EL4501 Просмотр технического описания (PDF) - Renesas Electronics
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EL4501 Datasheet PDF : 20 Pages
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EL4501
reference level. This improves start-up behavior and speeds
recovery after a signal drop-out. For ease of use, the EL4501
provides a buffered 1.3V DC level normally connected directly
to the restore loop reference input (REF IN). Alternatively, an
external voltage between 0V and 3.5V, connected to REF IN,
can be used to set the restored level.
0.1µF
VIN
~1.8V
+
-
CH
VOUT
GBWP
=
-----g---M-------
2CH
+
S/H
-
gM
VREF_IN
FIGURE 36. DC-RESTORE AMPLIFIER AND S/H
CONFIGURATION
INPUT
VIDEO
SIGNAL
BACK
PORCH
OUTPUT
CH1=500mV/DIV
CH3=5V/DIV
M=10µs
FIGURE 37. NTSC VIDEO SIGNAL WITH BACK PORCH
OUTPUT
Auto-Zero Loop Bandwidth
The gain bandwidth product (GBWP) of the auto-zero loop is
determined by the size of the hold capacitor and the
transconductance (gM1) of the sample and hold amplifier.
GBWP = gM1/(2 * CH), gM1 is about 1/(29k), for
CH = 270pF, GBWP is 20kHz. For CH = 100pF, GBWP is about
55kHz.
Charge Injection and Hold Step Error
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, it is typically 6fC. This charge changes the hold
capacitor voltage by V = Q/CH and will shift the output
voltage of the video amplifier by V. However, this shift is small
and can be negligible for the EL4501 (see the Hold Step
Voltage Error vs Hold Capacitance curve). Assuming CH =
100pF, V is about 60µV. There will be 60µV change at the
video amplifier output.
Droop Rate
When the S/H amplifier is in the hold mode, there is a small
current that leaks from the switch to the hold capacitor. This
quantity is called the droop current. This droop current
FN7327 Rev 3.00
November 12, 2010
produces a ramp in the hold capacitor voltage, which in turn
produces a similar voltage at the video amplifier output. The
droop rate at the video amplifier output can be found using the
following equation:
DroopRate
=
-----V----R----A----M-----P--
t
Assuming CH = 100pF, from the Droop Rate vs Hold
Capacitance curve, the droop rate is about 0.31mV/ms at the
video amplifier output at room temperature. In NTSC
applications, there is about 60µs between auto-zero periods.
Thus, there is (0.31mV/ms) * 60µs = 18.6µV. It is much less
than 0.5IRE (3.5mV). This drift is negligible.
Choice of Hold Capacitor
The EL4501 allows the user to choose the hold capacitor as
low as 1pF and it is still stable. A smaller hold capacitor has a
faster acquisition time and faster auto-zero loop response, but
would increase the droop and hold step error. Also, if the
acquisition time is too fast, it would probably give an image
with clamp streaking and low frequency noise with noisy
signals. Increasing the hold capacitor would increase the
acquisition time, lower the auto-zero loop response, lower the
droop and hold step error. See the performance curves for the
trade-off. Normally, in video (NTSC and PAL) applications, a
smooth acquisition might takes about 10 to 20 scan lines. For a
hold capacitor equal to 270pF, the acquisition time is about 10
lines. In the worse case, ambient temperature is 85°C, the
droop current is 2.2nA which causes the output voltage ramp
to about 0.49mV for 60µs. This drift is negligible in most
applications. Figure 38 shows the input and output waveforms
of the video amplifier while the S/H is in sample mode.
Applying a 1V step to the video amplifier input, the output of
the video amplifier jumps to 2.3V. Then, the auto-zero system
tries to drive the video output to the reference voltage, which is
1.3V. The acquisition time takes about 10 NTSC scan lines.
CH=270pF
VIDEO
AMP
OUTPUT
VIDEO
AMP
INPUT
CH1=500mV/DIV
CH2=1V/DIV
M=100µs
Auto-zero mechanism restores amplifier output to
1.3V after +1V step at input
FIGURE 38. INPUT AND OUTPUT WAVEFORMS WITH S/H IN
SAMPLE MODE
Page 16 of 20
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