Reduce LED Noise and Distortion for Accurate Pulse Oximetry

This article discusses the electronic blocks of a typical pulse oximeter before introducing appropriate LED drive solutions and their application toward the design of a low-noise, low-distortion LED drive circuit.

Pulse oximeter electronics

LED driver circuit

Where D = DAC register, the decimal data word, and N = number of DAC bits.

LED driver noise analysis

• The DAC’s integral and differential nonlinearity
• The voltage reference’s 1/f noise and the amplifier’s 1/f noise
• The amplifier’s common-mode distortion

These noise sources are worth a closer look.

From the Figure 4 and Figure 5 nonlinearity graphs, the maximum analog noise can be one-third of the worst-case nonlinearity, or 0.6 LSB which equals:

Using Equation 3, the total 1/f noise going into the DAC REFF pin is:

The noise for the voltage reference (U1) and the buffer amplifier (U2) is markedly smaller than the DAC’s LSB size.

Noise measurements

• • 16-bit DAC
• • 2.5 volt output reference
• • Zero common-mode crossover distortion
• • Zero common-mode crossover distortion

The actual noise measurement of this circuit is done using a noise measuring box with a gain of 10,000 V/V combined with a 0.1 Hz to 10 Hz filter (Figure 8).

The uncorrelated noise from the two systems combines with the RSS formula to give:

Konklusion