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Version 1.0 – July 2020 – D1

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3 Sensor Output Signal Description

3.1 Raw Signal Measurement

Calling the sgp40_measure_raw command launches/continues the VOC measurement mode. It starts one measurement of the

raw signal (SRAW) which is returned after 30 ms. Raw signal value in ticks is provided as one 16-bit word followed by one CRC

byte (Figure 6). The raw signal in ticks is proportional to the logarithm of the sensor resistance. This signal is used as input for

Sensirion’s VOC Algorithm to provide a processed VOC Index (see section 3.2 VOC Index Measurement).

Figure 6 Process flow for performing a raw signal measurement with the SGP40. For enabling on-chip humidity compensation,

current relative humidity and temperature values have to be supplied as part of the measurement command.

The SGP40 features an automatic on-chip humidity compensation of the raw signal. The feature remains disabled as long as

default values for both humidity and temperature are sent together with the sgp40_measure_raw command. For enabling the

compensation, the measurement command includes two words (+ respective CRC bytes) where the first word represents the

relative humidity and the second one represents the temperature. Calling the sgp40_heater_off command stops the VOC

measurement mode, switches off the hotplate, and puts the sensor in idle mode.

3.2 VOC Index Measurement

For best performance in indoor-air-quality use cases, Sensirion recommends processing the (humidity-compensated) raw signal

SRAW through Sensirion’s powerful VOC Algorithm. The VOC Algorithm (Figure 7) automatically adapts its output to any indoor

environment and maps all VOC events to a VOC Index scale ranging from 0 to 500 VOC Index points (Figure 8). The value 100

refers to the typical indoor gas composition over the past 24 h. While values between 100 and 500 indicate a deterioration,

values between 0 and 100 inform about improvement of the air quality. The VOC Algorithm is included in the VOC Index driver

package available on Sensirion’s webpage (see footnote 1 on page 3). SGP40 raw signal needs to be provided to the VOC

Algorithm at a sampling rate of 1 Hz.

Figure 7 Schematic of the VOC Index driver.

Figure 8 Explanation of the VOC Index.