The performance of even the best sensor can be degraded by poor design of the signal conditioning circuit that interfaces its output to the digital processors at the heart of the system.
Equally, over-specifying analogue devices can add cost.
In designing a battery-powered analogue signal conditioning board with a ZigBee communications interface, we evaluated a range of different analogue front-end design options.
System architecture
The signal conditioning board consists of a digital “motherboard” based on a nano-Watt PIC24F16KA102 with a Telegesis ETRX2 ZigBee module for communications, and powered from two AA cells.
A 40-way connector interfaces to the analogue modules, and there is a common interface specification allowing other boards to be plugged in following the same standard. The first of these to be created is a low-power passive infrared (PIR) board with a target power consumption of 100µA.
Analogue architecture
In deciding where to put the ADC, we faced something of a conundrum. If we put the ADC on the digital motherboard, we would need to specify a device with sufficient sensitivity and bandwidth to support any conceivable analogue module that we would subsequently wish to connect – potentially driving up the cost and power consumption of the simplest applications, such as our initial case of a PIR board.
For the most precise sensors, this would also have the drawback of necessitating taking potentially delicate analogue output signals off the daughterboard and through a set of connectors before reaching the ADC.
The best trade-off appeared to be to provide a very low-power, but comparatively low-performance ADC on the digital motherboard to serve low-end sensor modules, and use higher-performance parts located on the analogue module where the sensor required greater performance.
The 40-way connector includes separate analogue and digital signal channels offering appropriate routing to the two types of signal.
For the ADC on the motherboard, we sought an op-amp that offered reasonable performance and the absolute minimum of power consumption. We found it in an Intersil part, the ISL28194, which has a 330nA (typ) supply current with a 3.5kHz gain bandwidth product.
The part is designed for single-supply operation from 1.8V to 5.5V, making it suitable for applications with two 1.5V alkaline batteries, and it offers rail-to-rail input and output swing, giving reliable operation even with a fading battery. Another feature is the shutdown pin, which reduces power consumption to just 2nA when the part is not required, giving essentially zero power consumption when the ADC function is being carried out on the daughterboard.
For the PIR module, operation is based on detecting a threshold voltage, rather than on any more precise measurement, and the ISL28194 is more than equal to the task.
The PIR receiver selected in this case is the Murata IRSA-200ST01, a surface-mount device that is an ideal choice for this application as it runs down to 2V. This allows it to be driven from the same 3.3V rail as most digital ICs, eliminating the need for a separate power rail.
Using this device, and by eliminating the ADC from the module, we achieved a power consumption of just 26µA.
Amplifier stage
The amplifier stage consists of two ISL28194s, and the output of this stage feeds two further ISL28194s configured as a window comparator. By changing reference divider resistors the sensitivity can be altered.
The 3.3V digital output signal is fed to the Zigbee comms module. On receipt of a trigger, the module wakes up and sends an alert to the host, which, depending on the application, can trigger an alarm or other event.
A 0-3.3V analogue signal from the output of the second amplifier can be derived to feed directory into a microcontroller’s ADC. This signal is fed to the Telegesis Zigbee board. The 10-bit A/D converter built into the ZigBee module can sample this signal giving 3.22mV per step.
This design is intended to work with a range of different sensors. If used with a PT100-type temperature sensor, for example, a higher-accuracy op-amp like the ISL28156 would be required on the daughterboard to make the most of the temperature measurement capabilities of the sensor.
If a high-precision PT100 sensor, a microphone, or similar more demanding sensor was to be used, a faster op-amp with a wider bandwidth, such as the ISL28190 or the dual ISL28290, would be needed.
If we had specified the higher-performance op amp for the PIR sensor module, we would have ended up with a device using more than 25,000 times more power and delivering performance that we didn’t need. This illustrates the need to deliver a “good-enough” analogue design that is fit for purpose, rather than trying to achieve the best performance every time.
Author Martin Davey is senior design engineer at Anglia Components