EL4501 Просмотр технического описания (PDF) - Intersil
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EL4501 Datasheet PDF : 19 Pages
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EL4501
prevents the amplifier output drifting significantly from the
DC-restore 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 = 2----π-g---MC-----H--
+
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/(2π * CH), gM1 is about 1/(29kΩ), 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 ΔV = ΔQ/CH and will shift the output
voltage of the video amplifier by ΔV. 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, ΔV 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
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 = -Δ----V----R-Δ---A-t---M-----P--
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
16
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