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ADS8558: Don't see Busy signal (Daisy Chain Mode)

Jorge Corro
Prodigy 10 points
Part Number: ADS8558

Hello,

I'm currently working in a design that uses two ADS8558 connected in Daisy Chain and configured to be in Software Mode. According to Figure 42 in ADS8558 datasheet, a Busy signal pulse should be seen after a reset signal while nFS is high. Currently I can't see that initial pulse to start the configuration through SDI, am I misinterpreting the Datasheet? I don't see any Busy signal when a Convst_A is generated either. Please, could you review my Schematic and tell me if I did something wrong with the connections? Signals 3COM and ACOM are return signals and are connected together, they have different names only for reference. 

Thank you.

Jorge Corro

  • 0
    TI__Mastermind 45715 points

    Hello Jorge,
    Your schematic looks good, you are using Serial interface, Software and Daisy-Chain mode, and you are only using two data outputs for CHA and CHB pairs without SDO_C which is supported for Daisy-Chain mode on this ADC. When the rising edge of CONVST signal is applied to initiate a conversion, the BUSY signal from low to high (on Conversion) should be observed with default C20=C21=0 in CR register, have you checked both BUSY signals(BS1_PSC and BS2_PSC) when applying CONVST signal? Can you please grab a screenshot of SCLK and SDI along with FS/CS during your initialization cycle? Thanks.
    Other two things:
    1. Your are separated ground(ACOM and 3COM), please make sure that the ground voltage between AGND and BGND should be less than +/-0.3V.
    2. Maybe you have front-end amplifiers or not before the ADC input, because I can't see them in this schematic, generally they are required to drive this switched-capacitor input structure ADC and settle the signal properly, otherwise, it will degrade ADC's performance. This is not related to your issue but just a reminder.
    Thanks.

    Best regards
    Dale Li