Several parameters are important in the selection of voltage references. But the two that potentially contribute the greatest errors are initial accuracy, or error, and temperature coefficient (TC). Initial accuracy is the deviation of the actual output voltage from the desired specification. This article shows how to greatly reduce the errors caused by these specifications.
Many voltage references have trim pins that designers can use to set or pre-adjust the output in an attempt to offset the reference’s inherent initial accuracy error. The output is adjusted, or fine tuned, by setting the ratio of resistance above and below the “wiper” or trim pin. Unfortunately, the limited selection of normalized standard resistance values constrains the number of unique ratios attainable with fixed, 0.1% resistors.
Moreover, proper resistor divider selection only addresses initial accuracy. The TC of the voltage reference must also be considered. For example, the ADR425A from Analog Devices offers good performance at 25°C, but its TC may make it unsuitable for many temperature-critical applications. The system must maintain its precision over its entire required operating temperature range.
The ADR425A’s datasheet specifies an initial accuracy of ±3 mV (±0.15%) and a TC of 10 ppm/°C. (Our actual measurements revealed 12 ppm/°C.) But many applications require a lower TC, often approaching 1 ppm/°C.
Rather than spending several dollars for a top-of-the-line reference, the ADR425A can be combined with a low-cost Rejustor divider whose resistance ratio and resistance TC can be infinitely trimmed. The MBT-143-E eTC Rejustor was trimmed to have a negative temperature coefficient of resistance (TCR) closely matching the slope of the positive TC of the ADR425A. The absolute values of the two resistances were also trimmed to improve the initial accuracy of the output voltage.
The result is an initial error of only 0.0025% at 25°C, which is an improvement of almost two orders of magnitude. The nearly equal but opposite TCR of the Rejustor divider (see the figure) virtually nulled out the TC of the ADR425A, yielding a solution with a total TC of just 0.8 ppm/°C from 0°C to 85°C. The output voltage was measured using an Agilent 34420A 7.5-digit nano voltmeter. An Omega HH21 microprocessor thermometer with thermocouple K measured the temperature.