How to Reduce Reference Noise by Half - AN3531

A low-noise, 2.5V reference is constructed by stacking four 2.5V reference ICs in series. The resulting 10V output is divided back to 2.5V, and a low-noise unity-gain buffer added. This stacked-reference design reduces noise because the noise from each IC, being random and uncorrelated, partially cancels each other out. The low-frequency 1/f noise (LF noise) generated by a voltage reference at frequencies below 10Hz is very hard to reduce. One design approach reduces LF noise by implementing a lowpass RC filter, but that requires highvalued capacitors and resistors. High-value capacitors have lower value leakage resistance, which causes voltage error and long-term instability due to an uncontrolled voltage divider with the high-value series resistors. Highvalue series resistors cause error by undesired voltage drops due to leakage currents and by generating their own thermal noise. Stacking multiple references will reduce LF noise. By using multiple voltage sources, each composed of a DC source and a noise source, the DC voltages increase linearly and the noise sources (if not correlated) sum geometrically in an RMS fashion. Adding four references, for example, each consisting of a VREF generator and a VNOISE generator, produces the following outputs: The original ratio of noise voltage to DC reference voltage is divided in half. Figure 1 shows how to stack multiple references to generate a single, less noisy reference voltage. The resistors are parts of a high-stability metal-film network, and the op amp is selected for low noise, very low input-offset voltage, and low offset tempco.