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Voltage references in embedded design

2017-11-22 10:15  
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Embedded systems that are dealing with DAC require a good voltage references. Despite all voltage references have their nominal values there are specific tolerances that indicates how much values can vary from nominal value. References as a rule are semiconductors that characteristics are effected by temperature.

For instance if we decide to use a 2.5V voltage reference then (zener) diode LM336A-2.5 may be a good choice for this.

These diodes have low temperature coefficient. It can guarantee that voltage will be in range between 2.44 and 2.54 at 25?oC. This precise zener diode has a third lead that can be used to adjust break down voltage. In the following figure you may see diode with potentiometer where breakdown voltage can be adjusted in range of ?±120mV:

Adjusting break down voltage can be important feature where precision ADC are required. As we mentioned real reference voltage value can diverge from nominal so the ADC readings. And the bigger voltage needs to be read – the bigger error occurs.

We know that compensation can al so be done in software by adding some known offset value. But in many cases compensation leads to floating point operations where regular microcontrollers would waste their resources – especially in time critical measurements. As alternative there can be a reference table stored in memory, where calculations need to be made only once. But again for 10bit ADC readings we would need 1024 table entries – so this is a 1kB of non-volatile memory. Such implementations would also need a calibration in order to fill table with calibration values.

So probably it is better to avoid errors in hardware level than correct them in software. In precise embedded systems I would really recommend using precise voltage references. If you need more precision than zener diode can provide you can chose Maxim MAX6225 2.5V reference where value is in range between 2499V and 2.501V at 25?oC. MAX6225 also has ability to connect potentiometer that can adjust voltage range by about 40mV.

And probably I don’t need to mention that some microcontrollers have internal voltage reference sources that can be used for ADC capture. But if you need accuracy you should avoid using them.

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