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

[jkhoo]

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

[AqidAyman_Intel]

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

[FvM]

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.

[jkhoo]

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?

[jkhoo]

Hi Aqid,

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

Regards, JJ

[Farabi]

lets have discussion.

regards,

Farabi