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AD7934 Intermittent BUSY signal

Question asked by Calloutman on Feb 3, 2018
Latest reply on Feb 15, 2018 by Calloutman

Hi there,

 

I'm having some issues reading from the AD7934. I will firstly describe my configuration, and then state the issue.

 

I have the ADC on a custom PCB, talking to an FPGA board over some trailing wires, not great for noise, but it mostly seems okay.

 

My ADC is configured W/B mode set to Word.

Post control register configuration, I set CS and RD low permanently so the output data bits are always driven with the latest data.

I have CLKIN free running. My CONVST falls on the first rising edge to kick off the conversion, the busy signal is detected to rise on the first (next) falling edge.

 

This is where I start to have a series of intermittent and linked issues. The busy signal should fall on the 14th falling edge of CLKIN. It normally does and a new reading is detected, however sometimes one of two things happen. Firstly the busy signal will fall on the 13th rising edge, not the 14th falling edge. Secondly, the busy signal will fall on the 13th rising edge and then rise again on the 14th falling edge. It will stay risen until the CONVST signal goes high again to end the conversion. I'm not sure what would happen if I delay the rising edge of the CONVST signal. When the busy signal is in the incorrect place, no new conversion is observed on the data bits.

 

I am not sure if this is due to some timing issue, if I'm not complying with figure 34 in the datasheet somehow. Could it be a noise or manufacturing issue on the PCB as it is intermittent?

 

Linked are some screenshots from my oscilloscope, The green trace is CONVST, yellow is BUSY, and red is CLKIN

A normal conversion, with busy signal rising on 14th falling edge of CLKIN. A new reading is achieved.

 

The busy signal falls on the 13th rising edge of CLKIN, and then rises again on the next falling edge. No new reading is achieved.

 

The busy signal falls on the 13th rising edge, but does not rise again. No new reading is achieved.

Any ideas?

By the way, I only have access to a terrible analogue scope for the next week, so if you want any new traces, they won't be so pretty.

 

EDIT. I appreciate that in the example in the pictures, the CLKIN signal is <700kHz, which is too low according to the datasheet, but this still happens if I put the frequency up.

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