Atmel SpaceWire Router SpW-10X - Manual (Page 36)

Ref.: UoD_SpW-10X_
UserManual
Issue: 3.4
SpW-10X
SpaceWire Router
User Manual Date: 11 th
July 2008
Preliminary
36
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 I
b
=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, R
B
, from bias supply V
DD
to ground, R
B
= V
DD
/ I
b
.
E.g. R
B
= 3.3 V/0.1 mA = 33 k Ω. Since R
T
is much smaller than this value it can be ignored.
3. Determine the ratio of R2 to the total resistance, R
B
.
The line common mode voltage, V
cm
, should be 1.25 V so the ratio of R2 to R
B
is R2/R
B
= 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 k Ω = 12.5 kΩ, so the nearest standard value is 12 kΩ (E24 series).
5. Now recalculate the value of R
B
to give the required line common mode voltage
E.g. R
B
= 12k Ω / 0.379 V = 31.6 kΩ.
6. Calculate the value of R1, R1 = R
B
- R2, and round to a standard value.
E.g. R1 = 31.6 k Ω – 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. V
n
= 3.3 V x 100 Ω / 32 kΩ = 10.3 mV and V
cm
= 3.3 V x 12 k Ω /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.