The total line to line voltage change (ΔV ) can then be calculated by adding the hum
T
component (ΔV
) and the droop component (V
). This calculation results in two
HUM
DROOP
cases:
∆V
T
∆V
T
Case A
Case B
∆VT = ∆VHUM + VDROOP
To correct for ΔV in case A, the input stage must be able to charge the input capacitor
T
ΔV volts in 4.7μs. This is not a constraint as the typical clamping current of 650μA can
T
accomplish this for practical values of coupling capacitor.
The only way to compensate for ΔV in case B is to make V
>ΔV
. V
is
T
DROOP
HUM DROOP
increased by decreasing the input coupling capacitor value. Therefore the video
designer can increase hum rejection by decreasing the value of this capacitor. The
following is a numerical example:
choosing C = 0.022μF
c
... VDROOP
=
(63.5μ - 4.7μ) = 29.4mV
11
0.022
the maximum amount of 60 Hz hum that could be rejected would be when:
∆VDROOP
... VP =
= ∆VHUM = VP 2πƒHUM TLINE
∆VDROOP
29.4mV
=
= 1.23VPEAK HUM
2πƒHUMTLINE
2π(60) (63.5μ)
verifying that there is enough clamping current
∆Vt = 29.4mV + 29.4 mV = 58.8mV
58.8mV
4.7μ
... i = 0.022μ
= 275μA
(
)
which is less than 650μA.
5.2 Filtering
In order to keep the input to output delay small and temperature stable, no chrominance
filtering is done within the device. External filtering may be necessary if the input signal
contains large chrominance components (less than 77mV from sync tip) or has
significant amounts of high frequency noise. This filter can be a simple low pass RC
network constructed by a resistance (R ) in series with the source and a capacitor (C ) to
S
ƒ
ground. A single pole low pass filter having a corner frequency of approximately
500kHz will provide ample bandwidth for passing sync pulses with almost 18dB
attenuation at 3.58MHz. Care should be taken in choosing the value of the series resistor
in the filter since the source resistance seen by the sync separator affects its
performance. See Figure 5-4.
GS1881, GS4881, GS4981 Monolithic Video Sync
22 of 29
Separators
Data Sheet
Proprietary & Confidential
6926 - 5
November 2009
GENNUM [ GENNUM CORPORATION ]
