Echelon LonWorks Router - Manual (Page 72)

Figure 31. Differential Driver Circuit
Table 13. Bill of Materials for the Differential Driver Circuit
Designator Value
C6 470 pF
R14 10 kΩ, 1%
U101 SN74HCT240
Comparator Circuit
Figure 32 shows a differential comparator circuit for connecting a Router 5000
to a TPT/XF-1250 transceiver. The differential comparator circuit drives the
Router 5000’s receive (CP0) signal based on the TPT/XF-1250 transceiver’s
differential receive signals (CP0 and CP1).
The heart of the differential comparator circuit is a dual, high speed voltage
feedback operational amplifier (such as an Analog Devices AD826 Low Cost,
High Speed, Low Power Dual Operational Amplifier) and a high-speed
comparator (such as a Linear Technology LT1016 Ultra Fast Precision 10ns
Comparator). The operational amplifiers buffer the differential receive signal
and form a low-pass filter. The comparator interfaces directly to TTL/CMOS
logic while operating off the same 5 V power supply as the TPT/XF-1250
transceiver or a separate 5 V analog power supply (VA).
Important: Because capacitor C4 with resistors R7 and R8 act as a low-pass
filter for the differential signal, be sure to keep the traces between them and
U103 as short as possible. Excessive trace capacitance can lower the filter’s
cutoff frequency, which can cause signal loss from the TPT/XF-1250 transceiver.
62 Developing a LONWORKS Router