9

FN6247.3

April 13, 2007

Applications Information

Product Description

The ISL28190 and ISL28290 are voltage feedback operational

amplifier designed for communication and imaging applications

requiring very low voltage and current noise. Both parts feature

low distortion while drawing moderately low supply current. The

ISL28190 and ISL28290 use a classical voltage-feedback

topology which allows them to be used in a variety of

applications where current-feedback amplifiers are not

appropriate because of restrictions placed upon the feedback

element used with the amplifier.

Enable/Power-Down

The ISL28190 and ISL28290 amplifiers are disabled by

applying a voltage greater than 2V to the ENABLE pin, with

respect to the V- pin. In this condition, the output(s) will be in

a high impedance state and the amplifier(s) current will be

reduced to 13µA/Amp. By disabling the part, multiple parts

can be connected together as a MUX. The outputs are tied

together in parallel and a channel can be selected by the

ENABLE pin. The ENABLE pin also has an internal pull

down. If left open, the ENABLE pin will pull to the negative

rail and the device will be enabled by default.

Input Protection

All input terminals have internal ESD protection diodes to both

positive and negative supply rails, limiting the input voltage to

within one diode beyond the supply rails. Both parts have

additional back-to-back diodes across the input terminals (as

shown in Figure 25). In pulse applications where the input

Slew Rate exceeds the Slew Rate of the amplifier, the

possibility exists for the input protection diodes to become

forward biased. This can cause excessive input current and

distortion at the outputs. If overdriving the inputs is necessary,

the external input current must never exceed 5mA. An

external series resistor may be used to limit the current as

shown in Figure 25.

Using Only One Channel

The ISL28290 is a Dual channel op-amp. If the application

only requires one channel when using the ISL28290, the

user must configure the unused channel to prevent it from

oscillating. Oscillation can occur if the input and output pins

are floating. This will result in higher than expected supply

currents and possible noise injection into the channel being

used. The proper way to prevent this oscillation is to short

the output to the negative input and ground the positive input

(as shown in Figure 26).

Current Limiting

The ISL28190 and ISL28290 have no internal current-

limiting circuitry. If the output is shorted, it is possible to

exceed the Absolute Maximum Rating for output current or

power dissipation, potentially resulting in the destruction of

the device. This is why output short circuit current is

Power Dissipation

It is possible to exceed the +125°C maximum junction

temperatures under certain load and power-supply

conditions. It is therefore important to calculate the

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 as follows:

where:

•

θ

application

FIGURE 25. LIMITING THE INPUT CURRENT TO LESS THAN

5mA

FIGURE 23. LIMITING THE INPUT CURRENT TO LESS THAN

5mA

-

+

R

FIGURE 26. PREVENTING OSCILLATIONS IN UNUSED

CHANNELS

-

+

T

JMAX

T

MAX

θ

()

+

=

PD

MAX

2*V

S

I

SMAX

V

S

(

- V

V

OUTMAX

R

L

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

×

+

×

=

ISL28190, ISL28290