Voltage reference pairs with Microbridge rejustor

The Analog Devices ADR425A low-noise, 5V XFET voltage reference is specified to have an initial accuracy of +/-3mV (+/-0.15 per cent) and a TC of 10ppm/C.

To calibrate and temperature compensate the ADR425A, a 120K ohm resistor and an MBT143E 9:1 eTC rejustor divider from Microbridge was used.

The MBT143E is a member of the eTC (electrically adjustable temperature coefficient) family of rejustors.

The ADR425A was characterised over the temperature range of 0C to +85C.

Characterisation 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).

Based on these 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 per cent 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.

The use of an external rejustor has markedly improved the ADR425A as a cost-efficient voltage reference.

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