Hardware
The process of obtaining readings from the Biosensor involves the utilization of an LCR meter, which forms the crux of this operation. The hardware component of this project encompasses an LCR meter meticulously engineered around an STM32 microcontroller. This microcontroller unit adeptly generates a high-frequency PWM signal, which undergoes filtration through a low-pass filter to ensure signal integrity. Subsequently, the filtered signal is channeled into the input of a voltage divider assembly, comprising a known resistor and the device under test, which is none other than the Biosensor itself.
The Biosensor operates analogously to a capacitor, thus its behavior can be characterized by measuring both the voltage and the phase difference between the known resistor and the capacitor. By skillfully analyzing these parameters, specifically the impedance of the Biosensor, we gain profound insights into its capacitance. This, in turn, equips us with the ability to draw meaningful conclusions pertaining to the sample under scrutiny. The synergy between these hardware and measurement techniques empowers us to extract valuable data and insights from the Biosensor, thereby advancing our understanding of the sample at hand.
A basic diagram showcasing the working principle of the hardware is shown below.
By measuring the amplitudes of the ADC1 and ADC2 voltages, as well as their phase difference, we can compute the impedance of DUT, based on the following formulas:
where V1 is the voltage measured by ADC1, V2 is the voltage measured by ADC2, φ is the phase of V2, and the phase of V1 is 0, because the voltage is applied to an ohmic load.
An analytical schematic describing the above functions is shown here:
Measurement
To enhance precision in our testing procedures, a rigorous methodology was employed. Each resultant value was computed by aggregating data from a substantial 512 measurements. To ensure uniformity and to account for variations, all measurements were normalized by dividing them by the maximum value recorded during the initial measurement phase, effectively anchoring subsequent data points to the baseline value.
Furthermore, a comprehensive comparative analysis was conducted, juxtaposing our bespoke LCR meter with a specialized LCR instrument, the Extech 380193 Meter. The outcomes of this comparison revealed a remarkable median relative difference between the two meters. Specifically, for resistance values, the median relative difference was a mere 0.15%, attesting to the impressive accuracy of our developed unit. For capacitance values, the median relative difference was slightly higher at 1.09%, yet still indicating a notably high level of precision. These findings underscore the efficacy and reliability of our custom LCR meter, which excels in measuring capacitance and resistance with an exceptional degree of accuracy.
