EL2082CS datasheet(9/14 Pages) INTERSIL

9 / 14 page

phase lag of the EL2082 detracts from the phase margin of

the op-amp, and some overall bandwidth reduction may

result. The EL2082 appears as a 3.0ns delay, well past

100MHz. Thus, for a 20MHz loop bandwidth, the EL2082 will

subtract 20MHzx 3.0nsx 360 degrees= 21.6 degrees. The

loop path should have at least 55 degrees of phase margin

for low ringing in this connection. Loop bandwidth is always

reduced by the ratio RIN/(RIN + RF) with voltage mode op-

amps.

Current-feedback op-amps again solve the summing-

junction capacitance problem in this connection. The loop

bandwidth here becomes a matter of transimpedance over

frequency and its phase characteristics. Unfortunately, this

is generally poorly documented in amplifier data sheets. A

rule of thumb is that the transimpedance falls to the value of

the recommended feedback resistor at a frequency of

F- 3dB/4 to F-3dB/2, where F-3dB is the unity-gain closed-

loop bandwidth of the amplifier. The phase margin of the op-

amp is usually close to 90° at this frequency.

In general, RF is initially the recommended value for the

particular amplifier and is then empirically adjusted for

amplifier stability at maximum VGAIN, then RIN is set for the

overall circuit gain required. Sometimes a very small CF can

be used to improve loop stability, but it often must be in

series with another resistor of value around RF/2.

A virtue of placing the EL2082 in feedback is that the input-

referred noise will drop as gain increases. This is ideal for

level controls that are used to set the output to a constant

level for a variety of inputs as well as AGC loops.

Furthermore, the EL2082 has a relatively constant input

signal amplitude for a variety of input levels, and its distortion

will be relatively constant and controllable by setting RF.

Note that placing the EL2082 in the feedback path causes

the circuit bandwidth to vary inversely with gain.

The next circuit shows use of the EL2082 in the feedback

path of a non-inverting op-amp:

This example has the same virtues with regards to noise and

distortion as the preceding circuit; and its bandwidth shrinks

with increasing gain as well. The typical 12pF sum of

EL2082 output capacitance in parallel with stray capacitance

necessitates the inclusion of CF to prevent a feedback pole.

Because of this 12pF capacitance at the op-amp -input,

current-feedback op-amps will generally not be useable. As

before, the loop bandwidth and phase margin must

accommodate the extra phase lag of the EL2082.

The VIN pin can be used instead of the IIN pin so:

This connection is useful when a high input impedance is

required. There are a few caveats when using the VIN pin.

The first is that VIN has a 250V/µs slew rate limitation. The

second is that the inevitable CSTRAY across RG causes a

gain zero and gain INCREASES above the 1/(2

π CSTRAY

RG) frequency and can peak as much as 20dB with large

CSTRAY. A graph of gain vs. frequency for several CSTRAYS

is included in the typical performance curves. In general, if

wide bandwidth and frequency flatness is desired, the IIN pin

should be used.

The VIN pin does make an excellent ground reference pin,

for instance when low-frequency noise is to be rejected. The

next schematic shows the EL2082 VIN pin rejecting possible

60Hz hum induced on an RF input cable:

This example shows VIN rejecting low-frequency field-

induced noise but not adding peaking since the 0.01µF

bypass capacitor shunts high-frequency signals to local

ground.

GAIN

--------

Ω

-------------------------







FIGURE 4. EL2082 IN FEEDBACK NON-INVERTING GAIN

OUT

-------------

-VG

-----------

Ω

--------------------------





FIGURE 5. THE VIN PIN USED AS SIGNAL INPUT

FIGURE 6. USING THE VIN PIN AS A GROUND REFERENCE

TO REJECT HUM AND NOISE

EL2082

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