Here is the first draft of the glucose meter schematic:
Mathematically interesting is only the op amp integrator (IC1D). The formula for determining voltage output for the integrator is as follows:
As the non-inverting input is not connected to GND (VR = 0V) but to VR = +2.5V:
I changed the MAX4575, which resets the op amp integrator by shortening the capacitor by the quad bilateral switch CD4066 (IC2), because this chip is much cheaper. Because integration involves a known start time and end time, a reset circuit must be included to establish the start time before each integration time period. The integration end time occurs when the measurement is read. The CD4066 simply short-circuits the capacitor C1 if digital output D6 goes HIGH. The not used control inputs are tied to ground. Unused CMOS input pins must never be unconnected, because they tend to
float towards the dangerous region which is in the middle between VDD
The low-dropout regulator TPS76925DBVT (U1) provides a fixed output voltage of 2.5V.
The output voltage of the TPS76901 adjustable regulator (U2) is programmed using an external resistor divider. The output voltage is calculated using:
Where: Vref = 1.224V typ (the internal reference voltage).
As the applied voltage across two of the test strip electrodes should be about 400 mV, I chose R3 = 130k and R4 = 180k which results in a TPS76901 output voltage of 2.108V.
As mentioned above IC1D is configured as an op amp integrator. Currently I don't use the op amp integrator because the op amp integrator output voltage is not directly or indirectly proportional to the glucose concentration when using ONETOUCH test strips. This might be the case when using other test strips.
IC1B is configured as a transimpedance amplifier, where basically
The feedback capacitor C8 is required to improve stability.
IC1A and IC1C are configured as unity gain buffer amplifiers. In this configuration, the entire output voltage is fed back into the inverting input. The difference between the
non-inverting input voltage and the inverting input voltage is amplified
by the op-amp. This connection forces the op-amp to adjust its output
voltage simply equal to the input voltage:
For IC1A this is only the case if an electrolytic bridge (blood) between the reference and counter electrode is applied.
As long as the strip is not inserted, input pin D2 is LOW. If a test strip is inserted, the two strip detect pins on the strip connector getting connected and D2 goes HIGH.
C4 and C5 are decoupling capacitors.