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Newbie tbriese
Newbie
857 Views
Registered: ‎01-16-2018

ZC706 XADC odd behavior

We are using the ZC706 board and seeing some odd behavior.  Here is what happens:

 

We are connecting the Zynq PL to the XADC input pairs 0 and 8 on J63.  We are also using SW7 as an input to the FPGA PL.  

 

What we are seeing is that if we connect our external drivers to the N channel of the XADC, it's as if the FPGA doesn't see SW7.  With the N channels XADC inputs floating, SW7 feeds into our logic and works...the custom modules that consume these two inputs share a few position values that are generated based on the XADC value, but are otherwise decoupled.  We are simply synchronizing the SW7 signal using a few flip flops.

 

This behavior is extremely odd to us.  These seem that they should have no effects on each other, but they seem to...We just started debugging this issue, but I wanted to post here for any ideas as to why this might be happening.  Thanks!

 

Thad

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5 Replies
Scholar austin
Scholar
782 Views
Registered: ‎02-27-2008

Re: ZC706 XADC odd behavior

Check your grounding,

 

Check the signal is no smaller than -0.5v, and no larger than 1.0v.

 

Sounds like there is a voltage out of the allowed values due to a bad or missing ground.  Even a differential input requires a solid common ground reference.

Austin Lesea
Principal Engineer
Xilinx San Jose
Visitor tcbriese
Visitor
750 Views
Registered: ‎10-24-2017

Re: ZC706 XADC odd behavior

Thanks Austin.  I haven't had a lot of time to debug this.  I will take some measurements on the XADC signals today/tomorrow and let you know what I find.

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Scholar austin
Scholar
740 Views
Registered: ‎02-27-2008

Re: ZC706 XADC odd behavior

Check you have a solid electrical common (ground) to the XADC ground for the Vp/Vn pins from the source,

 

That is more of a visual check that a measurement.  Maybe also use an ohm meter....

 

 

Austin Lesea
Principal Engineer
Xilinx San Jose
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Newbie aadamian
Newbie
726 Views
Registered: ‎01-24-2018

Re: ZC706 XADC odd behavior

Hi, I am working with Thad on this issue. Thanks for your help..

 

We are connecting the source inputs directly to VAUX0 and VAUX8 of the ZC706 (J63 pins3/6 for VAUX0 and pins 7/8 for VAUX8). We have tried also connecting the grounds of the source (the "N" channel of the XADC input) to the signal ground of the ZC706 to ensure they are at the same potential, and that made no difference. FYI we are using the inputs in unipolar mode.

 

I did however play with the input voltage on the VAUX8 input and noticed something interesting. When the input voltage is less than ~910mV the system works fine. Once I increase the input voltage to above that voltage, the logic behind that button press (SW7) doesn't work. Note: I am measuring directly on the VAUX8P/N input. I still get valid data from the XADC when the input voltage goes above 910mV (up to about 1V), but whatever is going on inside the Zynq is causing that button not to work.

 

Thanks again for your help!

Ara

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Scholar austin
Scholar
724 Views
Registered: ‎02-27-2008

Re: ZC706 XADC odd behavior

What are the Vcc_int for PL and PS?

 

If the vcc_int is smaller than an input, you might be getting current flow through protection diodes (check current flowing into the XADC -- it should be very very small ( the sample and hold capacitor is a high impedance).

 

The Vcc_int should be 1.0v, so XADC voltages below 1.0v should be fine.

Austin Lesea
Principal Engineer
Xilinx San Jose
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