CY2212
Document #: 38-07466 Rev. *A
Page 5 of 10
Figure 3 below shows the clock driver implemented as a
push-pull driver. When stimulating the output driver, the trans-
mission lines shown in Figure 3 can be replaced by a direct
connection to the termination resistors, RT. The values for the
external components are given in Table 1.
As mentioned previously, the clock driver’s output impedance
matches the channel impedance. To accomplish this, each of
the output driver devices are sized to have an ROUT of about
20 ohms when fully turned on. ROUT is the dynamic output
resistance, and is defined in the DC Device Characteristics
Table on page 3 of this data sheet. Since ROUT is in series with
RS, and that combination is in parallel with RP, the effective
output impedance is given by:
RP(RS + ROUT)/(RP + RS + ROUT).
This calculation results in a effective output impedance of
about 27 ohms for the values listed in Table 1. Since the total
impedance is dominated by the external resistors, a large
possible range of ROUT is allowed. When the output is transi-
tioning, the impedance of the CMOS devices increases
dramatically. The purpose of RP is to limit the maximum output
impedance during output transitions.
In order to control signal attenuation and EMI, clock signal
rise/fall times must be tightly controlled. Therefore, external
filter capacitors CF are used to control the output slew rate. In
addition, the capacitor CMID is used to provide AC ground at
the mid-point of the RP resistors.
Table 1 gives the nominal values of the external components
and
their
maximum
acceptable
tolerance,
assuming
ZCH = 28 ohms.
Table 1. Output External Component Values
Parameter
Description
Value
Tolerance
Unit
RS
Series Resistor
68
±5%
ohm
RP
Parallel Resistor
39
±5%
ohm
CF
Edge-rate Filter Capacitor
15
±10%
pF
CMID
AC Ground Capacitor
0.01
±20%
µF
RS
RS
RP
RP
ZCH
ZCH
RT =ZCH
RT =ZCH
Measurement Point
Measurement Point
DRCG Lite
CF
CF
CMID
CMID
Figure 3. Output Driver
RS
RS
RP
RP
ZCH
ZCH
RT = ZCH
RT = ZCH
Measurement Point
DRCG Lite
CF
CF
CMID
CMID
ZCH
ZCH
RT = ZCH
RT = ZCH
Measurement Point
CMID
RX
RX
RX
RX
Figure 4. Output Driving Two Channels
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