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Why "1.5V IO standard" output pin with 50ohm load not 1.5V when Termination is set to OFF

jkhoo
Employee
525 Views

Hi all,

I set a FPGA pin as follow:

IO pin = Output Pins

I/O Standard = 1.5V

 

  1. When i scope physical pin with FPGA pin Termination = (default) series 50 ohm without calibration
    1. with my scope's port coupling set to DC1M : scope measured 1.6V
    2. with my scope's port coupling set to DC50 : scope measured 0.75V
  2. When i scope physical pin with FPGA pin Termination = OFF
    1. with my scope's port coupling set to DC1M : scope captured 2.0V (Overshoot included)
    2. with my scope's port coupling set to DC50 : scope measured 1.0

I understand the behaviour of bullet 1a, 1b and i can accept 2a. 

What i don't understand is bullet 2b

Since DC50 is just 50ohm load and the series 50ohm is turn OFF on the input pin, i expect the full 1.5V voltage drop on the 50ohm load.

But why am i not measuring 1.5V but instead, i am measuring only 1.0V?

 

Thanks, JJ

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AqidAyman_Intel
Employee
476 Views

Hello JJ,

I would like to get some clarification from you first.

What is the FPGA device that you use?

What is the difference between DC1M and DC50?


Regards,

Aqid


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FvM
Valued Contributor III
456 Views

Hi,
50 ohm load termination (oscilloscope setting "DC50") creates essentially a voltage divider between source and load impedance. In case of 50 ohm source termination, you get about 50% output level.

67 % output level (1.0/1.5) indicates 25 ohm output impedance, sounds plausible for higher drive strength setting. Expected 1.5 V would be achieved with zero output impedance which isn't feasible.

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jkhoo
Employee
439 Views

Hi FvM,

Yes, your explanation make sense. The current drive strength i am using is 12mA, which is at the high end of the setting.

 

Hi Aqid,

I am using Stratix V : 5SGXMA3K3F40C4. 

DC1M = 1Mohm load and DC50 = 50 ohm.

 

Where in stratix V spec that state down that with output pin termination set to off, the impedance of the output port is depend on the current strength setting and is there a table in the spec listing down what is the impedance/termination for the output port relative to the current strength (2mA to 12mA) for 1.5V IO standard and other standard?

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jkhoo
Employee
404 Views

Hi Aqid,

Appreciated if you can help me out on the question i have above.

Regards, JJ

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Farabi
Employee
289 Views

lets have discussion.


regards,

Farabi


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Farabi
Employee
96 Views

Hi JJ,


Can we discuss this internal and close this case?


regards,

Farabi


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Farabi
Employee
96 Views

We have been discussing this in email for quiet some time may be can continue to support you with email. Reason is we want to focus on other customers with the right business justification on this platform.


regards,

Farabi


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