What is the forward voltage at a specific current for the Virtex DXP/DXN pins? I want to develop my own temperature sensor. What is the current equation for these pins?
Virtex-II and Virtex-II Pro
For Virtex-II, the DXN/DXP diode characteristic is similar to a 1N914 or a 1N914 emitter base diode.
There is no simulation data available.
For the 1N914 diode characteristic, see:
On Page 3, you can use the data from the graph labeled "Forward Voltage vs. Ambient Temperature" (top right) to calibrate the diode. As the diode is connected to the die of the FPGA, the ambient temperature of the diode = junction temperature of the FPGA.
Note that the diode characteristic will vary with process variation, so the IV curve should not be taken as an absolute voltage vs. temperature reading. The measurement should involve either the temperature sensing chip or calibration using two currents as described below.
Virtex and Virtex-E
From Spice Simulations
1. At I = 10 uA, T = 150: V = 0.39 (spec is > 0.25V, Maxim)
2. At I = 100 uA, T = -55: V = 0.82 (spec is < 0.95V, Maxim)
These values vary from process variation, but do not exceed the Maxim specifications. Refer to the Maxim 1617 Data Sheet, page 2 at:
The (log I)/V curves roll off at ~500 uA. This provides plenty of margin room around the 100 uA maximum current that a Maxim, National, or Analog chip will send through the diode.
See (Xilinx Answer 5738) for more information on additional considerations and a list of temperature sensing diode.