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ADS1259: Data invalid immediately after DRDY_N asserts. Must insert delay to get valid data.

Part Number: ADS1259

Hello,

I am using Pulse mode with continuous read. Everything is working well, I just can't reconcile the need for this delay with the datasheet. I am using a state machine in an FPGA, so I have good control over the timing. I found that I needed to introduce a delay between receivng the DRDY_N signal and reading the data. My understanding is that the DOUT should take the state of the DRDY_N upon assertion of the CS_N.

In this trace I have about 800 ns delay between DRDY_N and asserting CS_N. I need about twice that for it to read properly. The input is shorted, so the high order bits should be zero. You can see that the most significant bit is a one.

If I wait about 1.6 us, the data is correct.

I am meeting all my speed and accuracy requirements, so this isn't a big problem. I just don't understand it.

I'm using an external 8MHz clock.

START is asserted for 5 clock cycles. (625 ns)

SPICLK is 4 MHz.

Any ideas?

Thanks,

Chris

  • Hi Chris Harthan,

    Do you have any digital isolation in your system that could potentially be causing propagation delays?

    Also, where are you measuring these signals with the scope? At the ADC, or at the FPGA (or both perhaps)?

    -Bryan