Ref.: | |||
SpaceWire Router |
| UserManual | |
Issue: | 3.4 | ||
User Manual | |||
Date: | 11th July 2008 | ||
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The bias resistor values are determined as follows:
1. Determine the amount of noise protection required.
E.g. if the maximum noise voltage expected is less than 10 mV then the bias current required is Ib =10 mV/100 Ω = 0.1 mA. Note: the bias current should be at least an order of magnitude lower than the 3 mA current loop used for normal LVDS operation.
2. Determine the total resistance, RB, from bias supply VDD to ground, RB = VDD / Ib.
E.g. RB = 3.3 V/0.1 mA = 33 kΩ. Since RT is much smaller than this value it can be ignored.
3. Determine the ratio of R2 to the total resistance, RB.
The line common mode voltage, Vcm, should be 1.25 V so the ratio of R2 to RB is R2/RB = 1.25 V/3.3 V = 0.379.
4. Calculate the value of R2 and round down to a standard value.
E.g. R2 = 0.379 x 33 kΩ = 12.5 kΩ, so the nearest standard value is 12 kΩ (E24 series).
5.Now recalculate the value of RB to give the required line common mode voltage E.g. RB = 12kΩ / 0.379 V = 31.6 kΩ.
6.Calculate the value of R1, R1 = RB - R2, and round to a standard value.
E.g. R1 = 31.6 kΩ – 12 kΩ = 19.6 kΩ, so the nearest standard value is 20 kΩ (E24 series) or 19.6 kΩ (E48 series).
7. Check the maximum noise voltage and common mode voltage.
E.g. Vn = 3.3 V x 100 Ω / 32 kΩ = 10.3 mV and Vcm = 3.3 V x 12 kΩ /32 kΩ = 1.24 V
If the noise on the disconnected inputs is likely to be higher than 10 mV then other resistor values need to be calculated.
For further details see RD1 and RD2.
Preliminary | 36 |
