The IR receiver AFE design is highly streamlined and optimized. R1 converts the IR light energy induced current into voltage and applies to the inverting input of the comparator. Because a reverse biased IR LED is used as the IR receiver, a higher I/V transimpedance gain is required to boost the amplitude of reduced current. A 10M resistor is used as R1 to support a 1-V, 100-nA transimpedance gain. This is made possible with the picoamps Input bias current IB (5pA typical). The RC network of R2 and C1 establishes a reference voltage Vref which tracks the mean amplitude of the IR signal. The RC constant of R2 and C1 (about 4.7 ms) is chosen for Vref to track the received IR current fluctuation but not the actual data bit stream. The noninverting input is connected to Vref and the output over the R3 and R4 resistor network which provides additional hysteresis for improved guard against spurious toggles.
To reduce the current drain from the coin cell battery, data transmission must be short and infrequent.