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Posted on 23 July 2019

Get <1ppm Performance from a 10ppm Precision Reference

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Rejustor Technology turns a good voltage reference into an ‘industry best’

Precision References require the user to make major cost/performance tradeoffs. Until now, ultra-high performance came at a high price. By using a low cost, tunable resistor divider (Rejustor) from Microbridge, precision references can be made more accurate and their temperature coefficients can be offset to offer unmatched performance over their operating temperature range.

By Bob Frostholm, VP Marketing – Microbridge Technologies Corp

 

Selecting precision voltage references can be an arduous task. Today, there are hundreds of products offered by dozens of chip companies, offering various combinations of initial accuracy, temperature coefficients and cost. Selecting the right part for the right application requires considerable research into the details of the datasheets as well as sourcing volume pricing quotations.

While several parameters are important in the selection of voltage references, the two that stand out as potentially contributing the greatest errors are initial accuracy and temperature coefficient (TC). Noise, thermal hysteresis, line and load regulation and long term stability should not be neglected when making a selection, but their contributions to error are shadowed by initial accuracy and TC. It is not surprising therefore, that chip manufactures concentrate their marketing efforts to proclaim best in class performance achievements in these two domains.

Initial Error

Initial error is the deviation of the actual output voltage from the desired specification. Many voltage references have trim pins that allow the user to set or pre adjust the output value in an attempt to offset the inherent initial error of the reference chip. (See Fig. 1) The output is adjusted by setting the ratio of resistance above and below the “wiper” shown in the figure.

Typical external resistor configurations to trim a precision voltage reference

Of course the ability to precisely select the desired output voltage then becomes an issue of finding the correct resistance values, taking into consideration their initial accuracies (or perhaps more correctly, their inaccuracies) as well as their contribution to the TC of the output voltage. The specific ratios attainable are constrained by the limited selection of fixed resistance values.

In this application we have set about to show how to improve the performance of an average precision voltage reference, the Analog Devices ADR 425A, by more than an order of magnitude.

Analysis of the circuit requirements showed that by replacing the 470KÙ ballast resistor with a 120KÙ, resistor, and using the MBT143E 1:9 ratio Rejustor, the Vout could be adjusted quite precisely. (See Fig 2).

Fully calibrated and temperature compensated precision reference

To assure the best possible performance, the trimming is accomplished at the assembled board level. During this calibration process, software algorithms provided by Microbridge automatically adjust the two resistances of the MBT143E down from their as manufactured values based on the real time feedback from Vout, taking about 1-2 seconds, until the desired output accuracy is achieved. The rejustor values will be trimmed to slightly different resistances for each ADR425A to compensate for both the minor manufacturing variances of the precision reference as well as the differing non idealities associated with each board layout. Thus the use of precision fixed resis- tors would become an almost impossible task without massive reiterative measurement and hand selection.

Temperature Coefficient

Temperature coefficient is the measure of the stability of the output of the precision voltage reference with temperature changes. Precision means nothing if it cannot be maintained over the useful operating temperature range of the system into which the precision voltage reference is designed. TC is referred to as the second most important specification to consider when selecting a reference. First-order temperature correction (or linear temperature correction) is the largest contributor to errors associated with temperature variations.

Datasheet examination of the ADR 425A reveals the device is specified to have an initial accuracy of +/- 3mV (+/- 0.15%) and a relatively linear TC of 10ppm/°C, making it an average performer among several competitive devices. See Figure 4.

Microbridge Rejustors provide designers with a new tool with which to craft and adjust an application circuit. Rejustors are passive precision resistor dividers whose ohmic resistance and TC can be trimmed independently. Once trimmed these values remain constant and require no power to maintain their values. The TC-Offset vs. Offset characteristics of MBT143E divider are shown in Figure 3. The Offset is the deviation of the divider output voltage Vin*(R1/(R1+R2)), measured in mV per volt of divider input voltage Vin, away from Vin*(R1o/(R1o+R2o)), where R1o and R2o are the nominal unadjusted divider resistance values.

A typical plot of the achievable sets of values for voltage divider

The TC-Offset is the temperature coefficient of that divider output voltage, measured in uV per degree-C (K) per volt of divider input voltage. Microbridge’s electrical TC adjustment software allows one to pick target values for Offset and TC-Offset as a point within the roughly-parallelogram-shaped region shown in Figure 3. For example, if initially the divider input voltage were low by 5% (50mV/V) from its designed value, and, additionally, it has an undesired +75uV/VK temperature variation, and if it is desired that the drive level be temperature- stable at the nominal Vin*(R1o/(R1o+R2o), then one programs the divider to the point (+50mV, -75uV/KV), as shown in the figure.

The adjustment software allows you to pick a target spot within this roughly parallelogram-shaped region. One specific example point is shown, at Offset = +50mV/V and TC-Offset = -75uV/VK, (+50mV/V, - 75uV/VK).

For the purposes of this example the ADR425A was characterized over the temperature range of +0°C to +85°C. Characterization and analysis of the temperature stability of the ADR425A reveals an output voltage variation of ~5000uV over the temperature, or ~12ppm/°C , close to the specification for the A grade product (10ppm/°C) shown as the curve in Figure 4 identified as “ADR425A as it is”.

Typical performance of ADR425A precision voltage reference

Based on these characterization results, Rejustor calibration targets were set and the MBT143E was adjusted “in circuit” using Rejust-it software from Microbridge at the same time that the configuration was being tested.

By adjusting the ohmic value of the two rejustor resistances to set the “perfect” divider ratio, the initial output of the ADR425A was improved from 4.99612V to 5.000125V or 0.0025% initial error. By adjusting the TCR of the two resistances in the divider, the majority of the positive temperature coefficient was eliminated, improving the TC from 12ppm/°C to 0.8ppm/°C.

Conclusion

You don’t have to pay a lot more for performance. For less than $0.50 Rejustor technology enhances the performance of a low cost precision voltage reference has been demonstrated to outperform devices costing several dollars more.

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