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:

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

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.

where:

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

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

C

FIGURE 50.

-

+

FIGURE 51.

V

RX

4kTRx

=

(EQ. 1)

V

ON

BW

=

VN

2

1

R

1

R

2

-------

+

⎝⎠

⎜⎟

×

IN-

2

R

1

2

IN+

2

R

3

2

1

R

1

R

2

-------

+

⎝⎠

⎜⎟

×

×

+

×

4K

T

R

1

4KTR

2

R

1

R

2

-------

⎝⎠

⎜⎟

×

×

×

×

+

×

×

×

4KTR

3

1

R

1

R

2

-------

+

⎝⎠

⎜⎟

×

×

×

×

++

+

⎝

⎠

⎜

⎟

⎛

⎞

×

(EQ. 2)

EL2126