10
FN7306.7
August 25, 2010
Description of Operation and Application
Information
Product Description
The EL5175 and EL5375 are wide bandwidth, low power
and single/differential ended to single-ended output
amplifiers. The EL5175 is a single channel differential to
single-ended amplifier. The EL5375 is a triple channel
differential to single ended amplifier. The EL5175 and
EL5375 are internally compensated for closed loop gain of
+1 orgreater. Connected in gain of 1 and driving a 500Ω
load, the EL5175 and EL5375 have a -3dB bandwidth of
550MHz. Driving a 150Ω load at gain of 2, the bandwidth is
about 130MHz. The bandwidth at the REF input is about
450MHz. The EL5175 and EL5375 is available with a
power-down feature to reduce the power while the amplifier
is disabled.
Input, Output and Supply Voltage Range
The EL5175 and EL5375 have been designed to operate
with a single supply voltage of 5V to 10V or a split supplies
with its total voltage from 5V to 10V. The amplifiers have an
input common mode voltage range from -4.3V to 3.3V for
±5V supply. The differential mode input range (DMIR)
between the two inputs is approximately -2.3V to +2.3V. The
input voltage range at the REF pin is from -3.6V to 3.3V. If
the input common mode or differential mode signal is outside
the above-specified ranges, it will cause the output signal to
become distorted.
The output of the EL5175 and EL5375 can swing from -3.9V
to 3.5V at 500Ω load at ±5V supply. As the load resistance
becomes lower, the output swing is reduced respectively.
Overall Gain Settings
The gain setting for the EL5175 and EL5375 is similar to the
conventional operational amplifier. The output voltage is
equal to the difference of the inputs plus V
REF
and then
times the gain.
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 EL5175 and EL5375 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. For A
V
= 2 and R
F
= R
G
= 806Ω, the BW is about
190MHz and the frequency response is very flat.
The EL5175 and EL5375 have a gain bandwidth product of
200MHz. For gains ≥5, its bandwidth can be predicted by
using Equation 2:
Driving Capacitive Loads and Cables
The EL5175 and EL5375 can drive 15pF capacitance in
parallel with 500Ω load to ground with less than 4.5dB of
peaking at a 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.
Disable/Power-Down
The EL5175 and EL5375 can be disabled and its outputs
placed in a high impedance state. The turn-off time is about
1.2µs and the turn-on time is about 80ns. When disabled,
the amplifier's supply current is reduced to 80µA for I
S
+ and
120µA for I
S
- typically, thereby effectively eliminating the
V
O
V(
IN
+V
IN
-V
REF
+ ) 1
R
F
R
G
--------
+
⎝⎠
⎜⎟
⎛⎞
×–=
(EQ. 1)
-
+
-
+
Σ G/B V
O
EN
V
IN
+
V
IN
-
V
REF
FB
R
G
R
F
FIGURE 25.
Gain BW 200MHz=×
(EQ. 2)
EL5175, EL5375
