Design of a 5-Bit Current Steering DAC for Driving High Forward Voltage LEDs

Over past decades, light-emitting diodes (LEDs) have been identified as an ordinary part of many industrial and biomedical applications and many attempts done to enlarge their versatility. This paper proposes a 5-bit current steering DAC with the capability of driving two LEDs in a commercial OSRAM photoplethysmography (PPG) sensor which have different forward voltages. The DAC operates based on the thermometer-code conversion and is designed for 65 nm TSMC technology. Combined with a LED driver circuit it is able to convert a 5-bit digital input to an LED current signal. Results indicate that the implemented DAC can reach up to 50 M samples per second (MS/s) and changing its input by 1 LSB leads to 940 µA variation in the LED current. It is shown that the DAC system can independently drive two LEDs with the forward voltages of 1.8 V and 2.8 V at different time instants. According to the binary input signal of the DAC, the amplitude of the driving current signal, which identifies the brightness of LEDs, can be varied between 3.29 mA and 32.45 mA at a maximum frequency of 50 KS/s.