Datasheet

LTM2881
19
2881fg
For more information www.linear.com/LTM2881
APPLICATIONS INFORMATION
Figure 18. Cable Length vs Data Rate
The PCB layout in Figures 16a to 16e show the low EMI
demo board for the LTM2881. The demo board uses a
combination of EMI mitigation techniques, including both
embedded PCB bridge capacitance and discrete GND to
GND2 capacitors. Two safety rated type Y2 capacitors
are used in series, manufactured by Murata, part number
GA342QR7GF471KW01L. The embedded capacitor ef
-
fectively suppresses emissions above 400MHz, whereas
the discrete capacitors are more effective below 400MHz.
EMI performance is shown in Figure 17, measured using
a Gigahertz T
ransverse Electromagnetic (GTEM) cell and
method detailed in IEC 61000-4-20, “Testing and Mea
-
surement Techniques – Emission and Immunity Testing
in T
ransverse Electromagnetic W
aveguides.”
Cable Length versus Data Rate
For a given data rate, the maximum transmission distance
is bounded by the cable properties. A typical curve of
cable length versus data rate compliant with the RS485
standard is shown in Figure 18. Three regions of this
curve reflect different performance limiting factors in data
transmission. In the flat region of the curve, maximum
distance is determined by resistive loss in the cable. The
downward sloping region represents limits in distance and
rate due to the AC losses in the cable. The solid vertical
line represents the specified maximum data rate in the
RS485 standard. The dashed line at 250kbps shows the
maximum data rate when SLO is low. The dashed line at
20Mbps shows the maximum data rate when SLO is high.
RF, Magnetic Field Immunity
The LTM2881 has been independently evaluated and has
successfully passed the RF and magnetic field immunity
testing requirements per European Standard EN 55024,
in accordance with the following test standards:
EN 61000-4-3 Radiated, Radio-Frequency,
Electromagnetic Field Immunity
EN 61000-4-8 Power Frequency Magnetic Field
Immunity
EN 61000-4-9
Pulsed Magnetic Field Immunity
Tests were performed using an unshielded test card de
-
signed per the data sheet PCB layout recommendations.
Specific limits per test are detailed in Table 1.
Table 1
TEST FREQUENCY FIELD STRENGTH
EN 61000-4-3, Annex D 80MHz to 1GHz 10V/m
1.4MHz to 2GHz 3V/m
2GHz to 2.7GHz 1V/m
EN 61000-4-8, Level 4 50Hz and 60Hz 30A/m
EN 61000-4-8, Level 5 60Hz 100A/m*
EN 61000-4-9, Level 5 Pulse 1000A/m
*Non IEC Method
2881 F18
DATA RATE (bps)
CABLE LENGTH (FT)
10k 1M 10M100k 100M
100
1k
10
10k
LOW-EMI MODE
MAX DATA RATE
RS485 MAX
DATA RATE
NORMAL
MODE MAX
DATA RATE