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# AFE4404: AFE4404 PPG Ambient Data

Part Number: AFE4404

Hi.

I'm measuring PPG on my wrist with AFE4404.

I look at the datasheet, 2A to 2C is LED1VAL, LED3VAL, LED2VAL.

But if you look at 2D to 2F, you get the value of ambient.

What is ambient?

I can get the value by lighting the LED from 2A to 2C, but where do you get the value from for 2D to 2F?

And what's the difference between 2D and 2F? If you look at the description of the two, it says ambient1 output, LED1-ambient1 output, so can you tell me the difference?

Thank you.

• Hello Kim,

When the AFE is reading in values from the photodiode, any extra light from the environment will also be read, leading to false data. Therefore, the LED1 phase really receives the reflected signal of LED1 + the ambient environmental light leaking in.

However since ambient light is the environmental light received by the photodiode that is not from the output of the LED driven by the AFE, ambient light is unwanted data. Therefore, to avoid skewed data, we take the received light when the LED is on and subtract the ambient light data.

The register 2F is doing the calculation for you ahead of time; it's the LED1 phase minus the ambient light, leaving just the signal received from LED1.

Kind regards,
Nick Z.

1. So registers 2E and 2F are LED2 - ambient2 and LED1 - ambient1?
2. Then, does 2D have a pure ambient1 value?
3. Then, can't I get the ambient value for l LED2 and LED3?

Thank you.

• Hello Kim,

1. Yes, register is 0x2E is LED2-Ambient2 and 0x2F contains LED1-Ambient1.

2. Yes, register 0x2D is the pure ambient 1 value, described in Table 60:

3. Ambient 2 / LED 3 share the same register depending on your application and implementation of the AFE4404.

Kind regards,
Nick Z