11

FN7504.6

March 17, 2008

effects of the disabled amplifiers’ feedback networks must be

considered when evaluating the active amplifier’s

performance in Mux Amp configurations.

Note that feed through from the IN+ to IN- pins occurs on

any Mux Amp disabled channel where the input differential

best suited for small signal applications. In any application

where two or more amplifier outputs are muxed, use series

the feed through current low enough to minimize the impact

on the active channel. See “Usage Implications” on page 11

for more details.

IN+ and IN- Input Protection

In addition to ESD protection diodes to each supply rail, the

EL8178 has additional back-to-back protection diodes across

the differential input terminals. If the magnitude of the

these diodes will conduct. For elevated temperatures, the

leakage of the protection diodes (see Circuit 1 in “Pin

Descriptions” on page 10) increases, resulting in the increase

USAGE IMPLICATIONS

If the input differential voltage is expected to exceed 0.5V, an

external current limiting resistor must be used to ensure the

input current never exceeds 5mA. For noninverting unity gain

applications, the current limiting can be via a series IN+ resistor,

or via a feedback resistor of appropriate value. For other gain

configurations, the series IN+ resistor is the best choice, unless

sufficiently large to limit the input current to 5mA.

Large differential input voltages can arise from several

sources:

1. During open loop (comparator) operation. The IN+ and

IN- input voltages don’t track.

2. When the amplifier is disabled but an input signal is still

the varying IN+ signal creates a differential voltage. Mux

Amp applications are similar, except that the active

3. When the slew rate of the input pulse is considerably

up with the IN+ signal, a differential voltage results, and

visible distortion occurs on the input and output signals.

To avoid this issue, keep the input slew rate below

0.2V/µs, or use appropriate current limiting resistors.

Large (>2V) differential input voltages can also cause an

EN I

nput Protection

The EN input has internal ESD protection diodes to both the

positive and negative supply rails, limiting the input voltage

range to within one diode beyond the supply rails

(see “Circuit 2” diagram on page 9). If the input voltage is

expected to exceed V+ or V-, then an external series resistor

should be added to limit the current to 5mA.

Output Current Limiting

The EL8178 has no internal current-limiting circuitry. If the

output is shorted, it is possible to exceed the “Absolute

Maximum Rating” for “operating junction temperature”,

potentially resulting in the destruction of the device.

Power Dissipation

It is possible to exceed the +150°C maximum junction

applications to determine if power supply voltages, load

conditions, or package type need to be modified to remain in

the safe operating area. These parameters are related in

Equation 1:

where:

•

θ

application

Proper Layout Maximizes Precision

To achieve the optimum levels of high input impedance

(i.e., low input currents) and low offset voltage, care should

be taken in the circuit board layout. The PC board surface

must remain clean and free of moisture to avoid leakage

currents between adjacent traces. Surface coating of the

circuit board will reduce surface moisture and provide a

humidity barrier, reducing parasitic resistance on the board.

When input leakage current is a paramount concern, the use

of guard rings around the amplifier inputs will further reduce

leakage currents. Figure 31 shows a guard ring example for

a unity gain amplifier that uses the low impedance amplifier

output at the same voltage as the high impedance input to

eliminate surface leakage. The guard ring does not need to

be a specific width, but it should form a continuous loop

around both inputs. For further reduction of leakage

currents, mount components to the PC board using Teflon

standoffs.

T

JMAX

T

MAX

θ

()

+

=

(EQ. 1)

PD

MAX

V

S

I

SMAX

V

S

(

- V

V

OUTMAX

R

L

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

×

+

×

=

(EQ. 2)

EL8178