Design of a fertility monitor system using SoC
A fertility device can utilize either the LH-testing or BBT testing methodology. For higher precision testing, both can be included. Figure 4 shows a block diagram of this application.
Figure 4: Block diagram of a fertility monitor in a programmable SoC
Click on image to enlarge
Feeding input from the LH test strip and the temperature sensor
As discussed earlier, in the LH testing, the tested band and the reference band’s color/ darkness need to be compared to measure the result. Therefore, a few external LEDs driven by a particular voltage using a VDAC (can be mux-ed or driven always) are needed to make light rays fall on the test strip. The light is absorbed by the bands on the strip, depending upon their darkness.
The tested LH strip is placed in between the plane where the LEDs are placed and the plane where two photo diodes are placed (refer to Figure 5). In general, the photo-diodes are placed just below the position where the test band and reference band appear. The two photodiodes corresponding to the test band and reference band generate currents depending on the intensity of the LED light falling on them. These photodiodes are connected to a trans-impedance amplifier to convert the current readings to suitable voltages, which are then fed to a Del-Sig ADC through an analog MUX for further processing.
A temperature sensor such as an RTD can be connected externally that feeds the sensed BBT reading (which is also muxed along with the photo-diode readings) to the ADC.
Figure 5: Placing LEDs and Photo-diodes above and below the LH test pad
The digital output of the tested band and reference band readings are compared and, if the reading of the tested band is lower than that of the reference band, this means the test band has darkened more than the reference band and the ovulation period has been reached.
For the BBT method, the range of the digital equivalent of the BBT values during ovulation is stored permanently in Flash or EEPROM as a reference value. The measured BBT value is compared to the reference value and the fertility level is noted.
The resolution of the ADC depends upon the characteristics of the photo-diode used and also on the actual measurement accuracy needed for the application.