Other Parts Discussed in Thread: STRIKE
Hi all,
I am using the AFE4404 for SpO2 calculations, sampling red and IR LEDs connected to LED1 and LED2 inputs respectively as well as ambient light. I am using an nRF52832 SoC to control the AFE4404.
To save power, LED3 is disabled. Following Praveen's answer here: e2e.ti.com/.../546473 post, I have set the LED3 current to 0 mA as well as giving the conversion phase for LED3 a single clock cycle. I'm developing on custom devboards.
We observe a lot of high-frequency noise in the red and IR data. The noise is present for a while before it vanishes, after which the signal is fairly clean. The time it takes for the noise to disappear varies between devboards -- for some the noise is always present. See below for the register values. All other registers contain their reset values.
reg. name reg. adr value description LED2LEDSTC 0x9 0 LED2 on LED2LEDENDC 0xa 399 LED2 off LED2STC 0x1 100 begin sample LED2 LED2ENDC 0x2 399 end sample LED2 ADCRSTSTCT0 0x15 401 begin ADC reset cycle 0 ADCRSTENDCT0 0x16 407 end ADC reset cycle 0 LED2CONVST 0xd 409 begin convert LED2 LED2CONVEND 0xe 868 end convert LED2 LED3LEDSTC 0x36 401 LED3 on LED3LEDENDC 0x37 401 LED3 off LED3STC 0x5 401 begin sample LED3 LED3ENDC 0x6 401 end sample LED3 ADCRSTSTCT1 0x17 870 begin ADC reset cycle 1 ADCRSTENDCT1 0x18 876 end ADC reset cycle 1 LED3CONVST 0xf 878 begin convert LED3 LED3CONVEND 0x10 879 end convert LED3 LED1LEDSTC 0x3 401 LED1 on LED1LEDENDC 0x4 800 LED1 off LED1STC 0x7 501 begin sample LED1 LED1ENDC 0x8 800 end sample LED1 ADCRSTSTCT2 0x19 881 begin ADC reset cycle 2 ADCRSTENDCT2 0x1a 887 end ADC reset cycle 2 LED1CONVST 0x11 889 begin convert LED1 LED1CONVEND 0x12 1348 end convert LED1 ALED1STC 0xb 902 begin sample ALED1 ALED1ENDC 0xc 1201 end sample ALED1 ADCRSTSTCT3 0x1b 1350 begin ADC reset cycle 3 ADCRSTENDCT3 0x1c 1356 end ADC reset cycle 3 ALED1CONVST 0x13 1358 begin convert ALED1 ALED1CONVEND 0x14 1817 end convert ALED1 CLKDIV_PRF 0x39 0 clk division setting PRPCT 0x1d 39999 compare value for prf PDNCYCLESTC 0x32 2617 begin power down cycle PDNCYCLEENDC 0x33 39199 end power down cycle TIA_FEEDBACK 0x21 3 TIA gain and bandwidth ILED 0x22 325 LED current control CONTROL1 0x0 256 NUMAV, TIMEREN CONTROL2 0x23 512 OSC_ENABLE, DYNAMICn
Below is an image showing the noise and how it vanishes about halfway. In this particular case, the data was noisy for a good 2-3 minutes before clearing up.
If I take the sample register settings from Table 11 in the datasheet and only change LED3 current to 0 mA, there is considerably less noise and it clears itself up much faster for all devices. This fact leads me to believe that this might be an issue regarding the register settings shown above, though I realize that it might very well be a hardware issue as well. Nonetheless, I would like to get a second opinion on the register settings I have chosen.
Best,
Fredrik
