APPLICATION NOTES

HEAT SINKING

To determine if a heat sink is necessary for your application and

if so, what type, refer to the thermal model and governing equation

below.

Governing Equation:

This example demonstrates a worst case analysis for the op-amp

output stage. This occurs when the output voltage is 1/2 the power

supply voltage. Under this condition, maximum power transfer oc-

curs and the output is under maximum stress.

Conditions:

Ω

For a worst case analysis we will treat the +8Vp sine wave

as an 8VDC output voltage.

1.) Find Driver Power Dissipation

=(16V-8V) (8V/100

Ω)

=0.64W

4.) R

5.) R

6.) Rearrange governing equation to solve for R

θSA

R

=((125°C -90°C)/0.64W) - 45°C/W - 0.15°C/W

=54.7 - 46.15

=8.5°C/W

Rev. C 9/06

3

Where

R

R

R

Potentiometer values ranging from 1K

Ω to 1MΩ can be used with

only a small amount of degradation (typically 0.15 to 0.25µV/°C) of

input offset voltage drift. If the input offset voltage is to be trimmed

to a value other than "zero", the following formula can be used to

approximate the change in input offset voltage drift:

Recommended External Component Selection Guide

APPROXIMATE

DESIRED GAIN

RI(+)

RI(-)

Rf(Ext)

Cf

-1

499

Ω

1K

Ω

1K

Ω

-2

330

Ω

499

Ω

1K

Ω

-5

169

Ω

200

Ω

1K

Ω

-8

100

Ω

124

Ω

1K

Ω

90.9

Ω

100

Ω

1K

Ω

-20

-10

100

Ω

100

Ω

2K

Ω

1 The positive input resistor is selected to minimize any bias current induced offset

voltage.

2 The feedback capacitor will help compensate for stray input capacitance. The value of

this capacitor can be dependent on individual applications. A 2 to 9 pf capacitor is

usually optimum for most applications.

Thermal Model:

The heat sink in this example must have a thermal resistance of

no more than 8.5°C/W to maintain a junction temperature of no

more than +125°C.

OFFSET NULL

Typically, the MSK 738(B) has an input offset voltage of less than

±25µV. If it is desirable to adjust the offset closer to "zero", or to

should be a ten-turn 10K

Ω potentiometer. Typical offset adjust is

±5mV.

2

2

2

2

2

2

1

1

1

1

1

1

1µf

1µf

1µf

1µf

1µf

1µf

C1