Oscilloscope Voltage Calibration

Overview

Voltage calibration ensures accurate amplitude measurements. We'll build precision voltage references using cheap ICs and measure scope accuracy at each voltage range.

Key Idea: Use $2 precision voltage reference ICs as "voltage rulers" to check scope accuracy.

Building Precision Voltage References

Parts List

Qty Part Voltage Accuracy Cost Source
1 LM4040-1.2 1.225V ±0.1% $1.50 Mouser, Digikey
1 LM4040-2.5 2.500V ±0.1% $1.50 Mouser, Digikey
1 LM4040-5.0 5.000V ±0.1% $2.00 Mouser, Digikey
2 Resistor 10kΩ 0.1% - ±0.1% $1.00 Mouser, Digikey
1 9V battery ~9V ±5% $1.00 Local store
1 Breadboard - - $3.00 Local store

Total: ~$10-15

Circuit Schematic

LM4040 Basic Circuit:

        9V Battery (+)
                            │
                            ├─────→ To other refs
                            │
                         10kΩ
                            │
                            ├────→ OUTPUT (1.225V, 2.5V, or 5V)
                            │       │
                        ┌───┴───┐   │
                        │ LM4040│   ├───→ Scope probe
                        │       │   │
                        └───┬───┘   │
                            │       │
                           GND ─────┴───→ Scope ground

Complete Reference Board (4 voltages):

9V Battery
                   │
                   ├─ 10kΩ ─┬─ LM4040-1.2 ─→ 1.225V output ─→ Banana jack RED
                   │        │    │
                   ├─ 10kΩ ─┼─ LM4040-2.5 ─→ 2.500V output ─→ Banana jack YEL
                   │        │    │
                   ├─ 10kΩ ─┼─ LM4040-5.0 ─→ 5.000V output ─→ Banana jack GRN
                   │        │    │
                   │        │    │
                   │    Precision voltage divider:
                   │        │
                   │      10kΩ (0.1%) ─┬─→ 10.00V output ─→ Banana jack BLU
                   │                   │   (from 5V ref × 2)
                   │                 10kΩ (0.1%)
                   │                   │
                   └──────────────── GND ─────────────────→ Banana jack BLK

Building the Circuit

Step 1: LM4040 Connections (30 minutes)

  1. Identify LM4040 pins: ``` LM4040 (TO-92 package, looking at flat side):

[Cathode] [Anode] [NC] 1 2 3

Pin 1 (Cathode) = Output voltage Pin 2 (Anode) = Ground Pin 3 (NC) = Not connected ```

  1. Breadboard layout: ``` Row 1: 9V+ Row 2: 10kΩ → LM4040-1.2 cathode → 1.225V output Row 3: LM4040-1.2 anode → GND

Row 5: 9V+ Row 6: 10kΩ → LM4040-2.5 cathode → 2.500V output Row 7: LM4040-2.5 anode → GND

Row 9: 9V+ Row 10: 10kΩ → LM4040-5.0 cathode → 5.000V output Row 11: LM4040-5.0 anode → GND ```

  1. Connect outputs to binding posts (for easy scope connection)

Step 2: 10V Divider (15 minutes)

From 5V reference to 10V:

Actually, easier method: Use two 5V refs in series!
                
                9V+ ─ 10kΩ ─ LM4040-5.0 (REF1) ─┬─ 5V output
                                                  │
                             10kΩ ─ LM4040-5.0 (REF2) ─┬─ 10V output (REF1+REF2)
                                                        │
                                                       GND

Or use precision divider:

5V reference ─ 10kΩ (0.1%) ─┬─ 10V output
                                             │
                              10kΩ (0.1%) ──┴─ GND
                
                Wait, this gives 2.5V, not 10V!
                
                Correct circuit:
                Need op-amp to create 10V from 5V, or use two refs in series.
                
                Simplest: Two LM4040-5.0 in series = 10V

Testing References with DMM

Verify each reference:

  1. 1.225V reference: DMM reading: 1.224V to 1.226V (±0.1%)

  2. 2.500V reference: DMM reading: 2.498V to 2.502V (±0.1%)

  3. 5.000V reference: DMM reading: 4.995V to 5.005V (±0.1%)

  4. 10.00V (if built): Two 5V in series: 9.99V to 10.01V (±0.1%)

If readings are off: Check wiring, polarity, battery voltage >7V

Calibration Procedure

Step 1: Prepare Oscilloscope

  1. Warm up: 15-30 minutes powered on
  2. Probe: 10:1 probe (or 1:1 for low voltages)
  3. Settings:
  4. Coupling: DC
  5. Bandwidth: Full (not limited)
  6. Trigger: Normal, not Auto

Step 2: Measure Each Reference

1.225V Reference:

  1. Connect:
  2. Scope probe to 1.225V output

  3. Ground clip to GND

  4. Scope settings:

  5. V/div: 500 mV/div (to see ~1.2V signal)

  6. Timebase: Doesn't matter (DC signal)

  7. Read voltage:

  8. Use MEASURE → DC voltage

  9. Or use cursor at trace level

  10. Record: Reference: 1.225V Scope reads: _____ V Error: _____ V Error %: _____ %

Repeat for all references: - 2.500V - 5.000V - 10.00V (if built)

Test multiple V/div settings:

For 5V reference, test: - 1V/div (scope reading: ) - 2V/div (scope reading: ) - 5V/div (scope reading: ___)

Each setting may have different error!

Step 3: Calculate Errors

Example measurements:

Reference: 5.000V
                V/div Setting    Scope Reads    Error (V)    Error (%)
                ─────────────────────────────────────────────────────
                1V/div           5.12V          +0.12        +2.4%
                2V/div           5.08V          +0.08        +1.6%
                5V/div           5.05V          +0.05        +1.0%
                
                Conclusion: Error varies by V/div setting!
                Scope is generally reading HIGH (+1 to +2.4%)

Step 4: Document Corrections

Create calibration table:

DSO1013D CH1 Voltage Calibration
                
                Reference    V/div    Reading    Error    Correction
                ─────────────────────────────────────────────────────
                1.225V       500mV    1.24V      +1.2%    ×0.988
                2.500V       1V       2.54V      +1.6%    ×0.984
                5.000V       2V       5.08V      +1.6%    ×0.984
                5.000V       5V       5.05V      +1.0%    ×0.990
                
                Average error: +1.35%
                Average correction factor: ×0.987
                
                CONCLUSION: Scope reads 1.35% HIGH
                Multiply displayed voltage by 0.987 for actual voltage

Repeat for CH2! (Each channel may differ)

Probe Compensation

Critical for AC measurements!

Using Built-in Cal Signal

Most scopes have ~1 kHz square wave output:

  1. Locate cal output:
  2. Usually labeled "CAL", "PROBE COMP", or "1kHz"

  3. Typically 1 kHz, 5V peak-to-peak square wave

  4. Connect probe:

  5. Probe tip to CAL signal

  6. Ground clip to GND

  7. Observe square wave: ``` Good (compensated): ┌──┐ ┌──┐ │ │ │ │ ┘ └──┘ └──

Under-compensated: ┌──┐ ┌──┐ ╱ │ ╱ │ ┘ └──┘ └── (Rounded edges, looks like RC filter)

Over-compensated: ┌─┐ ┌─┐ │ ╲╱ │ ╲╱ ┘ └──┘ └── (Overshoot, ringing) ```

  1. Adjust probe:
  2. Find small trimmer capacitor on probe body
  3. Use plastic screwdriver
  4. Adjust until square wave has flat top
  5. Should look crisp, no overshoot

Do this for EVERY probe, EVERY time you connect it!

Without Built-in Cal

Build external square wave:

Simple 555 timer circuit:

555 timer configured as astable
                Frequency: ~1 kHz
                Output: 5V square wave
                
                       +5V
                        │
                     ┌──┴──┐
                     │ 555 │
                     │timer│
                     └──┬──┘
                        │
                     Output → Scope

Or use function generator (if available)

Verification

Cross-Check with Known Voltages

Battery voltage:

Fresh AA battery: 1.5V - 1.65V
                Measure with scope and DMM:
                Should agree within ±3%

USB 5V:

USB port: 4.75V - 5.25V (spec)
                Measure with scope
                Compare to DMM

Known signal:

If you have signal generator:
                Set to 1V RMS sine wave
                Scope should show 2.83V peak-to-peak (1V × 2√2)

Complete Example

Calibrating DSO1013D Voltage

Equipment: - DSO1013D oscilloscope - LM4040-2.5 reference (built) - DMM (for verification)

Procedure:

  1. Built 2.5V reference
  2. LM4040AIZ-2.5
  3. 9V battery, 10kΩ resistor

  4. Breadboard

  5. Verified with DMM: DMM reading: 2.501V ✓ (within ±0.1% spec)

  6. Connected to scope CH1:

  7. 10:1 probe
  8. V/div: 1V (to see ~2.5V clearly)

  9. Coupling: DC

  10. Measured on scope: Scope reading: 2.58V

  11. Calculated error: Reference: 2.501V (DMM-verified) Scope: 2.58V Error: +0.079V Error %: (0.079 / 2.501) × 100 = +3.2%

  12. Tested other V/div settings: ``` V/div Scope Reading Error % 500mV 2.56V +2.4% 1V 2.58V +3.2% 2V 2.54V +1.6% 5V 2.52V +0.8%

Average error: +2.0% ```

  1. Created correction table: ``` CH1: Reads ~2% HIGH Correction: Multiply by 0.98

Example: Scope shows 5.0V → Actual is 4.9V Scope shows 3.3V → Actual is 3.23V ```

  1. Labeled scope: "CH1: +2.0% error CH2: (test separately) Multiply readings by 0.98"

Result: Know voltage accuracy to ±0.5%!

Alternative: Zener Diode References

If can't get LM4040:

Common zener voltages: - 3.3V zener diode (cheap, ~5% accuracy) - 5.1V zener diode (cheap, ~5% accuracy)

Circuit:

9V ─ 1kΩ ─┬─ 5.1V zener ─ GND
                           │
                        Output (≈5.1V ±5%)

Less accurate but usable for rough calibration

Summary

Voltage Calibration Achieved

✓ Built precision references (±0.1%) ✓ Measured scope at multiple voltages ✓ Documented errors per V/div setting ✓ Created correction table ✓ Probe compensation verified

Key Findings

  1. Scope errors typical: ±2-5% uncalibrated
  2. Each V/div setting different: Must test all
  3. CH1 and CH2 differ: Calibrate separately
  4. Probe compensation critical: Do every time!

Accuracy Achieved

Before calibration: Unknown, possibly ±5% After calibration: Known to ±0.5-1%

Next: Complete example

Voltage calibration complete!