This circuit shows a simple method of achieving a voltage reference which can be trimmed using an 8bit DAC with an on-chip voltage reference. The analogue output from the DAC drives the trim pin of the ZNREF025 via a resistor, R,. When Rs = 0, this circuit will produce a trim range exceeding ± 5% of the nominal reference voltage. When Rs is greater than zero, the trim range is reduced. It was found that after dividing the trim range by two, the needed value for Rs was approximately 1 MO.
Digitally-controlled-voltage-reference - schematic

The reference voltage of the ZNREF025 can be set to an accuracy determi11ed by the trim range of the device itself and the accuracy of the DAC. Increasing Rs reduces the percentage trim range and hence increases the effective voltage resolution. Other voltage references from the ZNREF series can be used with this circuit if other voltages are required. This voltage reference can be used to set the value of VREF to a much tighter tolerance, than the data sheet specification of ±1%, in a much wider range of operating conditions. Applications could include any system with automatic self-cahbration of instrumentation, such as in electronic weighing scales.

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