2

dc2063af

DEMO MANUAL DC2063A

Quick start proceDure

operating principles

Demonstration circuit 2063A is easy to set up and evaluate

the performance of the LTC2875.

1. Verify the jumpers are in the following default positions:

JP1:SMB ON; JP1:TURRET OFF

JP2: SPLIT

JP3:MAX ON; JP3:ADJ OFF

and GND turrets.

3. Turn on the power at the input.

When TXD is low, the LTC2875 drives the dominant state

onto the CANH and CANL bus lines, with CANH pulled high

and CANL pulled low. When TXD is high, the LTC2875 is

in the recessive state, with both CANH and CANL drivers

in the high impedance state, and the differential voltage on

the CAN bus lines is returned to near zero by the termina-

tion resistors. The TXD pin is pulled high by an internal

500kΩ resistor when it is left floating.

The CAN bus must be terminated at both ends by resis-

tors matched to the impedance of the bus cable, typically

120Ω, while all other nodes on the CAN bus must be

unterminated.TheDC2063Aprovidesthreejumperselect-

able termination configurations: split termination, single

termination, and no termination. The split termination

is selected by placing two jumpers on JP2 in the SPLIT

(vertical) configuration. This provides 120Ω termination

consisting of two series 60Ω resistors with the LTC2875

SPLIT pin and a 4.7nF decoupling capacitor biasing the

termination center point. This configuration lowers the

EME of the transmitted signal by reducing common mode

voltage fluctuations. The single termination is selected by

placing two jumpers on JP2 in the SINGLE (horizontal)

configuration. This provides 120Ω termination consisting

of two series 60Ω resistors with no biasing of the center

point. No termination may be selected by removing both

jumpers from JP2.

The LTC2875 receiver senses the differential voltage on

the CAN bus lines and produces a digital output on the

RXD pin. When a dominant state is sensed on the bus

lines, the RXD output will pull low. When a recessive state

is sensed on the bus lines, the RXD output will pull high.

When the LTC2875 is in the shutdown state, the RXD pin

is pulled high by an internal 500kΩ resistor. The LTC2875

RXD pin is connected directly to the DC2063A RXD turret.

NOTE: Make sure the input voltage does not exceed 6V.

4. Connect a function generator to SMB jack J1 and set

quency to 100kHz (200kbps). Enable output of function

generator.

5. Connect oscilloscope to pin RXD and observe 100kHz

waveform. This demonstration shows that data applied

to TXD is transmitted to the CAN bus, looped back

through the receiver, and outputted on RXD.