Reducing lowfrequency (1/f) noise generated by an IC voltage reference can prove difficult. In theory, adding a lowpass filter to a reference's output reduces noise. In practice, a lowpass RC filter for suppression of noise frequencies below 10 Hz requires large values of series resistance and shunt capacitance. Unfortunately, a highvalue series resistor introduces resistance errors and thermal noise, and a shunt capacitor's leakage resistance forms an unpredictable and unstable shunt path. Together, the two components form a noisy and temperaturedependent voltage divider that directly affects the reference's accuracy and longterm stability. In addition, pcboard surface contaminants can add yet another possible leakage path and error source.
You can stack multiple voltage references in series to reduce their 1/f noise. The references' dc outputs add linearly, and their uncorrelated internal noise sources add geometrically. For example, consider a stack of four voltage references, each comprising a dc reference source, V_{REF}, in series with a randomnoise generator, V_{NOISE}. Adding four reference sources produces the following outputs:
and
The original ratio of noise voltage to dc reference voltage thus divides in half.
Figure 1 illustrates a method of adding multiple references to produce a single, less noisy reference voltage. The resistors are parts of a highly stable metalfilm network, and buffer amplifier IC_{5} offers low noise, low inputoffset voltage, and low offsettemperature coefficients.
Table 1.  Noise voltages measured in Figure 1 using four 2.5 V MAX6037 voltage references 


Table 1 and Table 2 present the noise voltages that result from stacking four each of two types of 2.5 V references. Each table shows the 0.1 to 10Hz noise voltage for each of the four references, IC_{1} through IC_{4}, and for the combination. Note that the dispersion in the ratios of rms to peaktopeak values relates to subjectivity in the method of measuring the values. In addition to lower 0.1 to 10Hz noise, the circuit also reduces longterm drift of the reference voltage.
Table 2.  Noise voltages measured in Figure 1 using four 2.5 V MAX6143 voltage references 

