14
FN7045.3
May 4, 2007
Noise Calculations
The primary application for the EL2125 is to amplify very
small signals. To maintain the proper signal-to-noise ratio, it
is essential to minimize noise contribution from the amplifier.
Figure 51 below shows all the noise sources for all the
components around the amplifier.
FIGURE 51.
•V
N
is the amplifier input voltage noise
•I
N
+ is the amplifier positive input current noise
•I
N
- is the amplifier negative input current noise
•V
RX
is the thermal noise associated with each resistor:
where:
• k is Boltzmann's constant = 1.380658 x 10
-23
• T is temperature in degrees Kelvin (273+ °C)
The total noise due to the amplifier seen at the output of the
amplifier can be calculated by using the equation below
(Figure 52).
As the equation shows, to keep noise at a minimum, small
resistor values should be used. At higher amplifier gain
configuration where R
2
is reduced, the noise due to IN-, R
2
,
and R
1
decreases and the noise caused by IN+, VN, and R
3
starts to dominate. Because noise is summed in a root-mean-
squares method, noise sources smaller than 25% of the largest
noise source can be ignored. This can greatly simplify the
formula and make noise calculation much easier to calculate.
Output Drive Capability
The EL2125 is designed to drive low impedance load. It can
easily drive 6V
P-P
signal into a 100Ω load. This high output
drive capability makes the EL2125 an ideal choice for RF, IF,
and video applications. Furthermore, the EL2125 is current-
limited at the output, allowing it to withstand momentary short to
ground. However, the power dissipation with output-shorted
cannot exceed the power dissipation capability of the package.
Driving Cables and Capacitive Loads
Although the EL2125 is designed to drive low impedance
load, capacitive loads will decrease the amplifier's phase
margin. As shown the in the performance curves, capacitive
load can result in peaking, overshoot and possible
oscillation. For optimum AC performance, capacitive loads
should be reduced as much as possible or isolated with a
series resistor between 5Ω to 20Ω. When driving coaxial
cables, double termination is always recommended for
reflection-free performance. When properly terminated, the
capacitance of the coaxial cable will not add to the capacitive
load seen by the amplifier.
Power Supply Bypassing And Printed Circuit
Board Layout
As with any high frequency devices, good printed circuit
board layout is essential for optimum performance. Ground
plane construction is highly recommended. Lead lengths
should be kept as short as possible. The power supply pins
must be closely bypassed to reduce the risk of oscillation.
The combination of a 4.7µF tantalum capacitor in parallel
with 0.1µF ceramic capacitor has been proven to work well
when placed at each supply pin. For single supply operation,
where pin 4 (V
S
-) is connected to the ground plane, a single
4.7µF tantalum capacitor in parallel with a 0.1µF ceramic
capacitor across pins 7 (V
S
+) and pin 4 (V
S
-) will suffice.
For good AC performance, parasitic capacitance should be
kept to a minimum. Ground plane construction again should
be used. Small chip resistors are recommended to minimize
series inductance. Use of sockets should be avoided since
they add parasitic inductance and capacitance which will
result in additional peaking and overshoot.
Supply Voltage Range and Single Supply
Operation
The EL2125 has been designed to operate with supply
voltage range of ±2.5V to ±15V. With a single supply, the
EL2125 will operate from +5V to +30V. Pins 4 and 7 are the
power supply pins. The positive power supply is connected
to pin 7. When used in single supply mode, pin 4 is
connected to ground. When used in dual supply mode, the
negative power supply is connected to pin 4.
As the power supply voltage decreases from +30V to +5V, it
becomes necessary to pay special attention to the input
voltage range. The EL2125 has an input voltage range of
0.4V from the negative supply to 1.2V from the positive
supply. So, for example, on a single +5V supply, the EL2125
has an input voltage range which spans from 0.4V to 3.8V.
The output range of the EL2125 is also quite large, on a +5V
supply, it swings from 0.4V to 3.6V.
-
+
V
ON
V
IN
I
N
+
I
N
-
R
2
R
3
R
1
V
N
V
R3
V
R2
V
R1
V
RX
4kTRx=
V
ON
BW= VN
2
1
R
1
R
2
-------+
⎝⎠
⎜⎟
⎛⎞
2
× IN-
2
R
1
2
IN+
2
R
3
2
1
R
1
R
2
-------+
⎝⎠
⎜⎟
⎛⎞
2
××+× 4KTR
1
4KTR
2
R
1
R
2
-------
⎝⎠
⎜⎟
⎛⎞
2
××××+××× 4KTR
3
1
R
1
R
2
-------+
⎝⎠
⎜⎟
⎛⎞
2
××××++ +
⎝ ⎠
⎜ ⎟
⎛ ⎞
×
FIGURE 52.
EL2125
