Build a Single-Supply, Low-Noise LED Current Source Driver Using a Current-Output DAC in Reverse Mode

Circuit functions and advantages

This circuit provides a low noise, single supply current driver for LEDs. Each device was selected to operate from a single 3.0 V supply while maintaining extremely low peak-to-peak noise. The signal chain is optimized for low power, low noise optical communication and medical applications.

In a typical pulse oximeter application, the LED receives a pulsed signal and changes from a high current level (eg 3/4 scale) to a low current level (eg 1/4 scale). The “on” times of these pulses are typically only a few hundred microseconds. The peak-to-peak 1/f noise superimposed on the LED brightness level during the “on” period affects the overall measurement accuracy. The R-2R of the current output DAC itself has a low noise of 0.1 Hz to 10 Hz, the resistive noise of a resistor ladder. The AD5452 current output DAC is used in “inverting” mode to support single-supply applications. When 1.25 V is applied to the IOUT pin, the voltage on the VREF pin for full-scale code is 1.25 V − 1 LSB, and the voltage on the VREF pin for zero-scale code is 0 V.

The key to keeping noise low in this signal chain is the ADR127 reference, which has a 0.1 Hz to 10 Hz noise of only 9 μV peak-to-peak. In addition, the AD8655 is the industry’s lowest noise precision CMOS amplifier (1.23 μV peak-to-peak). Therefore, the typical 0.1 Hz to 10 Hz noise of the entire circuit is only 14.7 μV peak-to-peak.

circuit description

The signal chain in Figure 1 shows the current output DAC AD5452 operating in reverse (voltage switching) mode to control the brightness of the LEDs. See the AD5452 data sheet for a description of reverse mode. In reverse mode, the device accepts a 1.25 V low noise positive reference input and provides a 1.25 V – 1 LSB positive full-scale output. It is important to note that with this configuration, the switches in the DAC resistor ladder do not have the same source-to-drain drive voltage, so this current output DAC can only accept low input voltages (

Figure 1. Low Noise LED Driver (Simplified Schematic: All Connections Not Shown)

The voltage generated by the 12-bit AD5452 DAC drives the noninverting input of the op amp. This voltage also appears across resistor R1 (which should be a high precision resistor) and produces the current required for the collector of the bipolar transistor. For the circuit shown in Figure 1, the diode current is nominally 56.6 mA at full scale. A low noise AD8655 is used to sense the current through R1.

AD5452 adopts R-2R structure, DAC core noise is very low. The dominant noise source in the signal chain is the ADR127 reference, which has a nominal 1/f noise of 9 μV peak-to-peak typically. Figure 2 shows the 0.1 Hz to 10 Hz voltage noise on the negative side of the AD8655, excluding the noise added by the LED (Hamamatsu L5766 in this case). This circuit uses a standard 2N3904 NPN bipolar transistor to drive an LED.

Figure 2. Measured 0.1 Hz to 10 Hz noise at point A

common changes

Other suitable DACs are the 16-bit single channel AD5543 or the 14-bit AD5446. Dual DACs are also suitable, such as the AD5447 (12-bit) and AD5545 (16-bit). The AD8656 is a dual-channel version of the AD8655.

Other low noise precision op amps are also suitable, such as the ADA4841-1 and ADA4841-2.

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