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High accuracy analogue signal measurement with MCU

Posted: 17 Oct 2013 ?? ?Print Version ?Bookmark and Share

Keywords:analogue design? low power? microcontrollers? MCU? data acquisition systems?

There are many design considerations when optimising a high precision analogue design for low power consumption. The solution we consider here with low power microcontrollers (MCU) assumes that the design aim is to achieve the best low noise precision for 16bit to 24bit applications focused on sensor and data acquisition. Typical applications for sensors include thermocouple or thermopile sensors and bridge sensors such as pressure, strain, flow, AMR and ultrasonic sensors. Of course, these applications can be used for general purpose analogue inputs in data acquisition systems as well.

All of the solutions in the design we consider here achieve 18bit to 22bit ENOB (noise free bit design) are notable for highest precision. The solutions system partition for signal conditioning includes the frontend gain stage (usually amplifiers or instrumentation amplifiers), ADC and precision voltage excitation or voltage references for ratiometric design.

Table 1: Intersil's key mixed signal analogue components for highest precision, low noise design.

Starting with the op-amp; biasing circuitry, input, output and compensation stages need to be examined.

The DC biasing circuitry for the op-amp must provide accurately determined and suitably regulated quiescent biasing currents at very low current levels. The current must be insensitive to changes in temperature, supply voltage and process tolerances. The configuration of the input stage will dictate whether the op-amp can be used in a single low voltage (1.2 V, or lower) supply application. Very low power supply operation using core devices is desirable (but not always possible). Using a Class B or AB output stage can reduce the quiescent power dissipation particularly in a leaky process.

Utilising products with Rail to Rail common Mode input range but with the capability to drive a small output load can be desirable to achieve lower power consumption. But of course we still need to consider maintaining the high precision, accuracy and keeping offsets low with high input impedance is imperative while managing a high CMRR (Common Mode Rejection Ration) and PSRR (Power Supply Rejection Ratio). These are just some of the main considerations for the op-amp but there are of course a number of components required in a high precision signal chain one of the most important being the ADC. Higher throughput requires higher power consumption.

Reducing Sample Rates will reduce power consumption, which can potentially reduce the precision capabilities of the circuit design. With suitable conditioning and buffering an optimum performance can still be achieved while continuing to manage power efficiency effectively.

Choosing individual analogue components including the ADC, op-amp, buffers and voltage regulators that work at lower supply voltages is a major consideration when optimising your design for reduced power consumption.

If you require your design to work in a hand held application an obvious advantage here is to use batteries, which produce less noise than standard power supplies.

Immediately your analogue signal path will have less interference from environmental and electrical disturbances. Also use of differential signal conditioning where possible can reduce noise in the design and also increase the dynamic range prior to ADC conversion.

The combination of the Intersil ISL28134 in differential configuration with the ISL26102 makes for one of the best implementations of precision, low noise design common in bridge sensor and data acquisition applications.

The components chosen for the reference design are chopper stabilised op amplifiers, the ISL28134; and the Delta Sigma ADC, ISL26102/104. The data acquisition solution shows 'DAQ on Stick' as an example. The solution employs the techniques mentioned previously to achieve lowest noise design for precision applications.

The 'DAQ on Stick' evaluation platform includes GUI software to select gain for various sensors including pressure, strain, flow, AMR and Ultrasonic sensors. Included in the demo is a Strain Gauge sensor (using Vishay's VPG precision resistors). Additional circuitry is used to be able to attach various sensors and to allow for gain selection. These components are optional and can be removed for more optimal design if necessary.

Figure 1: Simplified strain gauge schematic.

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