15
FN7046.4
May 2, 2007
Applications Information
Product Description
The EL2126 is an ultra-low noise, wideband monolithic
operational amplifier built on Elantec's proprietary high
speed complementary bipolar process. It features 1.3nV/√Hz
input voltage noise, 200µV typical offset voltage, and 73dB
THD. It is intended for use in systems such as ultrasound
imaging where very small signals are needed to be
amplified. The EL2126 also has excellent DC specifications:
200µV V
OS
, 22µA IB, 0.4µA I
OS
, and 106dB CMRR. These
specifications allow the EL2126 to be used in DC-sensitive
applications such as difference amplifiers.
Gain-Bandwidth Product
The EL2126 has a gain-bandwidth product of 650MHz at
±5V. For gains less than 20, higher-order poles in the
amplifier's transfer function contribute to even higher closed-
loop bandwidths. For example, the EL2126 has a -3dB
bandwidth of 100MHz at a gain of 10 and decreases to
33MHz at gain of 20. It is important to note that the extra
bandwidth at lower gain does not come at the expenses of
stability. Even though the EL2126 is designed for gain ≥ 10.
With external compensation, the device can also operate at
lower gain settings. The RC network shown in Figure 50
reduces the feedback gain at high frequency and thus
maintains the amplifier stability. R values must be less than
RF divided by 9 and 1 divided by 2
πRC must be less than
200MHz.
Choice of Feedback Resistor, RF
The feedback resistor forms a pole with the input
capacitance. As this pole becomes larger, phase margin is
reduced. This increases ringing in the time domain and
peaking in the frequency domain. Therefore, RF has some
maximum value which should not be exceeded for optimum
performance. If a large value of RF must be used, a small
capacitor in the few pF range in parallel with RF can help to
reduce this ringing and peaking at the expense of reducing
the bandwidth. Frequency response curves for various RF
values are shown in the typical performance curves section
of this data sheet.
Noise Calculations
The primary application for the EL2126 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 shows all the noise sources for all the components
around the amplifier.
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 2.
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.
-
+
R
F
R
C
V
IN
V
OUT
FIGURE 50.
-
+
V
ON
V
IN
I
N
+
I
N
-
R
2
R
3
R
1
V
N
V
R3
V
R2
V
R1
FIGURE 51.
V
RX
4kTRx=
(EQ. 1)
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
××××++ +
⎝ ⎠
⎜ ⎟
⎛ ⎞
×
(EQ. 2)
EL2126
