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STMicroelectronics Confidential
Since the discharge time has some spread, and since the parasitic frequency may be any of the
horizontal frequencies in the range, there is no hope to choose such capacitor, to avoid coincidence
between the parasitic pulses and the critical point. The only way is:
●
to give minimal area to the receiving loop “capacitor/IC”,
●
to maintain minimal area and maximum distance for the loop which emits parasitic voltages.
5.2.4
Leakage on C
s
Considering again the schematic of the AGC system and supposing that C
s
has some leakage
current I
l
, the charges lost all along period T must be compensated during the sampling time t,
through R
s
:
where V
A
is the voltage on R
s
(provided by the amplifier with gain A = (R
1
+ R
2
) / R
1
.
If V
p
is the peak sawtooth voltage, we have
∆
V
A
= Ax
∆
V
p
, and combining the two equations:
For instance, at 50 Hz, a 1% variation of output amplitude will result if I
l
= 80 nA
For that reason, it is recommended to use a good quality film capacitor (rather than an electrolytic)
as sampling capacitor.
Figure 15: Vertical Vibration
>
<
70µs
2V
Vsync
Early switching
~5V
Ramp with early switching
Normal ramp
V
s
I
l
T
×
C
s
-------------
V
A
t
×
∆
R
s
C
s
×
---------------------
=
=
∆
V
p
I
l
T
t
---
×
R
s
A
-------
×
=
∆
