# Estimation of Mental Distress from Photoplethysmography
## 2 Materials and Methods  
### Monitoring Pulse Rate Variability  
In the proposed design, an optical plethysmogram technique is used to obtain blood volume information by means of a PPG sensor. In a PPG acquisition and signal conditioning circuit there are several key elements: light emitting diode (LED), LED driver circuitry, photo-detector (PD), and PD signal conditioning circuitry [22]. Although it is possible to address the development of these elements from scratch, nowadays there are commercial fully-integrated analog front-ends and optical sensorssuitable for PPG signals. In this regard, an analog front-end AFE4400 (Texas Instruments Incorporated, Dallas, TX, U.S.A.) and an optical sensor NJL5310R (New Japan Radio Corporation Ltd., San Jose, CA, U.S.A.) have been selected. The AFE4400 integrates a LED driver and a PD signal conditioning circuitries, and a timing control module in a single package. The NJL5310R consists of two green LEDs and a high sensitivity PD. Hence, they are suitable to build a circuit capable of measuring blood volume through reflective photometry. According to the manufacturer, the NJL5310R has been designed with an optimal LEDs and PD separation to get a high-quality PPG signal. In our design, the green LEDs have been connected front-to-front, so that they light up and off simultaneously. In reflective photometry, LEDs and PD are placed in the same plane as the body part, and the PD receives the reflected light from different depths under the skin (such as blood, tissue, and so forth). Furthermore, the AFE4400 has two clearly differentiated stages. On the one hand, the transmit stage is divided into LED driver and LED current control. The LED driver sets a reference current of each LED and has been configured in push-pull mode to turn both LEDs on and off simultaneously (since LEDs in the optical sensor have been wired in common anode). LED current control regulates and ensures that LED current follows its reference. On the other hand, the receiver stage is split into transimpedance amplifier (TIA), conditioning section and analog-to-digital converter (ADC). The differential current-to-voltage TIA converts PD input current into a voltage. The differential voltage at the output of the TIA includes the component from ambient light. So, after TIA a programmable digital-to-analog converter (DAC) provides the current to cancel the ambient component. Then, the resulting signal is amplified, passed through a low-pass filter, and, finally, buffered before driving the 22-bit sigma-delta analog-to-digital converter (ADC). In addition, power needs are different for transmit and receiver stages. For transmit stage, the supply voltage depends upon voltage drop in the photo sensor's LEDs and the transmit stage reference voltage (set as a compromise between low-power and better dynamic range). For these reasons, a supply voltage of 4.3 V has been selected for transmit stage. For receiver stage, the supply voltage is not so strongly warped by external elements, rather for the admissible analog front-end (AFE) supply voltage. Accordingly, a supply voltage of 3 V has been chosen for receiver stage. As shown in Figure 1, a low-input DC-DC boost switching converter, TPS61093 (Texas Instruments Incorporated, Dallas, TX, U.S.A.), steps up incoming voltage to 6.5 V. The converter's input voltage range allows to power the design by two alkaline batteries, a single Li-ion battery or through a traditional 3.3 V and 5 V regulated power supply. A couple of 150 mA ultra-low-noise linear regulators, TPS7A4901 (Texas Instruments Incorporated, Dallas, TX, U.S.A.), generate 4.3 V and 3 V supply for transmit and receiver stages, respectively. The choice of a linear regulator as power supply final stage allows filtering out the output voltage ripple inherent to DC-DC switching conversion. Figure 1: (**a**) Photoplethysmogram sensor, and (**b**) major building blocks: analog front-end, power rails, clock, and protection circuitry. DC: direct current; SPI: serial peripheral interface.