Low-Power, Slew-Rate-Limited
RS-485/RS-422 Transceivers
_________________________________________________________________Test Circuits
Y
VOD
Z
R
R
VOC
RECEIVER TEST POINT
OUTPUT
CRL
15pF
1kΩ
1kΩ
VCC
S1
S2
Figure 4. Driver DC Test Load
Figure 5. Receiver Timing Test Load
3V
DE
DI
Y
VID
Z
CL1
A
RDIFF
B
CL2
RO
RE
OUTPUT
UNDER TEST
500Ω
CL
VCC
S1
S2
Figure 6. Driver/Receiver Timing Test Circuit
Figure 7. Driver Timing Test Load
HWD483/HWD487/HWD488/HWD489:
Reduced EMI and Reflections
The HWD483 and HWD487–HWD489 are slew-rate limit-
ed, minimizing EMI and reducing reflections caused by
improperly terminated cables. Figure 12 shows the dri-
ver output waveform and its Fourier analysis of a
150kHz signal transmitted by a HWD481, HWD485,
HWD490, HWD491, or HWD1487. High-frequency har-
monics with large amplitudes are evident. Figure 13
shows the same information displayed for a HWD483,
HWD487, HWD488, or HWD489 transmitting under the
same conditions. Figure 13’s high-frequency harmonics
have much lower amplitudes, and the potential for EMI
is significantly reduced.
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