Voltage reference (reference voltage source IC)

01

Pick topology, output voltage and accuracy

TI's REF35 datasheet describes a family of nanopower, low-drift series references with output options of 1.024, 1.2, 1.25, 1.6, 1.8, 2.048, 2.5, 3.0, 3.3, 4.096 and 5.0 V, initial accuracy of ±0.05% maximum, temperature coefficient of 12 ppm/°C maximum (−40 to 105 °C, SOT-23) or 10 ppm/°C (−40 to 85 °C, WCSP), 1/f output noise of 3.3 ppm p-p (0.1–10 Hz), long-term stability of 40 ppm at 1,000 hours and thermal hysteresis of 70 ppm. [1]

TI's LM4040 datasheet describes a precision micropower shunt reference with fixed reverse-breakdown voltages of 2.048, 2.5, 3, 4.096, 5, 8.192 and 10 V; for the 2.5 V A grade it lists ±0.1% output tolerance at 25 °C, 35 µV rms noise (10 Hz–10 kHz), an operating current range of 60 µA to 15 mA and 100 ppm/°C maximum temperature coefficient, needing no output capacitor and tolerating capacitive loads. [2]

Editorial calculation: for a 2.5 V REF35, ±0.05% is ±1.25 mV and 12 ppm/°C over a 60 °C swing adds up to 2.5 × 12 × 60 × 10⁻⁶ ≈ 1.8 mV; a 16-bit ADC on a 2.5 V reference has an LSB of about 38 µV, so reference error, not ADC resolution, sets absolute accuracy unless the system is calibrated. [1]

Give the supplier the output voltage, series or shunt topology, initial accuracy, temperature coefficient over your range, noise band, long-term stability and hysteresis, supply voltage and load current (or shunt current range), quiescent current budget, enable or noise-reduction pins, temperature range, qualification grade, package and quantity.

02

Check current, stability, temperature and package

The REF35 lists 650 nA (typ.) quiescent current, output current of +10 mA source and −5 mA sink, input voltage from VREF plus dropout up to 6 V, an NR pin to reduce noise and an EN pin to cut shutdown current, operation from −55 to 125 °C, and 6-pin SOT-23 or 4-pin WCSP packages, for flow transmitters, glucose monitors, servo drives, power-quality analysers and oscilloscopes. [1]

The LM4040 comes in SOT-23, TO-92 and SC70 packages, in industrial (−40 to 85 °C) and extended (−40 to 125 °C) grades, with an AEC-Q100 automotive version. [2]

Editorial note: a shunt reference draws its current through a series resistor whether or not the load needs it, so compare the total supply current of a shunt design against a series reference's quiescent current for battery products. [2] [1]

03

Add up drift, accuracy and noise into an error budget

SGMICRO's product page for its SGM4029 low-noise, low-drift precision series reference lists temperature drift of 5 ppm/°C (typ.) and 10 ppm/°C maximum over −40 to +125 °C, and 3 ppm/°C (typ.) over 0 to 85 °C; initial accuracy of ±0.05% (typ.) and ±0.1% maximum; noise of 4.5 µVp-p per volt of output; ±10 mA output current (source or sink); and a green SOIC-8 package. [3] It lists data acquisition, test and lab instruments, industrial process control, medical equipment and temperature and pressure transmitters as applications. [3]

Comparing two makers' parts on the same lines shows the trade-off between initial accuracy, drift, current and power: the REF35 is specified for 650 nA quiescent current, while the SGM4029 is positioned for low noise and ±10 mA drive. [3] [1]

Editorial calculation: for an example 2.5 V output, ±0.1% initial error is ±2.5 mV; a 10 ppm/°C maximum drift over the 165 °C span from −40 to +125 °C adds up to 1,650 ppm (about 4.1 mV); noise of 4.5 µVp-p/V is about 11 µVp-p. For a 16-bit converter with a 2.5 V range, one LSB is about 38 µV, so drift and initial error dominate unless calibrated out.

State the output voltage, the temperature range the system sees, whether one-point calibration is possible, and the converter resolution, and ask for initial accuracy, drift (with method), noise, long-term stability and current figures for the exact grade.

04

When the budget is tighter: a laser-compensated zener reference for 16- to 18-bit converters

Apex Microtechnology's VRE302 page describes a zener-based +2.5 V reference whose accuracy comes from a proprietary multipoint laser compensation technique. The headline lists ±0.40 mV (0.016%) output accuracy, drift as low as 1.0 ppm/°C, 1.5 µVp-p noise from 0.1 to 10 Hz, 6 ppm/V (typ.) line regulation and output trim capability; the description gives ±0.6 mV (0.024%) initial accuracy for the series. [4] Apex recommends the series as a reference for 14-, 16- or 18-bit D/A converters that need an external precision reference, and for calibrating the scale factor of high-resolution A/D converters. The two listed variants, VRE302JD (8-pin DIP) and VRE302JS (8-pin SMT), are each ±0.6 mV initial error and 2.2 ppm/°C over −40 to 85 °C. [4]

Editorial calculation: for a VRE302JD, 2.2 ppm/°C over the 125 °C span from −40 to 85 °C allows up to 2.5 V × 2.2 × 125 × 10⁻⁶ ≈ 0.69 mV, and with ±0.6 mV initial error the worst case is about 1.3 mV. On the same 2.5 V output the SGM4029 example above reached about 2.5 mV initial plus 4.1 mV drift over a wider span. Against a 16-bit, 2.5 V converter's 38 µV LSB, even the zener part's 0.69 mV drift is about 18 LSB, so specify whether the system calibrates at one temperature or must hold accuracy uncalibrated. [4] [3]

Read the page's own figures carefully: its headline and its variant table give different initial accuracy (±0.40 mV against ±0.6 mV) and drift (1.0 against 2.2 ppm/°C), because the best figures apply to particular models and grades. Order by full part number and ask the supplier for the guaranteed values of that grade, not the series headline. [4]

05

SGMICRO and 3PEAK reference families, and the grade data to ask any maker for

SGMICRO lists the SGM4020, SGM4027, SGM4029, SGM4030 and SGM4040 references; 3PEAK lists shunt references including the TPR433-S and TPR6040.

Ask each supplier for the datasheet with initial accuracy, temperature coefficient and its test method (box or slope), noise, long-term stability, hysteresis and current figures for the grade you order. [1] [2]

06

Reference core, series output stage, shunt element, NR and EN pins, package

The package row reaches into the drift figure: TI quotes the REF35 at 12 ppm/°C maximum in SOT-23 over −40 to 105 °C and 10 ppm/°C in WCSP over −40 to 85 °C. [1]

07

Output voltage, initial accuracy, drift, noise, stability, hysteresis, current and package for a reference order

Rows with two figures give the REF35 series value first and the LM4040 shunt value second; the last row adds SGMICRO's SGM4029. [1] [2] [3]

08

Send the voltage, accuracy and current data together

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Voltage: [V]
Topology: [series; shunt]
Accuracy: [%; ppm/°C; range]
Noise/stability: [ppm p-p; ppm/1000 h]
Current: [quiescent; load; shunt range]
Features: [EN; NR]
Temp/grade/package: [°C; AEC-Q100; package]
Quantity: [units]

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