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AFE4490SPO2EVM: ADC_RDY not toggling when in Bypass mode

Part Number: AFE4490SPO2EVM
Other Parts Discussed in Thread: AFE4490,

I am evaluating the AFE4490 for a new product application. I am using the evaluation kit AFE4490EVM. Section 10.4 of the User Manual (10.4 Using an External ADC (Bypass ADC Mode) (Available Only for AFE4490 Device) describes how to use the device in Bypass mode. It states
"In this mode, the internal ADC of AFE4490 is disabled, one of the internal ADC_RESET clocks is brought out on the PD_ALM pin (PD_ALM is monitored at series jumper resistor R37). This signal is used to convert each of the four phases (with every pulse repetition period). Additionally the ADC_RDY signal (ADC_RDY is monitored at series jumper resistor R26) is used to synchronize the external ADC with the AFE".

I do see the output voltage from the AFE at the RX_OUTP and RX_OUTN pins. I also see the PD_ALM toggling with each cycle. However, I'm not seeing the timing signal at ADC_RDY.

Within the GUI I have set the En Bypass ADC option and the CLKs on ALM pin option. I am monitoring at R37 (PD_ALM) and R26 (ADC_RDY). I see a signal at R37 (PD_ALM) but NOT at R26 (ADC_RDY). I have purchased another eval board (AFE4490SpO2EVM) and see the same issue again. How can I enable an output at R26 (ADC_RDY)?

  • Hi Andrew,

    We would like to ask for couple of days to test this and try to find the root cause.

    We will get back to you by early next week with the test result and workaround (if any).

    Regards,

    Prabin.

  • Thanks, I look forward to your reply.

  • My correction to, or additional information for this issue:

    I see a 25nsec pulse on the ADC_RDY pin at the pulse repetition frequency (500 hz).

    I see a 500usec pulse on the PD_ALM pin at the pulse repetition frequency (500 hz)

    Section 10.4 of the User Manual (10.4 Using an External ADC)  states “This signal is used to convert each of the four phases (with every pulse repetition period)”. However, the signals don’t distinguish between the LED 1 ON, LED 2 ON, Ambient 1, and Ambient 2 periods. 

    So my question should have been how do I use the ADC_RDY and the PD_ALM signals to mark the times of the LED 1 ON, LED 2 ON, Ambient 1, and Ambient 2 periods?

  • Hi Andrew,

    My apologies for the delayed response.

    Form your last response it looks like the pulse on PD_ALM ( ADC_RESET signal) is 500uS wide and repeats at 500Hz.

    However from the Figure 76 of the datasheet, the pulse on PD_ALM pin will be small ( as set by ADCRST registers i.e. 0x15 to 0x1C) and the frequency of this pulse will be 4 times then the pulse repetition frequency.

    So I would suggest you to check your register settings so that the pulses on ADC_RDY and PD_ALM comes as described in Figure 76 of datasheet. You can also try the default register settings of AFE4490EVM.

    Also as mentioned in the datasheet , this particular mode is only for debug purposes and is not recommended for production.

    Regards,

    Prabin.

  • I have attached the screenshots of the registers.  I'm still looking for a solution.

    Thanks,

    Andy

  • Hi Andrew,

    To get ADC_RESET signal on PD_ALM pin, you need to set "RST_CLK_ON_PD_ALM" bit. This bit is D16 of 0x23 register.

    Once you set this bit , you should get  pulses on PD_ALM pin with frequency" 4 times of PRF" (i.e. 4*500 = 2KHz). 

    Similarly the signal on RDC_RDY pin will be same as PRF , so within a cycle of ADC_RDY signal there will be 4 pulses of PD_ALM pin.

    Also, the 4 pulses of PD_ALM represents the start of 4 convert instances in a particular order as LED2_CNV, AMB2_CNV, LED1_CNV and AMB1_CNV.

    Now if you use pulses on PD_ALM pin to start conversion in an external ADC, then the 4 data  will be in order LED2, AMB2, LED1 and AMB1.

    Regards,

    Prabin.

  • Thank you for your response.  I will be out of the office until March 4th (vacation and business travel).  However, I have asked a colleague to test out your suggested solution.  It may be a few weeks before either he executes the test, or I return to the office to complete the testing.