Voltage References: Series vs. Shunt Comparison — A Practical Tutorial

In precision analog and mixed-signal designs, the voltage reference (VREF) is the heartbeat of accuracy. Whether you’re designing a high-resolution ADC circuit or a bias circuit for a precision sensor, the performance of your reference IC directly impacts your entire system. When selecting a voltage reference for your application it is important to understand the two different voltage reference architectures:

  • Series Voltage References
  • Shunt Voltage References

Though they serve the same purpose — providing a stable reference voltage — they differ significantly in operation, application, and design considerations.

🔌 What Is a Voltage Reference?

A voltage reference is a device that outputs a constant, precise voltage over variations in:

  • Supply voltage
  • Temperature
  • Time (long-term drift)
  • Load current

🧬 The Two Main Types

1. Series Voltage Reference

🔧 How It Works:

A series reference behaves like a low-dropout regulator. It sits between the power supply and the load and actively regulates the output voltage by controlling the current flow.

📘 Equivalent Circuit Diagram:

Figure 1. Block diagram of the 3-terminal series voltage reference.

The internal circuitry uses a feedback loop to compare the output to a precision internal reference and adjusts current flow through a pass element (usually a BJT or MOSFET that acts as an adjustable resistor) to maintain an accurate output voltage. May include an internal precision buffer amp on the output to improve transient response.

✅ Pros:

  • Excellent line and load regulation
  • No external current tuning resistor required
  • Can deliver better accuracy
  • Can have less drift over temperature changes
  • Efficient at higher output currents
  • Can supply relatively large currents (10s of mA)

❌ Cons:

  • Typically does not have a large input voltage range and Vin > Vout + Vdropout
  • Often more expense
  • More pins (usually at least 3: Vin, GND, Vout)

📌 Use Cases:

  • ADC and DAC references
  • Biasing precision analog signal chains
  • Low-noise instrumentation

Note: all Anabit products use series based voltage references

2. Shunt Voltage Reference

🔧 How It Works:

A shunt reference behaves like a precision Zener diode. It is placed in parallel with the load, and it regulates voltage by sinking current to maintain a constant voltage across itself.

📘 Equivalent Circuit Diagram:

Figure 2. Block diagram of the 2-terminal shunt voltage reference.

You must use an external resistor (R1) to limit the current into the shunt regulator and ensure it stays within its operating range.

✅ Pros:

  • Can operate with wide input voltages, but Vin > Vout
  • Only two pins (VREF, GND) — simple to implement
  • Ideal for negative or floating references
  • Typically lower cost

❌ Cons:

  • Needs an external resistor
  • Inefficient at high load currents
  • Line regulation depends on resistor sizing
  • Load current and resistor current must be managed carefully
  • Lower accuracy specifications compared to series 

📌 Use Cases:

  • Low-current biasing
  • Voltage clamping
  • Negative voltage generation
  • Low-power sensor interfaces

🧐 When to Use Each Type?

Feature Series Reference Shunt Reference
Supply Voltage Range Limited (needs headroom) Wide (can work with high Vin)
Output Current Higher (good for ADCs, DACs) Lower (good for sensing, bias)
Pin Count Usually 3 (Vin, GND, Vout) Usually 2 (Vref, GND)
External Components No resistor required Requires current-limiting R
Power Efficiency More efficient Less efficient at high current
Simplicity More pins Requires current tuning R
Common Use Cases Precision analog systems Voltage clamping, low-current biasing

🔏 Cost, Accuracy, and Drift Comparison

Feature Series Reference Shunt Reference
Cost Higher part cost but no external resistor needed Lower part cost but needs external resistor
Accuracy Very high (up to, ±0.01%), factory trimmed Moderate to low (±0.5% to ±2%), may need trimming
Drift Low tempco (e.g., 3–15 ppm/°C), excellent long-term stability Higher tempco (30–100+ ppm/°C), more drift over time

📌 Design Summary:

Choose a series reference when:

  • Accuracy and stability are critical
  • Load current is variable or significant
  • Power supply is regulated and headroom is available

Choose a shunt reference when:

  • Cost or simplicity matters more than precision
  • The reference is used for supervisory, bias, or clamping
  • Input voltage is wide or poorly regulated

🎓 Real-World Examples:

  • Series Ref Example: ADR4525 (2.5V) — Low noise, high accuracy, low drift, great for precision ADCs.
  • Shunt Ref Example: TL431 — Classic adjustable shunt reference used for biasing, SMPS control, and regulation.

✅ Conclusion

Understanding the difference between series and shunt voltage references helps you make the right design tradeoffs.
Use a series reference when you need clean, precise, high-current output.
Use a shunt reference when you want simplicity, wide voltage compatibility, or low current draw.

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