10

FN6631.0

June 5, 2009

Applications Information

Functional Description

The ISL28107 and ISL28207 are single and dual, very low

1/f noise (14nV/

√Hz @ 10Hz) precision op-amps. These

amplifiers feature very high open loop gain (50kV/mV) for

excellent CMRR (145dB), and gain accuracy. Both devices

are fabricated in a new precision 40V complementary bipolar

DI process.

The super-beta NPN input stage with bias current cancellation

provides bipolar-like levels of AC performance with the low

input bias currents approaching JFET levels. The temperature

stabilization provided by bias current cancellation removes the

high input bias current temperature coefficient commonly

found in JFET amplifiers. Figures 7 and 8 show the input bias

current variation over temperature.

voltage and temperature with the common mode voltage at

0V for split supply operation.

The complimentary bipolar output stage maintains stability

driving large capacitive loads (to 10nF) without external

compensation. The small signal overshoot vs. load

capacitance is shown in Figure 28.

Operating Voltage Range

The devices are designed to operate over the 4.5V (±2.25V)

to 40V (±20V) range and are fully characterized at 10V (±5V)

and 30V (±15V). Both DC and AC performance remain

virtually unchanged over the complete 4.5V to 40V operating

voltage range. Parameter variation with operating voltage is

shown in the “Typical Performance Curves” beginning on

page 4. The input common mode voltage range sensitivity to

temperature is shown in Figure 9 (±15V).

Input ESD Diode Protection

The input terminals (IN+ and IN-) each have internal ESD

protection diodes to the positive and negative supply rails, a

series connected 500

Ω current limiting resistor followed by an

anti-parallel diode pair across the input NPN transistors

(Circuit 1 in “Pin Descriptions” on page 9).

The resistor-ESD diode configuration enables a wide

differential input voltage range equal to the lesser of the

Maximum Supply Voltage in the “Absolute Maximum

Ratings” on page 2 (42V) or, a maximum of 0.5V beyond the

V+ and V- supply voltage. The internal protection resistors

eliminate the need for external input current limiting resistors

in unity gain connections and other circuit applications where

large voltages or high slew rate signals are present. Although

the amplifier is fully protected, high input slew rates that

exceed the amplifier slew rate (±0.32V/µs) may cause output

distortion.

Output Current Limiting

The output current is internally limited to approximately

±40mA at +25°C and can withstand a short circuit to either

rail as long as the power dissipation limits are not exceeded.

This applies to only 1 amplifier at a time for the dual op-amp.

Continuous operation under these conditions may degrade

long term reliability.

Output Phase Reversal

Output phase reversal is a change of polarity in the amplifier

transfer function when the input voltage exceeds the supply

voltage. The ISL28107 and ISL28207 are immune to output

phase reversal, even when the input voltage is 1V beyond

the supplies.

Using Only One Channel

The ISL28207 is a dual op-amp. If the application only

requires one channel, the user must configure the unused

channel to prevent it from oscillating. The unused channel

will oscillate 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

inverting input and ground the positive input (as shown in

Figure 29).

Power Dissipation

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

temperatures under certain load and power supply

conditions. It is therefore important to calculate the

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 using

Equation 1:

where:

Equation 2:

FIGURE 29. PREVENTING OSCILLATIONS IN UNUSED

CHANNELS

-

+

T

JMAX

T

MAX

θ

+

=

(EQ. 1)

PD

MAX

V

S

I

qMAX

V

S

(

- V

V

OUTMAX

R

L

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

×

+

×

=

(EQ. 2)

ISL28107, ISL28207