GS1881_09 Datasheet, PDF (22/29 Pages) Gennum Corporation – Monolithic Video Sync Separators

The total line to line voltage change (ΔV_T) can then be calculated by adding the hum
component (ΔV_HUM) and the droop component (V_DROOP). This calculation results in two
cases:
∆V_T
∆V_T
Case A
Case B
∆V_T = ∆V_HUM + V_DROOP
To correct for ΔV_T in case A, the input stage must be able to charge the input capacitor
ΔV_T volts in 4.7μs. This is not a constraint as the typical clamping current of 650μA can
accomplish this for practical values of coupling capacitor.
The only way to compensate for ΔV_T in case B is to make V_DROOP >ΔV_HUM. V_DROOP is
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_c = 0.022μF
.^..
V_DROOP =
11
0.022
(63.5μ - 4.7μ) =
29.4mV
the maximum amount of 60 Hz hum that could be rejected would be when:
∆V_DROOP = ∆V_HUM = V_P 2πƒ_HUM T_LINE
.^.. V_P = ^∆VDROOP =
29.4mV
= 1.23V_PEAK HUM
2πƒ_HUMT_LINE 2π(60) (63.5μ)
verifying that there is enough clamping current
∆V_t = 29.4mV + 29.4 mV = 58.8mV
( ) ^{... i = 0.022μ}
58.8mV
= 275μA
4.7μ
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_S) in series with the source and a capacitor (C_ƒ) to
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
Separators
Data Sheet
6926 - 5
November 2009
22 of 29
Proprietary & Confidential