UNCLASSIFIED
TM-CAL-006
PRECISION VOLTAGE REFERENCE
Band-Gap IC References (LM4040, LT1021) for DC Voltage Calibration
Prepared by: Mervyn Martin, KO6NNH
Merced, California  •  26 May 2026
Amateur Radio / Electronics — Not for commercial calibration use

Table of Contents

  1. Chapter 1 — General Information
  2. Chapter 2 — Theory of Operation
  3. Chapter 3 — Materials and Construction
  4. Chapter 4 — Assembly Procedures
  5. Chapter 5 — Calibration Procedure
  6. Chapter 6 — Tuning and Adjustment
  7. Chapter 7 — Verification
  8. Appendix A — Calculations and Formulas
  9. Appendix B — Example Results

CHAPTER 1 — GENERAL INFORMATION

1-1. SCOPE

This manual covers construction and use of a precision voltage reference based on IC band-gap reference circuits. The reference provides stable, accurate DC voltages traceable to the SI volt through the component manufacturer's calibration chain. Suitable for calibrating digital multimeters, analog panel meters, and ADC input circuits.

1-3. SAFETY

NOTEOperates from a 9–15 V DC supply. No shock hazard under normal conditions.
CAUTIONBand-gap reference ICs are sensitive to electrostatic discharge. Handle with ESD precautions. Do not allow output to be connected to voltages exceeding the supply rail.

CHAPTER 2 — THEORY OF OPERATION

2-1. BAND-GAP REFERENCE PRINCIPLE

Band-gap voltage references exploit the temperature-stable forward voltage of silicon junctions. A PTAT (proportional-to-absolute-temperature) current is summed with a complementary CTAT current; when properly ratioed, the sum equals the silicon band-gap extrapolated voltage of approximately 1.205 V. This voltage is independent of temperature to first order.

Precision reference ICs (LM4040, LT1021, LM399, LTZ1000) implement this principle with additional trimming and, in high-precision devices, temperature-controlled substrates.

Voltage Reference IC Comparison
ICVoltageInitial Acc.Temp. Coeff.Notes
LM4040A-5.05.000 V±0.1%100 ppm/°CShunt reg, low cost
LM4040B-5.05.000 V±0.2%100 ppm/°CEconomy grade
LT1021-55.000 V±0.05%5 ppm/°CExcellent, SOT-23
LM3996.95 V±0.3%0.3 ppm/°CHeated substrate, best TC
LTZ10007.0 V±0.05%0.05 ppm/°CUltra-precision, requires heater

CHAPTER 3 — MATERIALS AND CONSTRUCTION

3-1. BILL OF MATERIALS (LT1021-5 VERSION)

Components
QtyItemValue/Spec
1LT1021CCZ-5 or LT1021BCZ-55.000 V precision reference, TO-92 or DIP-8
1Voltage regulatorLM7809 or LM317, 9–12 V input
1Electrolytic capacitor10 μF / 25 V
2Ceramic capacitor100 nF, C0G/NP0 on reference input and output
1Resistor1kΩ 0.1%, current limiting for shunt types
1BNC or banana jackOutput connector
19 V battery or DC inputFor portable use

CHAPTER 4 — ASSEMBLY PROCEDURES

  1. Pre-regulate supply to 9 V using LM7809 with 100 nF input/output bypass capacitors.
  2. For LT1021 (series type): connect IN pin to 9 V through 1kΩ resistor. Connect OUT to output terminal. Connect GND to ground. Add 100 nF on output.
  3. For LM4040 (shunt type): connect through 1kΩ from supply to LM4040 anode. Output is taken across the LM4040. Load current must be <15 mA.
  4. Solder all components on perfboard with short, tidy leads. Shield board from air currents if TC performance is critical.
  5. Mount in small enclosure. Label output: '5.000 V REF ±0.1%'.
  6. Allow 15 minutes warm-up before first measurement.

CHAPTER 5 — CALIBRATION PROCEDURE

5-1. OUTPUT VOLTAGE MEASUREMENT

  1. Allow reference to warm up for 15 minutes.
  2. Measure output with the best available DMM (Fluke 15B+ or equivalent).
  3. Record: Vmeas, ambient temperature, supply voltage.
  4. Compare to nominal 5.000 V: error% = (Vmeas − 5.000) / 5.000 × 100%.
  5. For LT1021BCZ-5: accept ±0.05% (±2.5 mV). Trim with external resistor if needed.
  6. For LM4040A: accept ±0.1% (±5.0 mV). Not adjustable; select units.

5-2. USING THE REFERENCE TO CALIBRATE A DMM

  1. Connect reference output to DMM V input.
  2. DMM should read within its specification of the reference voltage.
  3. If DMM reads 4.985 V and reference is known 5.000 V, DMM error = −3.0%.
  4. Apply correction: actual voltage = DMM reading × (Vref / Vdmm).
  5. Verify at multiple voltage settings using a precision resistor divider to derive additional reference points.

CHAPTER 6 — TUNING AND ADJUSTMENT

The LT1021 can be trimmed by connecting a small resistance (<100 Ω) in series with the internal trim network (pins 4 and 5 on DIP-8 package) to adjust the output voltage up or down by up to 0.2%. A 50 Ω trimmer pot provides the full adjustment range. Set at room temperature after full thermal stabilization.

CHAPTER 7 — VERIFICATION

  1. Verify output at T = 25 ±5°C after 15-minute warm-up.
  2. Remeasure after 24 hours: long-term stability should be <50 ppm/month for LT1021.
  3. Verify supply rejection: change input supply voltage from 9 V to 12 V. Output should change <1 mV (<0.02%).
  4. Log all readings with date and temperature.

APPENDIX A — CALCULATIONS AND FORMULAS

Voltage error in ppmerrorppm = (Vmeas − Vnom) / Vnom × 106
Temperature error (LT1021, 5 ppm/°C)ΔV = Vnom × TC × ΔT = 5.000 × 5×10−6 × ΔT
Example: 10°C temperature change with LT1021ΔV = 5.000 × 5×10−6 × 10 = 0.00025 V = 0.25 mV

APPENDIX B — EXAMPLE RESULTS

Voltage Reference Output Log
DateICSupply (V)Temp (°C)Output (V)Error (mV)Error (ppm)
2026-05-26LT1021BCZ-59.12234.9998−0.2−40 ppm
2026-05-26LM4040A-5.09.12235.0038+3.8+760 ppm
2026-05-26LT1021BCZ-512.00235.00000.00 ppm
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