DC1746A-A Linear Technology, DC1746A-A Datasheet - Page 14

• Manufacturer Part Number: DC1746A-A
• Description: EVALUATION KIT LOW EMI LTM2881-3
• Manufacturer: Linear Technology
• Series: µModuler

Specifications of DC1746A-A

• Main Purpose: Interface, Transceiver RS-422/RS-485
• Embedded: No
• Utilized Ic / Part: LTM2881
• Primary Attributes: 3V Supply, Isolated, Transceiver + Power
• Kit Contents: Board, Datasheet
• Tool / Board Applications: Interface-Backplane, PCI, PCI Express, RS-XXX, Digital & Opto Isolator
• Mcu Supported Families: LTM2881
• Development Tool Type: Hardware - Eval/Demo Board
• Lead Free Status / RoHS Status: Not applicable / Not applicable
• Secondary Attributes: -

LTM2881
APPLICATIONS INFORMATION
Switchable Termination
Proper cable termination is very important for signal fidelity.
If the cable is not terminated with its characteristic imped-
ance, reflections will distort the signal waveforms.
The integrated switchable termination resistor provides
logic control of the line termination for optimal perfor-
mance when configuring transceiver networks.
When the TE pin is high, the termination resistor is enabled
and the differential resistance from A to B is 120Ω. Figure
11 shows the I/V characteristics between pins A and B
with the termination resistor enabled and disabled. The
resistance is maintained over the entire RS485 common
mode range of –7V to 12V as shown in Figure 12. The
integrated termination resistor has a high frequency re-
sponse which does not limit performance at the maximum
specified data rate. Figure 13 shows the magnitude and
14
Figure 13. Termination Magnitude and Phase vs Frequency
Figure 11. Curve Trace Between A and B with Termination
Enabled and Disabled
150
140
130
120
110
100
0.1
FREQUENCY (MHz)
1
MAGNITUDE
PHASE
2881 F13
2881 F11
10
10
0
–10
–20
–30
–40
phase of the termination impedance versus frequency.
The termination resistor cannot be enabled by TE if the
device is unpowered, ON is low or the LTM2881 is in
thermal shutdown.
Supply Current
The static supply current is dominated by power delivered to
the termination resistance. Power supply current increases
with data rate due to capacitive loading. Figure 14 shows
supply current versus data rate for three different loads
for the circuit configuration of Figure 4. Supply current
increases with additional external applications drawing
current from V
Figure 12. Termination Resistance vs Common Mode Voltage
Figure 14. Supply Current vs Data Rate
130
128
126
124
122
120
118
116
114
112
110
250
230
210
190
170
150
130
110
90
70
50
–10
CC2
0.1
LTM2881-3
R=54 CL=1000p
R=54 CL=100p
R=54 CL=0
LTM2881-5
R=54 CL=1000p
R=54 CL=100p
R=54 CL=0
.
–5
COMMON MODE VOLTAGE (V)
DATA RATE (Mbps)
0
1
5
10
10
2881 G11
2881 F14
15
2881fb