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 k
Ω,
so the nearest standard value is 12 k
Ω
(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 k
Ω
.
6. Calculate the value of R1, R1 = R
B
- R2, and round to a standard value.
E.g. R1 = 31.6 k
Ω
– 12 k
Ω
= 19.6 k
Ω
, so the nearest standard value is 20 k
Ω
(E24 series) or 19.6 k
Ω
(E48 series).
7. Check the maximum noise voltage and common mode voltage.
E.g. V
n
= 3.3 V x 100
Ω
/ 32 k
Ω
= 10.3 mV and V
cm
= 3.3 V x 12 k
Ω /
32 k
Ω
= 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.