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AFE4490 seems to go to saturation @ ~35mV?

user138497234
Intellectual 265 points
Other Parts Discussed in Thread: AFE4490

Hello there,

i have build a custom PCB for PPG measurement with the TI AFE4490. It has been designed with respect to all recommendations of the datasheet. 

Tx supply is 5V and Rx supply is 3V3. Osrams SFH7050 is used for red/infrared light source and reflection measurement. 

But i have two problems:

1. I am not able to see any pulse on the ADC_RDY pin. It seems to be the same problem as in this thread:  

I have double checked that all possible reasons for the signal to be not generated are not true in my case. Besides, the 3State Option is not used. Because i want to check if the other components are working as expected, i have used the option to brought the internal period of LED2 convert to the PD_ALM pin (CLKALMPIN in CONTROL1 register, p. 72). On every rising edge on PD_ALM, i then read out the values of the conversion.

2. With the PD_ALM-workaround for synchronization, i have another problem with the measured signal. When placing the thumb on the SFH7050, the signal seems to go to a kind of saturation very early. The LED1 channel voltage has a maximum of ~33mV, LED2 channel of ~22mV. Changing Rf and Rg has no effect other then reaching the 'saturation level' faster or slower. In the short GIF i prepared you can see the placing and releasing of my thumb and the resulting limitation of the measured signal:

I am assuming that the TIA and/or stage 2 amplifier are malfunctioning. The reason for this assumption is the fact that i can measure voltages of 80mV in ADC_Bypass mode, when the amplifiers are decoupled. I think that this 80mV is a kind of floating drift voltage indicating no limiting malfunction of the ADC itself. 

Regards

  • 0
    Intellectual 265 points
    I have noticed that you have to click on the image to see the animated GIF.
  • 0 in reply to user138497234
    TI__Mastermind 18285 points

    User,

    We have received your post and hope to have a response to you soon.

  • 0 in reply to user138497234
    TI__Mastermind 49956 points

    Hello,

    Could you post your schematics and the AFE register configuration?

    Did you check the power supply currents? Are they within the specs in the datasheet?

    Did you run the diagnostic mode? Did any of the fault flags get set?

  • 0 in reply to Praveen Aroul
    Intellectual 265 points

    Hello Praveen, 

    this is the relevant part of my schematics:

    My register configuration is:

    for Timing:

    #define AFE4490_REG_LED2STC_VAL                             15050
#define AFE4490_REG_LED2ENDC_VAL                            19998
#define AFE4490_REG_LED2LEDSTC_VAL                          15000
#define AFE4490_REG_LED2LEDENDC_VAL                         19999
#define AFE4490_REG_ALED2STC_VAL                            0050
#define AFE4490_REG_ALED2ENDC_VAL                           4998
#define AFE4490_REG_LED1STC_VAL                             5050
#define AFE4490_REG_LED1ENDC_VAL                            9998
#define AFE4490_REG_LED1LEDSTC_VAL                          5000
#define AFE4490_REG_LED1LEDENDC_VAL                         9999
#define AFE4490_REG_ALED1STC_VAL                            10050
#define AFE4490_REG_ALED1ENDC_VAL                           14998
#define AFE4490_REG_LED2CONVST_VAL                          0006
#define AFE4490_REG_LED2CONVENDC_VAL                        4999
#define AFE4490_REG_ALED2CONVST_VAL                         5006
#define AFE4490_REG_ALED2CONVENDC_VAL                       9999
#define AFE4490_REG_LED1CONVST_VAL                          10006
#define AFE4490_REG_LED1CONVENDC_VAL                        14999
#define AFE4490_REG_ALED1CONVST_VAL                         15006
#define AFE4490_REG_ALED1CONVENDC_VAL                       19999
#define AFE4490_REG_ADCRSTSTCT0_VAL                         0000
#define AFE4490_REG_ADCRSTENDCT0_VAL                        0006
#define AFE4490_REG_ADCRSTSTCT1_VAL                         5000
#define AFE4490_REG_ADCRSTENDCT1_VAL                        5006
#define AFE4490_REG_ADCRSTSTCT2_VAL                         10000
#define AFE4490_REG_ADCRSTENDCT2_VAL                        10006
#define AFE4490_REG_ADCRSTSTCT3_VAL                         15000
#define AFE4490_REG_ADCRSTENDCT3_VAL                        15006
#define AFE4490_REG_PRPCOUNT_VAL                            20000

    for general configuration:

    CONTROL1: 0x000608

    TIAGAIN: 0x008003

    TIAAMBGAIN: 0x000003

    LEDCNTRL: 0x011010

    CONTROL2: 0x000000

    ALARM: 0x000080

    I have not measured the supply current yet, but will do it tomorrow. 

    Reading of the fault flags gives me 0, so no faults at all. 

    Thanks!

  • 0 in reply to Praveen Aroul
    Intellectual 265 points

    I have checked the power supply current for the Rx part, i.e. the 3V3 domain.

    With loaded configuration, Stage 2 Gain disabled, and Timer disabled: 1037.0µA

    With loaded configuration, Stage 2 Gain enabled, and Timer disabled: 1119.2µA

    With loaded configuration, Stage 2 Gain disabled, and Timer enabled: 1054.4µA

    With loaded configuration, Stage 2 Gain enabled, and Timer enabled: 1136.7µA

    Does not look too bad from my point of view.

    I think, that my amplifier stage is malfunctioning in some way. To verify this assumption, i wanted to use the ADC bypass option. Therefore the ADC Reset Signal would be useful to bring it out to the PD_ALM pin. The datasheet tells me on page 49:

    Also, use the CLKALMPIN[2:0] bits on the PD_ALM pin register bit to bring out the ADC reset signal to the PD_ALM pin. ADC_RDY can be used to indicate the start of the pulse repetition period to the external ADC.


    But when looking to the Control1 register map page (p. 72f.) i could not see anything about ADC reset signals, only sample and conversion signals.

    I also tried to set the RST_CLK_ON_PD_ALM bit in Control2 register (even not mentioned in ADC bypass section, why?), but with this bit set, it seems to stop the whole measurement. After disabling this bit, the measurement continues. 

    So what is wrong with my settings? How could i bring out the ADC reset signal to PD_ALM?!