verifying that there is enough clamping current
The interfering hum component is defined by:
HUM(t) = VPcos(2πƒHUMt)
∆Vt = 29.4 mV + 29.4 mV = 58.8 mV
v
58.8 mV
... i = 0.022 µ
= 275 µA
where: VP = Peak voltage of AC hum
(
)
4.7 µ
ƒHUM = Frequency of hum (50 Hz or 60 Hz)
which is less than 650 µA.
The maximum rate of change of this hum signal occurs at the
zero crossing points and is:
(2) FIltering
dvHUM
= ± VP2πƒHUM
Inordertokeeptheinputtooutputdelaysmallandtemperature
stable, no chrominance filtering is done within the device.
External filtering may be necessary if the input signal contains
large chrominance components (less than 77 mV 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 (RS) in series with the source and a capacitor (Cƒ)
to ground. A single pole low pass filter having a corner
frequency of approximately 500 kHz will provide ample
bandwidth for passing sync pulses with almost 18 dB
attenuation at 3.58 MHz. Care should be taken in choosing
the value of the series resistor in the filter since the source
resistanceseenbythesyncseparatoraffectsitsperformance.
dt
π
2
3π
2
t =
,
Sincethehorizontalscanperiodismuchfasterthantheperiod
of the interference ( 63.5 µs << 1/ƒHUM)a good approximation
is to assume that the maximum line to line voltage change
resulting from the interfering hum is:
∆VHUM = ± VP2πƒHUM TLINE
where: TLINE = 63.5 µs
Thetotallinetolinevoltagechange(∆VT)canthenbecalculated
by adding the hum component (∆VHUM ) and the droop
component (VDROOP). This calculation results in two cases:
As the source resistance rises, the video input sync tip starts
to be clipped due to the clamping current during the sync.
This clamping current is relatively large due to the
non-symmetric duty cycle of video. To a good approximation
the amount of sync clamp current can be calculated as
follows:
∆V
T
∆V
T
Case A
Case B
( ICLAMP ) (TSYNC) = (IDIS) (TLINE - TSYNC
)
AVG
∆VT = ∆VHUM + VDROOP
ICLAMP (4.7 µs) = (11 µA) (63. 5 µs - 4.7 µs)
AVG
... ICLAMP
To correct for ∆VT in case A, the input stage must be able to
charge the input capacitor ∆VT volts in 4.7 µs. This is not a
constraint as the typical clamping current of 650 µA can
accomplish this for practical values of coupling capacitor.
= 137.6 µA
AVG
This clamp current flows in the source resistance causing a
voltage drop equal to :
The only way to compensate for ∆VT in case B is to make
VDROOP >∆ VHUM. VDROOP is increased by decreasing the input
coupling capacitor value. Therefore the video designer can
VCLIP = ( ICLAMP ) (RS)
AVG
= (137.6 µ) (RS)
increasehumrejectionbydecreasingthevalueofthiscapacitor.
The following is a numerical example:
ICLAMP
choosing C = 0.022 µF
VIDEO
INPUT
c
RS
11
8
6
... VDROOP
=
(63.5 µ - 4.7 µ) = 29.4 mV
2
-
+
0.022
V
CC
CLIP
the maximum amount of 60 Hz hum that could be rejected
would be when:
4
75Ω
C
ƒ
680k
0.1µ
∆VDROOP
=
∆VHUM = VP 2πƒHUM TLINE
Fig. 22 Simple Chrominance Filtering
∆VDROOP 29.4mV
... VP =
=
=1.23vPEAK HUM
2πƒHUMTLINE 2π(60) (63.5 µ)
520 - 23 - 03
12
GENNUM [ GENNUM CORPORATION ]
