EL2257C Elantec Semiconductor, EL2257C Datasheet - Page 17

• Manufacturer Part Number: EL2257C
• Description: 125 MHz Single Supply/ Clamping Op Amps
• Manufacturer: Elantec Semiconductor

Video Sync Pulse Remover Application
All CMOS Analog to Digital Converters (A/Ds) have a
parasitic latch-up problem when subjected to negative
input voltage levels. Since the sync tip contains no use-
ful video information and it is a negative going pulse, we
can chop it off. Figure 7 shows a unity gain connected
amplifier A of an EL2257C. Figure 8 shows the com-
plete input video signal applied at the input, as well as
the output signal with the negative going sync pulse
removed.
Multiplexing with the EL2257C/EL2357C
The ENABLE pins on the EL2257C/EL2357C allow for
multiplexing applications. Figure 9 shows an EL2357C
with all 3 outputs tied together, driving a back termi-
nated 75
amplitudes and frequencies are applied to the three
video load. Three sinewaves of varying
Figure 7.
Figure 8.
17
inputs. Logic signals are applied to each of the ENABLE
pins to cycle through turning each of the amplifiers on,
one at a time. Figure 10 shows the resulting output
waveform at V
ns. Notice the outputs are tied directly together. Decou-
pling resistors at each output are not necessary. In fact,
adding them approximately doubles the switching time
to 100 ns.
Short Circuit Current Limit
The EL2257C/EL2357C have internal short circuit pro-
tection circuitry that protect it in the event of its output
being shorted to either supply rail. This limit is set to
around 100 mA nominally and reduces with increasing
junction temperature. It is intended to handle temporary
shorts. If an output is shorted indefinitely, the power dis-
sipation could easily increase such that the part will be
destroyed. Maximum reliability is maintained if the out-
put current never exceeds ±90 mA. A heat sink may be
125 MHz Single Supply, Clamping Op Amps
EL2257C/EL2357C
OUT
. Switching is complete in about 50
Figure 10.
Figure 9.