# Low-Cost, Compact, and Rapid Bio-Impedance Spectrometer with Real-Time Bode and Nyquist Plots
## 3 Results and Discussion  
### Prototype of the BIS Instrumentation System  
The prototype of the developed BIS instrumentation system is given in Figure 6, where Figure 6a is photo of the hardware system and Figure 6b is main display of the software system. The main unit of the hardware system is current source and data acquisition system. The hardware system is equipped with connectors for biological samples and a connector to the PC as the main control device and data processing and display unit. To inject the current from the BIS device into the biological sample, two types of connectors can be chosen--i.e., female USB and BNC. The female USB connector is used for interdigitated electrodes (IDT), while BNC connectors is used for parallel plates or needle electrodes. In the software section, it provides several pre-acquisition menus such as selecting low and high frequency limits as well as determining the frequency samples, selecting the current value, and selecting the voltage amplification value. At the time of data acquisition, the measurement results are displayed in the form of Bode plots (magnitude and phase-angle), Nyquist plots, and also in MS-Excel table data for further purposes. In addition, the software system also displays the results of calculating the resistance and capacitance values of the samples that are carried out automatically by the program. Furthermore, it is important to state the validation results on the BIS instrumentation system that has been made, specifically the performance of the current source and the data acquisition system. Figure 7a shows the characteristics of the voltage generator, that stated in Figure 2, from frequencies of 1 Hz to 10 MHz obtained from the test results. As we can see, the signal has a good and perfect sine wave shape at frequencies of 1 kHz, 1 MHz, and 10 MHz. At low frequencies up to 2 MHz, the signal has an almost constant amplitude of 0.5 volts. However, starting at 2 MHz, the signal amplitude starts to decrease with increasing frequency. So that the voltage generator module can work well in low frequencies up to 2 MHz. In the design of a BIS instrumentation system, it is very important to measure the actual current value that is injected into the sample, which can change due to environment such as temperature. In this case, by using a reference resistor (\(R_{\text{Ref}}\)), the actual current value injected into the sample can always be monitored precisely. In terms of impedance measurement, voltage and current characteristics are also greatly influenced by the load connected to them. Figure 7b is characteristic of the current source due to impedance loads within frequency range of 10 Hz to 10 MHz, for current setting of 1 \(\upmu\)A, 10 \(\upmu\)A, and 100 \(\upmu\)A. We can see, although the voltage source to be changed has a flat shape at frequencies up to 2 MHz, but the current will decrease at high frequencies, along with the greater load. In here, the choice of a smaller current will provide benefits in terms of high frequency operations. However, keep in mind that biological tissue commonly has a small impedance value, so the use of a small current will also produce very small voltage. This is where a voltage amplifier device is needed. In addition, with regard to the operating frequency, the BIS system has effective performance in the frequency operation of 100 Hz to 100 kHz. Although the frequency range of 10 Hz to 1 MHz still can be achieved, but it will provide relatively large error, especially in the lower and upper frequency borders. Figure 6: Prototype of developed BIS instrumentation system: (**a**) hardware, and (**b**) software. Figure 7: Current-source characteristics: (**a**) frequency response of voltage generator, and (**b**) frequency response of several current values of _V/I_ converter output due to related load. Moreover, it is important to mention here how to make Bode and Nyquist plots in the BIS system that was developed.