EL5174, EL5374
The gain setting for EL5374 is:
R
F1
+
R
F2
-
V
ODM
=
(
V
IN
+
–
V
IN
-
) ×
1
+ ---------------------------
R
G
2R
F
-
V
ODM
=
(
V
IN
+
–
V
IN
-
) ×
1
+ ----------
R
G
V
OCM
=
V
REF
Driving Capacitive Loads and Cables
The EL5174 and EL5374 can drive 23pF differential
capacitor in parallel with 1kΩ differential load with less than
5dB of peaking at gain of +1. If less peaking is desired in
applications, a small series resistor (usually between 5Ω to
50Ω) can be placed in series with each output to eliminate
most peaking. However, this will reduce the gain slightly. If
the gain setting is greater than 1, the gain resistor R
G
can
then be chosen to make up for any gain loss which may be
created by the additional series resistor at the output.
When used as a cable driver, double termination is always
recommended for reflection-free performance. For those
applications, a back-termination series resistor at the
amplifier's output will isolate the amplifier from the cable and
allow extensive capacitive drive. However, other applications
may have high capacitive loads without a back-termination
resistor. Again, a small series resistor at the output can help
to reduce peaking.
Where:
R
F1
= R
F2
= R
F
R
F1
FBP
V
IN
+
V
IN
-
V
REF
R
G
I
N
+
I
N
-
REF
FBN
R
F2
V
O
-
V
O
+
Disable/Power-Down (for EL5374 only)
The EL5374 can be disabled and placed its outputs in a high
impedance state. The turn off time is about 1.2µs and the
turn on time is about 130ns. When disabled, the amplifier's
supply current is reduced to 1.7µA for I
S
+ and 120µA for I
S
-
typically, thereby effectively eliminating the power
consumption. The amplifier's power down can be controlled
by standard CMOS signal levels at the EN pin. The applied
logic signal is relative to V
S
+ pin. Letting the EN pin float or
applying a signal that is less than 1.5V below V
S
+ will enable
the amplifier. The amplifier will be disabled when the signal
at EN pin is above V
S
+ - 0.5V.
FIGURE 24.
Choice of Feedback Resistor and Gain Bandwidth
Product
For applications that require a gain of +1, no feedback
resistor is required. Just short the OUT+ pin to FBP pin and
OUT- pin to FBN pin. For gains greater than +1, the
feedback resistor forms a pole with the parasitic capacitance
at the inverting input. As this pole becomes smaller, the
amplifier's phase margin is reduced. This causes ringing in
the time domain and peaking in the frequency domain.
Therefore, R
F
has some maximum value that should not be
exceeded for optimum performance. If a large value of R
F
must be used, a small capacitor in the few Pico farad range
in parallel with R
F
can help to reduce the ringing and
peaking at the expense of reducing the bandwidth.
The bandwidth of the EL5174 and EL5374 depends on the
load and the feedback network. R
F
and R
G
appear in
parallel with the load for gains other than +1. As this
combination gets smaller, the bandwidth falls off.
Consequently, R
F
also has a minimum value that should not
be exceeded for optimum bandwidth performance. For gain
of +1, R
F
= 0 is optimum. For the gains other than +1,
optimum response is obtained with R
F
between 500Ω to
1kΩ.
The EL5174 and EL5374 have a gain bandwidth product of
200MHz for R
LD
= 1kΩ. For gains
≥5,
its bandwidth can be
predicted by the following equation:
Gain
×
BW
=
200MHz
Output Drive Capability
The EL5174 and EL5374 have internal short circuit
protection. Its typical short circuit current is ±60mA. If the
output is shorted indefinitely, the power dissipation could
easily increase such that the part will be destroyed.
Maximum reliability is maintained if the output current never
exceeds ±60mA. This limit is set by the design of the internal
metal interconnections.
Power Dissipation
With the high output drive capability of the EL5174 and
EL5374. It is possible to exceed the 135°C absolute
maximum junction temperature under certain load current
conditions. Therefore, it is important to calculate the
maximum junction temperature for the application to
determine if the load conditions or package types need to be
modified for the amplifier to remain in the safe operating
area.
The maximum power dissipation allowed in a package is
determined according to:
T
JMAX
–
T
AMAX
-
PD
MAX
= --------------------------------------------
Θ
JA
11
FN7313.5
August 8, 2005
INTERSIL [ INTERSIL CORPORATION ]
