Question: What is the required specification of each power supply of AFE4400 / AFE4490
- Analog Supply: input range 2.0 – 3.6V
- Digital Supply: input range 2.0 – 3.6V
- LED DRV Supply : input range 3.0 - 5.25V
- TX CTRL Supply : input range 3.0 - 5.25V
How much is the required noise specification on the power supplies?
Answer:
Analog and Digital supplies both have a range of 2-3.6V and can be tied together and driven by a single external supply.
Similarly, the LED_DRV_SUP can be tied together with the TX_CTRL_SUP.
LED driver supply has a range of 3-5.25V. For a given forward voltage of the LED and current setting of the LED driver, the minimum supply voltage on this supply can be determined by referring to Table 7 on Page 66 of the AFE4490 datasheet - http://www.ti.com/lit/ds/symlink/afe4490.pdf. In this table, VHR refers to the headroom (extra voltage) over and above the LED forward voltage that needs to be accounted for while deciding on the LED_DRV_SUP voltage.
For example, if the current setting is 75 mA, and the forward voltage of the LEDs at this current setting is 2.5V, and TX_REF is set to 0.75V, then VHR would be equal to 1.3V. For this case, the minimum LED driver supply voltage would be equal to 3.8V (which is 2.5V+1.3V).
The noise spec is important on both the Analog and LED driver supply. The exact impact of noise would depend on the frequency content of the noise and the signal bandwidth of interest (which for pulse ox is usually up to a few Hz). Assuming that the ripple on the supply would be at high frequency (100s of kHz and beyond), a 1 mVpp ripple would be acceptable.