9

Capacitive Feedback

The EL2071/EL2171 rely on their feedback resistor for

proper compensation. A reduction of the impedance of the

feedback element results in less stability, eventually resulting

in oscillation. Therefore, circuit implementations which have

capacitive feedback should not be used because of the

capacitor's impedance reduction with frequency. Similarly,

oscillations can occur when using the technique of placing a

capacitor in parallel with the feedback resistor to

compensate for shunt capacitances from the inverting input

to ground.

Printed Circuit Layout

As with any high frequency device, good PCB layout is

necessary for optimum performance. Ground plane

construction is a requirement, as is good power-supply

bypassing close to the package. The inverting input is

sensitive to stray capacitance, therefore connections at the

inverting input should be minimal, close to the package, and

constructed with as little coupling to the ground plane as

possible.

Capacitance at the output node will reduce stability,

eventually resulting in peaking, and finally oscillation if the

capacitance is large enough. The design of the

EL2071/EL2171 allow a larger capacitive load than

comparable products, yet there are occasions when a series

resistor before the capacitance may be needed. Please refer

to the graphs to determine the proper resistor value needed.

Disable/Enable Operation for EL2071

The EL2071 has a disable/enable control input at pin 8. The

device is enabled and operates normally when pin 8 is left

open or returned to ground. When the voltage at pin 8 is

The output becomes a high impedance, the inverting input is

no longer driven to the positive input voltage, and the supply

current is reduced to less than 2.2.mA. There are internal

resistors which limit the current at pin 8 to a safe level

(~ ±500µA) if pin 8 is shorted to either supply.

Typically, analog and digital circuits should have separate

power supplies. This usually leads to slight differences

between the power supply voltages. The EL2071's disable

feature is dependent on the voltage at pins 8 and 7.

Therefore, to operate the disable feature of the EL2071

dependably over temperature, it is recommended that the

logic circuitry which drives pin 8 of the EL2071 operate from

the same +5V supply as the EL2071 to avoid voltage

differences between the digital and analog power supplies.

over temperature) be used to drive the disable pin of the

EL2071.

When disabled, (as well as in enabled mode), care must be

taken to prevent a differential voltage between the + and -

inputs greater than 5.0V. For example, in the figure below,

the EL2071 is connected in a gain of +7 configuration and is

disabled while the analog bus is driven externally to +5V.

then 5.71V appears between pins 3 and 2. Internally, this

75

Ω terminating resistor from pin 3 to ground will prevent

pin 3 from approaching -5V.

Using the EL2071 as a Multiplexer

An interesting use of the enable feature is to combine

several amplifiers in parallel with their outputs in common.

This combination then acts similar to a MUX in front of an

amplifier. A typical circuit is shown. The series resistance at

each output helps to further increase isolation between

amplifiers.

When the EL2071 is disabled, the DC output impedance is

>100k

Ω in parallel with 2pF capacitance.

To operate properly, the decoder that is used must have a

connected to the same power supply as the EL2071.

EL2071, EL2171