EL4093C Elantec Semiconductor, Inc. (acquired by Intersil), EL4093C Datasheet - Page 12

• Manufacturer Part Number: EL4093C
• Description: 300MHz Dc-restored Video Amplifier
• Manufacturer: Elantec Semiconductor, Inc. (acquired by Intersil)

Applications Information
With R
tained Note however that this is the small signal
GBWP As mentioned earlier the sample and
hold has special boost circuits built in which pro-
vides
These boost circuits turn on when the S H input
differential voltage exceeds
boosters are turned on gm1 greatly increases and
the circuit becomes nonlinear Thus some stabili-
ty issues are associated with the boosters and
they will be addressed in a later section
Charge Injection and Hold Step
Charge injection refers to the charge transferred
to the hold capacitor when switching to the
HOLD mode The charge should ideally be 0 but
due to stray capacitive coupling and other effects
is typically 0 1 pC in the EL4093 This charge
changes the hold capacitor voltage by V
C
stage transconductance (gm2) to produce a
change in S H output current This last quantity
is listed as the spec I
ing the following
EL4093C
300 MHz DC-Restored Video Amplifier
HOLD
I
STEP
g
F
8 5 mA of charge current during full slew
e
and this
e
C
750
HOLD
Q
V is multiplied by the output
a GBWP of 310 kHz is ob-
STEP
gm2
g
and is calculated us-
50 mV When the
Contd
e
Q
Figure 4
12
For C
change in S H output current flows through R
shifting the CFA output voltage However as we
shall soon see this shift is negligible Assuming
R
give (750 )(100 nA)
CFA output
Droop Rate
When the S H amplifier is in HOLD mode there
is a small current that leaks from the switch into
the hold capacitor This quantity is termed the
droop current and is typically 10 nA in the
EL4093 This droop current produces a ramp in
the hold capacitor voltage which in turn produc-
es a similar effect at the CFA output The Droop
Rate at the CFA output can be found using the
equation below
Droop
Assuming R
drift in the CFA output due to droop current is
about 7
tions there is about 60 s between autozero peri-
ods Thus there is 7 V
less than 0 1 IRE of drift over each NTSC scan
line This drift is negligible in most applications
I
F
STEP
e
HOLD
e
750
has a typical value of 100 nA This
I
C
V
DROOP
HOLD
F
e
e
s Recall that in NTSC applica-
I
STEP
2 2 nF and gm2
750
(gm2
e
is impressed across R
and C
0 08 mV of change at the
s)(60 s)
4093 – 40
R
F
HOLD
)
e
e
e
2 2 nF the
0 4 mV or
1 (500 )
F
to
F