EL2126CSZ datasheet(15/19 Pages) RENESAS

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EL2126

FN7046 Rev 4.00

Page 15 of 19

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 VOS, 22µA IB,

0.4µA IOS, 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

RC

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.

VN is the amplifier input voltage noise

IN+ is the amplifier positive input current noise

IN- is the amplifier negative input current noise

VRX 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 R2 is reduced, the noise due to IN-, R2,

and R1 decreases and the noise caused by IN+, VN, and R3

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.

VIN

VOUT

FIGURE 50.

VON

VIN

IN+

IN-

VR3

VR2

VR1

FIGURE 51.

VRX

4kTRx

(EQ. 1)

VON

-------





 2



IN-

IN+

-------





 2



4K T R1 4KTR2

-------





 2



4KTR3

-------





 2

















(EQ. 2)

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