Measurement of DC Voltage on One Analog Input

1. Aim

To measure a DC voltage using one selected analog input of SEELab3/ExpEYES (A1, A2, or A3) with respect to GND.

2. Apparatus / Components Required

3. Theory & Principle

The analog inputs of SEELab3 act as digital voltmeters (12 bit resolution).

Use only one input at a time for now:

Always measure voltage with respect to GND: \(V_{\text{measured}} = V(\text{Input}) - V(\text{GND})\)

4. Circuit Diagram / Setup

  1. Select one channel: A1 (or A2 or A3).
  2. Connect source negative to GND.
  3. Connect source positive to the selected input.
  4. Open the voltage-measurement tool in software.

5. Procedure

  1. Start with a small DC source (for example, a 1.5V cell).
  2. Note the reading on the selected input.
  3. Reverse leads once to observe sign change (+V becomes -V).
  4. For batteries in series:
    • Measure one cell.
    • Then measure two cells in series.
    • Then measure three cells in series (if expected value is within channel limit).
  5. In the mobile app, you can select the voltage range by clicking on the range button at the top right corner of the graph.
  6. Record all readings and compare with expected sums.

6. Observation Table

Selected Input Source Expected Voltage (V) Measured Voltage (V) Remarks
  Single cell (1.5V)      
  Two cells in series      
  Three cells in series      

7. Error Analysis

Possible causes of small mismatch:

8. Results and Discussion

9. Precautions

  1. Voltage limits are critical:
    • A1, A2: keep within about $\pm16V$
    • A3: keep within about $\pm3.3V$
  2. Battery checks:
    • 1 cell (AA): ~1.2 to 1.6V
    • 2 cells in series: ~2.4 to 3.2V (safe on A3, near upper side for fresh cells)
    • 3 cells in series: ~3.6 to 4.8V (do not use A3, use A1/A2)
  3. Use common GND.
  4. Do not leave input floating during observation .

10. Troubleshooting

Symptom Possible Cause Corrective Action
Reading stays near 0V Wrong/loose connection Recheck GND and selected input wiring
Reading clipped at limit Input over-range Shift from A3 to A1/A2, reduce source voltage
Reading lower than expected Loading by input impedance Use A3 for high-resistance sources
Noisy trace Floating input Connect input properly to source or GND
Device not found Connection issue. Reconnect the USB cable and restart the software.

11. Viva-Voce Questions

Q1. Why do we connect source negative to GND?

Ans: Voltage is measured relative to a reference. `GND` is that 0V reference for SEELab.

Q2. Which input is safer for 3 cells in series (~4.5V)?

Ans: `A1` or `A2`. `A3` should be kept within about $\pm3.3V`.

Q3. Why can readings differ between A1 and A3 for high-resistance sources?

Ans: Input impedance differs. A lower impedance meter loads the circuit more and can pull the measured node voltage down.

Q4. If a 3V source is connected to A1 through a 1MΩ series resistor, what will be displayed?

Ans: Model it as a divider: source -- $1M\Omega$ -- input impedance to ground. For A1, $R_{\text{in}} \approx 1M\Omega$: $$ V_{A1}=3\times\frac{1}{1+1}=1.5V $$ So the displayed voltage is approximately 1.5V.

Q5. For the same setup (3V through 1MΩ), what will A3 show if A3 input impedance is 10MΩ?

Ans: Again divider rule: $$ V_{A3}=3\times\frac{10}{1+10}=3\times\frac{10}{11}\approx2.73V $$ So A3 shows approximately 2.73V, closer to the true source voltage because of higher input impedance.