Selecting Programmable Voltage References

While designing your programmable reference voltages, you’ll want to select devices that give you the broadest range of options while maintaining the precision your design needs.

The Importance of Programmable Reference Voltages

Today’s industrial, automotive and communication systems use real-world inputs to produce precise control responses. The components in these systems and their analog subsystems require voltage references. Variations in process, components and the natural world may affect system performance, requiring changes in your voltage references.

We’ll explain how to choose devices for your programmable voltage references that help you:

Increase revenue

• Reducing board spins improves time to market, increasing revenue for your new product

Lower overall cost

• Revisions also cost money, so we’ll reduce them with a low-cost solution

Mitigate risk

• This simple, readily available and programmable solution improves precision and accuracy in analog subsystems

Spinning a board refers to the Printed Circuit Board Assembly (PCBA) development process, which consists of three stages: design, build and test. For all but the simplest designs, multiple iterations of this process are required to optimize the design. Adding programmable components to your design can help minimize spins and reduce cycle time.

How to Select Programmable Reference Voltages

While designing your programmable reference voltages, you’ll want to select devices that give you the broadest range of options while maintaining the precision your design needs.

When you are determining your requirement, you will typically know what voltage resolution that is required and the range that the voltage will vary between. For example, if the voltage resolution was 10 mV, and the application needed this resolution across 1 volt, then a device with 7-bit resolution across this range would be suitable. If the device’s output needed to go across the entire voltage range (such as most digital-to-analog converters or DACs) then 9-bit resolution would be required (5.0 / 512 = 9.8 mV). This would typically mean that a 10-bit device would be selected (since 9-bit resolution devices are not widely available).

This table shows how many steps there are over the voltage range at a given step size.

This table shows what voltage range can be achieved based on the device’s resolution at a given step size.

Although the digital potentiometer can window the output voltage, one must understand the issues to ensure that the design takes the issues into account.

The major issue is the variation of the RAB resistance. Typically the RAB resistance will vary ±20%, while typical resistors will be ±1% (although high-tolerance resistors are available).

In this example, R1 = 10k ohm (±1%), R2 = 5k ohm (±1%) and the RAB resistance is 10k ohm ±20%. So if we look at the nodes A and B, the typical voltage at these nodes (VA and VB) is 0.6*VDD and 0.2*VDD.

Looking at the worst case (min/max) combinations, then VA can be as high as 0.6327*VDD and VB can be as low as 0.1829*VDD. Also, VA can be as low as 0.5618*VDD and VB can be as high as 0.2201*VDD. The VA(MIN)/VB(MAX) is the range that one must ensure that the VOUT will range. If that is not sufficient, then the R1 and R2 values must be modified.

Note that when VA is at its minimum value, VB will be greater than the typical value. When VB is at its maximum value, then VA will be less than the typical value.

If VA and VB are forced to fixed voltages (such as VDD and VSS), then the wiper output voltage will be ratio metric to the selected code (all step resistors have the same variation).

Ourdigital potentiometersminimize development risk and increase system performance by providing reliable, well-documented functionality and quick fixes for design flaws and offer better precision because of their Integral Nonlinearity (INL), total unadjusted error specs and increase in resolution. The portfolio enables fine tuning with highly accurate, small form factor, digitally controlled resistor solutions.

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