EVAL-CN0370-PMDZ: Very Low Noise, 20mA

The circuit in Figure 1 is a complete single-supply, low noise LED current source driver controlled by a 16-bit digital-to-analog converter (DAC) in a PMOD form factor. The system maintains ±1 LSB integral and differential nonlinearity and has a 0.1 Hz to 10 Hz noise of less than 45 nA p-p for a full-scale output current of 20 mA.

The innovative output driver amplifier eliminates the crossover nonlinearity normally associated with most rail-to-rail input op amps that can be as high as 4 LSBs or 5 LSBs for a 16-bit system. This industry-leading solution is ideal for pulse oximetry applications where 1/f noise superimposed on the LED brightness levels affects the overall accuracy of the measurement.

Total power dissipation for the three active devices is less than 20 mW typical when operating on a single 5 V supply.

In a typical pulse oximetry application, an LED is pulsed from a high level of current (for example, 3/4 scale) to a lower level of current (for example, 1/4 scale). The on-time of these pulses is typically in the order of several hundred microseconds. Peak- to-peak 1/f noise superimposed on the LED brightness levels during the on-time affects the accuracy of the overall measurement and must be minimized.