13

FN7045.2

April 21, 2005

Applications Information

Product Description

The EL2125 is an ultra-low noise, wideband monolithic

operational amplifier built on Elantec's proprietary high

speed complementary bipolar process. It features

0.83nV/

√Hz input voltage noise, 200µV 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 EL2125 also has excellent DC

CMRR. These specifications allow the EL2125 to be used in

DC-sensitive applications such as difference amplifiers.

Gain-Bandwidth Product

The EL2125 has a gain-bandwidth product of 800MHz at

±5V. For gains greater than 20, its closed-loop -3dB

bandwidth is approximately equal to the gain-bandwidth

product divided by the small signal gain of the circuit. For

gains less than 20, higher-order poles in the amplifier's

transfer function contribute to even higher closed-loop

bandwidths. For example, the EL2125 has a -3dB bandwidth

of 175MHz at a gain of 10 and decreases to 40MHz 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 EL2125 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

FIGURE 50.

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 the in typical performance curves section

of this data sheet.

Pin Descriptions

5-PIN SOT-23

8-PIN SO

PIN NAME

PIN FUNCTION

EQUIVALENT CIRCUIT

16

VOUT

Output

CIRCUIT 1

2

4

VS-

Supply

3

3

VINA+

Input

CIRCUIT 2

4

2

VINA-

Input

Reference Circuit 2

5

7

VS+

Supply

-

+

R

C

EL2125