong correlation. The precision of the designed circuit is evaluated by the accuracy of testing the array resistor and its intrinsic precision. In this case, current values from 10 nA to 18 mA have a tolerance below 1.01% of the range due to the 0.1% tolerance of the tested resistors, with values ranging from 100 \(\Omega\) to 100 M\(\Omega\).  
The laboratory-designed system demonstrates significant promise for practical applications by achieving high precision in the 1 nA range, a crucial requirement for highly sensitive biosensors. In an electrochemical biosensor for the detection of glycerin that uses amperometry techniques the current difference between 0 mM and 25 mM only reaches around 25 nA [58]. A measurement device lacking sufficient resolution would not be able to detect such a low limit. Minimizing circuit interference and noise is critical for achieving measurement precision. This requires careful circuit design techniques and the selection of appropriate components. The chosen TIA LMP7721 plays a vital role in low-current measurement due to its exceptionally low input bias current of 3 femtoamperes. This characteristic makes it ideal for converting low currents to voltages. Additionally, the chosen ADC ADS122C04 allows for digital filtering of the 50/60 Hz power line noise, further reducing interference. However, the circuit design itself is paramount for optimal performance. The implementation of a guard region around the analog signal region isolates the analog circuitry from the activity of the MCU and other digital components on the same circuit board. In addition, a 4-layer PCB layout was employed, where the signal layer is placed between two dedicated ground planes on the outer layers. These design choices effectively reduce white noise from temperature, high-frequency noise from the digital components' activity, and power line interference, contributing to the system's high-precision capability.  
Figure 6b shows the spectrum of the excitation LED light source in this report. It has seven wavelengths: 385 nm, 405 nm, 470 nm, 525 nm, 590 nm, 630 nm, and 720 nm. The largest full width at half maximum (FWHM) is 37 nm, observed at the 470 nm wavelength.  
Figure 6: (**a**) The correlation between the mean of measured current and the theoretically calculated current (black points) with its RSD (red points) and (**b**) the spectrum of the designed excitation monochrome LED source.