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EETE SEPTEMBER 2012

DESIGN & PRODUCTS DATA ACQUISITION Low power system design optimizations with SAR A/D Converters By Shane O’Meara POwer cOnSuMPtiOn is one of the key requirements for result in lower power consumption. battery powered A/D converter applications today. the trends in All A/D converters on the market targeted at low power portable hand held instruments for the medical, consumer and applications have powerdown or standby modes to conserve industrial markets are for reductions in size and weight, longer energy during periods of inactivity. the ADc is either powered operating time per battery or per battery charge and cost reduc- down between single conversions, or a burst of conversions tions. this is often combined with an increased feature set. this can be performed at a high throughput rate and then the ADc is is equally true for non-battery applications where the benefits powered down between these bursts of conversions. For single of low power systems should not be overlooked. Low power channel converters controlling the operating modes is generally systems don’t require heat sinks or fans, making them smaller, integrated into the communication interface or occurs automati- lower cost, more reliable—and greener. cally once a conversion is complete. Many designers are faced with the challenge to design the advantage of integrating the mode control into the com- products with either reduced power budgets or products with munication interface is a reduction in pin count. this results in enhanced features or performance while maintaining existing reduced power consumption as there are fewer inputs to drive power budgets. and less leakage current. Smaller pin counts also lead to smaller with the huge selection of A/D converters on the market package sizes and less i/O required by the Mcu. whatever today choosing the correct part to meet specific system re- the control method being used, careful use of these modes will quirements is becoming ever more challenging. Once converter provide considerable power savings. performances have been evaluated if low power is a must Power is reduced in power-down modes as the name sug- there are even more specifications that need to be considered. gests by turning off parts of the ADC circuitry. The time to Understanding these specifications and how design decisions turn the circuitry that was shutdown back on determines the affect the power budget is essential for determining system throughput rate at which such modes can be used effectively. power consumption and battery life calculations. An A/D converter with an internal reference the restart time For an A/D converter the average power consumption is a will be determined by the time taken to recharge the reference combination of the power used during conversion, the power capacitor. A/D converters using an external reference require used while not converting and the amount of time spent in each enough time to track the analogue input correctly on restart. mode. this can be expressed by the following equation. For all A/D converters on the market today, power scales with throughput. the power consumed is a combination of static and dynamic power. Static power is constant while the dynamic power scales linearly with throughput. therefore power savings will be made by choosing the lowest throughput rate possible to suit the application. where: PAVG = Average power dissipated PcOnV = Power dissipated during conversion PStBY = Power dissipated during standby or shutdown mode tcOnV = time spent converting tStBY = time spent in standby or powerdown mode to reduce power consumption the power used during conversion and power down mode should be reduced. As the power used during conversion is much greater than the standby power the average power can be greatly reduced if the time in standby mode is increased. One of the biggest factors affecting system power usage is the choice of on-board power supplies. For battery applications the system will often be powered directly by a battery such as a 3V lithium coin cell. this avoids the need for a low dropout volt- age regulator thus saving on power, space and cost. non bat- tery applications also benefit from converters that have low Vdd supply ranges as power consumption scales with input voltage. choosing the lowest Vdd possible for the A/D converter will Shane O’Meara is Application engineer at Analog Devices - www.analog.com – he works in the Precision converters Fig. 1: Power vs. throughput for AD7091R A/D converter. Applications Group in Limerick, ireland. 36 Electronic Engineering Times Europe September 2012 www.electronics-eetimes.com


EETE SEPTEMBER 2012
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