9.2.2.5 Reference Voltages

Referring to Figure 32, the divider networks comprising R7, R8, R9, and R10, generate the reference voltages VREF_High and VREF_Low of the window comparator. The center point of the divider provides the bias voltage of the gain in the stage B through the connection to the noninverting input of the amplifier.

Due to the very low bias current of the TLV8544 device, it is possible to use very large values of resistors in the divider networks to minimize the current to ground through the resistors to a negligible amount. For R7 = R8 = R9 = R10 = 15 MΩ:

Equation 8.

Equation 9.

Low leakage ceramic capacitors C7, and C8, maintain constant threshold voltages, preventing potential chatter at the output of the comparators. It should be noted that using cheap electrolytic capacitors must be avoided as they have high (many µA) leakage current. The comparator outputs stay low in the absence of motion across the sensor. In the presence of motion, comparators C and D generate high output pulses as shown in Figure 33. The order of the pulses depends on the direction of the motion in front of the sensor.