12
For sourcing:
For sinking:
Where:
VS = Total supply voltage
ISMAX = Maximum quiescent supply current
VOUTi = Maximum output voltage of the application for
each channel
RLOADi = Load resistance tied to ground for each
channel
ILOADi = Load current for each channel
By setting the two PDMAX equations equal to each other, we
can solve the output current and RLOADi to avoid the device
overheat.
Power Supply Bypassing and Printed Circuit
Board Layout
As with any high frequency device, a good printed circuit
board layout is necessary for optimum performance. Lead
lengths should be as short as possible. The power supply
pin must be well bypassed to reduce the risk of oscillation.
For normal single supply operation, where the VS- pin is
connected to the ground plane, a single 4.7F tantalum
capacitor in parallel with a 0.1F ceramic capacitor from VS+
to GND will suffice. This same capacitor combination should
be placed at each supply pin to ground if split supplies are to
be used. In this case, the VS- pin becomes the negative
supply rail.
For good AC performance, parasitic capacitance should be
kept to a minimum. Use of wire wound resistors should be
avoided because of their additional series inductance. Use
of sockets should also be avoided if possible. Sockets add
parasitic inductance and capacitance that can result in
compromised performance. Minimizing parasitic capacitance
at the amplifier’s inverting input pin is very important. The
feedback resistor should be placed very close to the
inverting input pin. Strip line design techniques are
recommended for the signal traces.
Typical Applications
VIDEO SYNC PULSE REMOVER
Many CMOS analog to digital converters have a parasitic
latch up problem when subjected to negative input voltage
levels. Since the sync tip contains no useful video
information and it is a negative going pulse, we can chop it
off. Figure
32 shows a gain of 2 connections. Figure
33shows the complete input video signal applied at the input,
as well as the output signal with the negative going sync
pulse removed.
MULTIPLEXER
Besides the normal power down usage, the ENABLE pin of
the EL8202 can be used for multiplexing applications. Figure
34 shows two EL8202 with the outputs tied together, driving
a back terminated 75
video load. A 2VP-P 2MHz sine wave
is applied to Amp A and a 1VP-P 2MHz sine wave is applied
to Amp B. Figure 33 shows the ENABLE signal and the
resulting output waveform at VOUT. Observe the break-
before-make operation of the multiplexing. Amp A is on and
VIN1 is passed through to the output when the ENABLE
signal is low and turns off in about 25ns when the ENABLE
signal is high. About 200ns later, Amp B turns on and VIN2 is
passed through to the output. The break-before-make
operation ensures that more than one amplifier isn’t trying to
drive the bus at the same time.
PDMAX
VS ISMAX Σ VS VOUTi
–
()
VOUTi
RLi
-----------------
×
+
×
=
PDMAX
VS ISMAX Σ VOUTi VS-
–
() I
LOADi
×
+
×
=
FIGURE 32. SYNC PULSE REMOVER
5V
1K
VOUT
VIN
75
+
-
75
1K
75
VS+
VS-
FIGURE 33. VIDEO SIGNAL
1V
0.5V
0V
1V
0.5V
0V
M = 10s/DIV
VOUT
VIN
EL8202, EL8203, EL8403