Changing Measurement Resolution to Match Specific Requirements

With the ability to tune the measurement resolution from 9 to 16 bits, the SDP600 series differential pressure sensors enable users to adjust the measuring speed and precision to match their specific requirements.

Theory

The SDP600 series is equipped with a fourth-generation silicon sensor chip named SF04, which is composed of a thermal mass flow sensor element, interface, EEPROM memory Sigma-Delta A/D converter, amplifier and digital signal processing circuitry.

The sensor-internal signal conditioning process involves the amplification, digitization, and linearization (calibration) of the analog signal of the measuring cell. The Sigma-Delta-ADC yields a digital signal with a resolution range of 9-16 bits based on the integration time of the analog raw signal.

A higher resolution of the signal is associated with a longer integration time, which provides a smoother and more accurate differential pressure value. Similarly, a lower resolution is associated with a shorter integration time, which delivers measurement results with faster response times.

The analog raw signal is more responsive for low differential pressures, as observed in Figure 1. As a result, the resolution of the linearized sensor output is not homogenously distributed. At 12bit (default), the digital resolution is roughly 0.025Pa near zero and around 1.1Pa near full-scale (500Pa).

Figure 1. The analog raw signal is more responsive for low differential pressures.

Once the linearization process is completed, the sensor output remains as a16bit integer value that is scaled with the scale factor of the sensor of 60Pa^-1. Nevertheless, this 16 bit number relies on the internal digital resolution, which is user-selectable. For many applications, the recommended value by Sensirion is 12 bit resolution (default setting) to achieve fast response time and superior accuracy.

Steps for Changing Sensor Resolution

The advanced user register is stored with the resolution setting. The following steps need to be followed in order to change the measurement resolution of the sensor.

1. Read advanced user register (Figure 2).

2. Define the new register entry in accordance with the desired resolution.

3. Write the new value to the advanced user register.

Read Advanced User Register

After the header with R/_W=1, the register value to the bus is written by the sensor system. The first byte written is the most important byte, whereas the second byte is the least significant byte of the register. A CRC byte follows if the master keeps on clocking the SCK line subsequent to the second byte.

The sensor system ensures whether the master transmits an acknowledgement subsequent to each byte and terminates the transmission if not. Two transfer sequences are required; one for storing the command and the other for invoking the previously registered command in ‘READ’ mode.

Figure 2. Command is Read advanced user register. Hatched areas indicate that the sensor controls the SDA line.

Advanced User Register Content

The measurement resolution is stored in bits 11 to 9 of the advanced user register, while bit 0 indicates the least significant bit (LSB) and bit 15 relates to the most significant bit (MSB) (Figure 3).

Figure 3. Advanced user register content.

The coding of the resolutions is as shown in the following table:

Write Advanced User Register

The command ‘Write Advanced User Register’ is used to overwrite the register addressed by the command (Figure 4). After the command byte, the new register value is read from the bus by the sensor system. The first byte is recorded as the most significant byte, whereas the second byte is registered as the least significant byte of the register. The reception of each byte (ACK) is acknowledged by the sensor system.

Figure 4. Write advanced user register.

Default Resolution

After each soft reset or hard rest of the sensor, the default resolution is stored in the advanced user register. When working with a different setting, it is necessary to change the measurement resolution after every soft reset or power-on.

Response time

The sensor’s internal integration time, and consequently its response time, changes with the resolution. The following table summarizes the processing time for different resolutions. For the first measurement, there is an additional delay subsequent to the soft reset or power-on of the sensor.

This information has been sourced, reviewed and adapted from materials provided by Sensirion Inc.

For more information on this source, please visit Sensirion Inc.