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Digital Pots

It is highly desirable to eliminate or when that is not possible, replace all SIT resistors (or any other SIT part for that matter) with devices that allow automatic adjustment.

A common situation is the use of a digital trimpot to replace an SIT resistor when a specific requirement cannot be achieved economically using off the shelf components.

In the past 10 years or so, most power supplies and amplifiers I have designed that had a microprocessor on board have been using DACs under control of the microprocessor to perform such adjustments. The advantage has often gone beyond the simple elimination of an SIT components. With a particular High Voltage supply, being able to control the output voltage in software via a DAC allowed us to perform temperature compensation in software to eliminate thermal drift due to process variation at the HV Resistor manufacturer a year after the product was qualified. The software fix was designed and implemented over a week-end, whereas replacing the resistors would have taken months.

However, this solution poses a problem when the component to be adjusted is on a separate subassembly than the microprocessor because typical DACs must be initialized by the processor before they assume a specific value, which sometimes prevents to completely test the subassembly before the next level of integration with the processor.

Digital Pots overcome that problem because they generally have an internal non-volatile memory and automatically wake up in the same state they were when powered down.

Most prevalent digital pots have a simple Up/Down interface, where one pin is set high or low to indicate the direction of desired change, and the other pin is toggled until the right setting is achieved. When both pins are left open, the device retains the setting.

This interface, while simple and convenient to use manually (only two switches are required), is not processor friendly because debouncing logic (to allow simple switch control of the device) prevents fast operation. Also, this interface does not allow read-back, so there is no immediate way to know how far we are in the adjustment range, and it is difficult to track process variations until the range become insufficient to achieve the desired result, at which point the product does not ship until the problem is resolved. Finally, each digital pot requires it's own interface, which requires at least 3 connector pins (two active pins + ground), possibly protection circuitry to prevent damage to the device while connected, and access to that connector is required, which means the cover to the unit usually has to be removed while the part is adjusted.

An alternative that combines the best of both worlds is to use a processor friendly digital pot. An example of such part is the MAX5477 series. The MAX5477 series is a true digital potentiometer with an I2C interface. The I2C interface uses two wires which can be shared with multiple devices. Each device is identified with an address, which is selected by grounding selected pins of the device. The MAX5477 has three address inputs, allowing to select one out of 8 possible addresses, so up to 8 devices of the same part number can share the same bus.

The MAX5477 has read-back capability, meaning that the current setting of the part can be recalled through the processor interface bus and saved in the ATE for statistical analysis purposes if desired.

The MAX5477 is specified from -40 to +85 degree C and in most applications I have been involved with, the upper end is insufficient. If this part were used in applications where the upper range must be higher, the part should be qualified over the intended environment.

digital_pots.txt · Last modified: 2013/01/08 19:00 (external edit)
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