EL4421 Intersil Corporation, EL4421 Datasheet - Page 8

• Manufacturer Part Number: EL4421
• Description: Multiplexer/Amplifier, Video Mux 2:1, 80MHz, Gain=1, Muxd Dual Input
• Manufacturer: Intersil Corporation

Applications Information
General Description
The EL44XX family of video mux-amps are composed of two
or four input stages whose inputs are selected and control
an output stage. One of the inputs is active at a time and the
circuit behaves as a traditional voltage-feedback op-amp for
that input, rejecting signals present at the unselected inputs.
Selection is controlled by one or two logic inputs.
The EL4421, EL4422, EL4441, and EL4442 have all -inputs
wired in parallel, allowing a single feedback network to set
the gain of all inputs. These devices are wired for positive
gains. The EL4443 and EL4444, on the other hand, have all
+inputs and -inputs brought out separately so that the input
stage can be wired for independent gains and gain polarities
with separate feedback networks.
The EL4421, EL4441, and EL4443 are compensated for
unity-gain stability, while the EL4422, EL4442, and EL4444
are compensated for a fed-back gain of +2, ideal for driving
back-terminated cables or maintaining bandwidth at higher
fed-back gains.
Switching Characteristics
The logic inputs work with standard TTL levels of 0.8V or
less for a logic 0 and 2.0V or more for a logic 1, making them
compatible for TTL and CMOS drivers. The ground pin is the
logic threshold biasing reference. The simplified input
circuitry is shown in Figure 1 below.
The ground pin draws a maximum DC current of 6µA, and
may be biased anywhere between (V-) +2.5V and (V+)
-3.5V. The logic inputs may range from (V-)+2.5V to V+, and
are additionally required to be no more negative than V(Gnd
pin)-4V and no more positive than V(Gnd pin)+6V.
FIGURE 1. SIMPLIFIED LOGIC INPUT CIRCUITRY
EL4421, EL4422, EL4441, EL4442, EL4443, EL4444
8
For example, within these constraints, we can power the
EL44XX's from +5V and +12V without a negative supply by
using these connections.
The logic input(s) and ground pin are shifted 2.5V above
system ground to correctly bias the mux-amp. Of course, all
the signal inputs and output will have to be shifted 2.5V
above system ground to ensure proper signal path biasing.
A final caution: the ground pin is also connected to the IC's
substrate and frequency compensation components. The
ground pin must be returned to system ground by a short
wire or nearby bypass capacitor. In Figure 2, the 22kΩ
resistors also serve to isolate the bypassed ground pin from
the +5V supply noise.
Signal Amplitudes
Signal input and output voltages must be between (V-)+2.5V
and (V+)-2.5V to ensure linearity. Additionally, the differential
voltage on any input stage must be limited to ±6V to prevent
damage. In unity-gain connections, any input could have
±3V applied and the output would be at ±3V, putting us at
our 6V differential limit. Higher-gain circuit applications
divide the output voltage and allow for larger outputs. For
instance, at a gain of +2 the maximum input is again ±3V
and the output swing is ±6V. The EL4443 or EL4444 can be
wired for inverting gain with even more amplitude possible.
The output and positive inputs respond to overloading
amplitudes correctly; that is, they simply clamp and remain
monotonic with increasing +input overdrive. A condition
exists, however, where the -input of an active stage is
overdriven by large outputs. This occurs mainly in unity-gain
connections, and only happens for negative inputs. The
overloaded input cannot control the feedback loop correctly
and the output can become non-monotonic. A typical
scenario has the circuit running on ±5V supplies, connected
for unity gain, and the input is the maximum ±3V. Negative
input extremes can cause the output to jump from -3V to
FIGURE 2. USING
THE EL44XX MUX AMPS WITH +5V
AND +12V SUPPLIES