Wearable blood glucose test module circuit diagram

Filter 18.432M

L0504-Murata muRata common mode inductor 90Ω 150mA

Blood glucose test circuit analysis: In order to detect the number of free electrons generated by the reaction of glucose oxidase and glucose, it is necessary to apply a fixed bias voltage across the electrode, and then detect the current generated by the bias voltage to drive the free electrons to calculate the blood glucose mass concentration value. According to the chemical material applied on the test piece, the required bias voltage is also different. In this design, the bias voltage is obtained by dividing the voltage of the VLL1 pin of the liquid crystal drive module by a resistor divider. The voltage is around 273mV. The current generated by the reaction of normal human blood with glucose oxidase is nA to μA. In order to convert this current into a voltage amount and accurately measure it, a signal conversion and voltage amplification circuit is required. Figure 2 shows the amplifier circuit implemented by the comparator, using the comparator module of the LL16 chip itself. The positive terminal of the comparator is connected to a bias voltage of 273mV. The bias voltage is obtained by dividing the VLL1 provided by the LCD module of the LL16 through three resistors R6, R5 and R4. The filter capacitor C4 is used to filter out the VLL1 itself. High frequency noise to ensure the stability of the bias voltage. The negative end of the comparator is connected to the enzyme electrode of the test paper, that is, the fourth pin. The free electrons on the electrode are directional flow driven by the bias voltage, which is equivalent to a resistor Rx. The smaller the resistance value of Rx, the higher the blood sugar mass concentration. Calculate the equivalent resistance value and calculate the blood sugar mass concentration value.

In-depth analysis of the data: the resistance of the resistor Rx is in the range of 18k Ω to 300kΩ, and the minimum voltage of the 3V battery is about 2.3V. In order to ensure that the maximum output of the comparator cannot exceed the minimum supply voltage, the amplification factor of the amplifier circuit must not be too large. Therefore, take R3 as 120k Ω, so that when the resistance to be tested is 18kΩ, the output voltage of the amplifier circuit is 2.1V, which is less than the minimum voltage of the battery. Since the core device of the amplifying circuit in the design is a comparator, the comparator output in this circuit is a square wave signal, and the resistor R and the capacitor C1 at the output of the comparator are first-order low-pass filter circuits, which can harmonics in the high-frequency square wave. The signal is filtered to obtain a relatively stable DC signal; also to avoid the output of the comparator not exceeding the positive limit, the value of R1 should not be too large, and in order to achieve a better filtering effect, the value should not be too small, so Take a moderate 15k Ω. In order to filter out the high frequency signal as much as possible, the capacitance of C1 should not be too small, but if the value of C1 is too large, the negative side of the comparator will have a relatively large amplitude. After the actual test, C1 takes 1 μF to get a better overall effect. Although the filter circuit composed of R1 and C1 has filtered the output signal of the comparator to some extent, the signal at the positive end of C1 still has high frequency ripple. In order to ensure the accuracy and consistency of the blood glucose mass concentration measurement, R2 and C2 Once again, the output signal of the amplifying circuit is filtered. Since the high frequency harmonics are filtered out, considering the optimal filtering frequency range of the capacitor, C2 takes 10 μF and R2 takes 10 kΩ.