Precision Voltage References

#### What is a Precision Voltage Reference?

• Very high initial accuracy and low noise
• Flexibility: many voltage options and small packaging
• Good line and load regulation performances
• Good power supply rejection ratio
• Line regulation is a measure of how well a circuit maintains its output voltage with respect to changing input voltage. It is usually expressed as ppm/V. It's defined as:
• (ΔVout/VoutTyp)/ΔVin
• Load regulation is a measure of how well a circuit maintains its output voltage with respect to changing load conditions. It's defined as:
• (ΔVout/VoutTyp)/Δ(Iout)

Applications that require precision voltage reference:

The figure below shows a precision voltage reference used in an ADC application.

#### Why Should I Use a Precision Voltage Reference?

Accuracy
An Analog–to–Digital Converter's (ADC's) performance is affected by the voltage reference as the ADC Code is inversely proportional to the voltage reference. Certain Analog-to-Digital converters may also have other reference requirements including transient response, DC current requirements, startup timing, or capacitive loading so it is important to test a voltage reference with the specific ADC to be used in an application.

Using a voltage reference with tight initial error can help eliminate room-temperature systems calibration. Voltage reference performance affects an ADC’s overall accuracy.

Output Noise
Output noise should be smaller than the least significant bit (LSB) voltage (the minimum change in voltage that causes a change in the output code) to maintain converter signal-to-noise ratio. In many cases, it is possible to reduce the effective reference noise by including an RC filter between the reference and ADC and/or adding a buffer between the two chips.

The LSB for a 16-bit ADC with VREF = 4.096 V is 62.5 µV. The LSB for a 12-bit ADC with VREF = 4.096 V.

#### MCP1501 High-Precision Buffered Voltage Reference Key Features:

• Initial accuracy: 0.08%
• Temperature coefficient:
• 10 ppm/°C typical and 50 ppm/°C max
• Line regulation: 50 ppm/V
• Load regulation: 40-70 ppm/mA
• Output noise: < 110 µVrms
• 8 voltage options: 1.024 V, 1.25 V, 1.8 V, 2.048 V, 2.5 V, 3 V, 3.3 V, 4.096 V
• Operating current: 140 µA
• Extended temperature range: -40 °C to +125 °C
• Packages: very small 8L 2x2 WDFN, 6L SOT-23, 8L SOIC

The associated figure shows a basic MCP1501 circuit configuration. The input voltage is connected to the device at the VIN input, with an optional 2.2 μF ceramic capacitor. This capacitor would be required if the input voltage has excess noise. A 2.2 μF capacitor would reject input voltage noise at approximately 1 to 2 MHz. Noise below this frequency will be amply rejected by the input voltage rejection of the voltage reference. Noise at frequencies above 2 MHz will be beyond the bandwidth of the voltage reference and consequently not transmitted from the input pin through the device to the output.

### PIN FUNCTION TABLE

 Function Symbol Buffered VREF output OUT Buffered VREF feedback FEEDBACK System ground GND Shutdown Pin Active low SHDN Bar Power Supply input VDD Exposed thermal pad EP

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