Interfacing with sensors

Sensors are being incorporated into an increasing number of embedded systems, from adding accelerometers in mobile phones to adding water-vapour sensors in microwaves.

System designers that previously worked only in the digital domain now have to interface with analogue sensors. A sensor’s analogue signal needs to be digitised to be used by the system, and the signal path has to go through several stages: amplification, filtering, and digitisation (shown in Figure 1). Each of these stages is normally accomplished by several external components, but using the PSoC mixed-signal array offers all of these features in a single-chip solution.

 

 

Each sensor has a different output signal and range. The output signal can be voltage, current, resistive, capacitive, or frequency based. However, since a majority of sensors output a low-level voltage-based signal, this article will assume a voltage signal is being used.

The output of a sensor can be as small as several mV and as large as several volts. In order for the signal to be properly digitised it needs to be large enough for the ADC to effectively read the signal, so most sensor signals will need amplification.

As an example, a typical type K thermocouple outputs 41µV/°C, which needs to be greatly amplified if the user would like to read 1°C granularity. Amplifier selection is mainly based on the amount of gain needed. When selecting an amplifier, the designer must take the ADC resolution into account to ensure that signal is amplified sufficiently to obtain the desired granularity.

The gain of a basic operational amplifier is set by a network of resistors, whereas the gain of a Programmable-Gain Amplifier (PGA) is set digitally. PSoC offers several types of integrated PGAs which can be used individually to amplify the signal or cascaded together for increased amplification.

Noise can come from a number of sources including board layout, radios, thermal noise, and even the sensor itself. Signal noise not only causes inaccurate and unstable ADC readings, but amplified noise also exaggerates the error in the signal. Signal noise can be quantified as low frequency, high frequency, or a specific known frequency. Most often the noise is high frequency. To remove noise, the signal is filtered using low-pass or band-pass filters. Normally these filters are implemented using on-board passives, if possible sources of noise on the board are not taken into account, a board re-spin may be required.

PSoC's low-pass and band-pass filters are 2-pole filters with configurable corner frequencies. The filters can even be cascaded to create a filter with up to 8 poles, which is useful for designs that have strict cut-off needs.

In order to use the sensor’s filtered signal within the system, it is necessary to quantify the analogue signal into the digital domain using an ADC. Selection of an ADC mainly revolves around the system’s requirements for sampling speed and resolution. The sampling speed required for the system is related to the sensor’s bandwidth or how often the system needs to be updated. The resolution required for the system is dependent on the granularity needed to react to the sensor information. The system’s usage model defines this speed and resolution requirement. For example, an accelerometer used on a phone to change the screen orientation might only need a 10-bit ADC that updates several times a second, whereas a load cell used in a production line might require a 16-bit ADC updating several thousand times a second.

When selecting an ADC the user has the choice of using an external ADC or a microcontroller with an integrated ADC. External ADCs tend to be higher performance compared to integrated ADCs, but the majority of sensor application requirements can be met with microcontroller ADCs. Generally, microcontroller vendors only offer one type of ADC with their microcontroller that only has the ability to change speed and resolution. The PSoC offers several types of ADC. These have variable speed and resolution, and can be selected and programmed on the fly.

Once the signal has been digitised, the user can integrate the signal with a control system in the microcontroller, or they can pass the data to a host processor via a communication protocol. The signal path for a sensor may seem simple, but the implementation can be convoluted. Cypress’ s PSoC reduces the complexity of quantifying the sensor’s signal path by integrating the amplifier, filters, and ADC into a single device.

 

 

Cypress Semiconductor / Design Note