EL2071, EL2171
the EL2071 is connected in a gain of +7 configuration and is
disabled while the analog bus is driven externally to +5V.
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.
Pin 2 is consequently at +0.71V, and if V is driven to -5V,
IN
then 5.71V appears between pins 3 and 2. Internally, this
voltage appears across a forward biased V in series with
BE
a reverse biased V and is past the threshold for zenering
BE
the reverse biased V . In a typical application, a 50Ω or
BE
75Ω terminating resistor from pin 3 to ground will prevent
pin 3 from approaching -5V.
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.
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.
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.
When the EL2071 is disabled, the DC output impedance is
> 100kΩ in parallel with 2pF capacitance.
To operate properly, the decoder that is used must have a
V
> (V +) - 0.4V with I
= 750µA, and should be
OH
OH
S
Disable/Enable Operation for EL2071
connected to the same power supply as the 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
brought to within 0.4V of pin 7 (V +), the EL2071 is disabled.
S
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.
Since V
is temperature dependent, it is recommended
DIS
that 5V CMOS logic (with a V
> 4.6V sourcing > 750µA
OH
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,
9