CLC449AJE Datasheet_PDF文件下载_芯片资料查询

I

BN

+

R

eq1

V

os

CLC449

-

I

BI

R

f

R

eq2

V

o

R

L

Matching the output transmission line over greater

frequency ranges is accomplished by placing C

6

in

parallel with R

6

, reducing the output impedance to

compensate for the internal increase of the op-amp鈥檚 out-

put impedance with frequency.

Thermal Design

To calculate the power dissipation for the CLC449,

follow these steps:

Figure 5: DC Offset Model

DC Design (Output Loading)

R

L

, R

f

, and R

g

load the op amp output. The equivalent

closed-loop load impedance seen by the output in Figure

5 is:

鈥?/div>

Calculate the no-load op amp power:

P

amp

= I

cc

鈥?/div>

(V

cc

鈥?V

ee

)

鈥?/div>

Calculate the output stage鈥檚 RMS power:

P

o

= (V

cc

鈥?V

load

)

鈥?/div>

I

load

where V

load

and I

load

are the RMS voltage and

current across the external load.

鈥?/div>

R

L_eq

= R

L

|| (R

f

+ R

eq2

), non-inverting gain

鈥?/div>

R

L_eq

= R

L

|| R

f

, inverting gain

R

L_eq

needs to be kept large enough so that the

minimum available output current can produce the

required output voltage swing.

Capacitive Loads

Capacitive loads, such as found in A/D converters,

require a series resistor (R

s

) in the output to improve set-

tling performance. The

R

s

and Settling Time vs. C

L

plot

in the

Typical Performance Characteristics

section

provides the information for selecting this resistor.

Also, use a series resistor to reduce the effects of

reactive loads on amplifier loop dynamics. For instance,

driving coaxial cables without an output series resistor

may cause peaking or oscillation.

Transmission Line Matching

One method for matching the characteristic impedance of

a transmission line is to place the appropriate resistor at

the input or output of the amplifier. Figure 6 shows the

typical circuit configurations for matching transmission

lines.

R

1

V

1

+

-

R

4

V

2

+

-

Z

0

Z

0

R

3

R

2

R

g

R

5

C

6

+

鈥?/div>

Calculate the total op amp RMS power:

P

t

= P

amp

+ P

o

To calculate the maximum allowable ambient tempera-

ture, solve the following equation: T

amb

= 175 鈥?P

t

鈥?/div>

胃

JA

,

where

胃

JA

is the thermal resistance from junction to

ambient in 掳C/W and T

amb

is in 掳C. Thermal resistance

for the various packages are found in the

Package

Thermal Resistance

section.

Dynamic Range (Input /Output Protection)

Input ESD diodes are present on all connected pins for

protection from static voltage damage. For a signal that

may exceed the supply voltages, we recommend using

diode clamps at the amplifier鈥檚 input to limit the signals to

less than the supply voltages.

Dynamic Range (Input /Output Levels)

The

Electrical Characteristics

section contains the

Common-Mode Input Range and Output Voltage

Range; these voltage ranges scale with the supplies.

Output Current is also specified in the

Electrical

Characteristics

section.

Unity gain applications are limited by the Common-Mode

Input Range. At greater non-inverting gains, the Output

Voltage Range becomes the limiting factor. Inverting gain

applications are limited by the Output Voltage Range.

For transimpedance or inverting gain applications, the

current (I

inv

) injected at the inverting input pin of the op

amp needs to be:

V

|I

inv

|

鈮?/div>

max

R

f

where V

max

is the Output Voltage Range.

The voltage ranges discussed above are achieved as

long as the equivalent output load is large enough so that

the output current can produce the required output

voltage swing. See the

DC Design (Output Loading)

sub-section for details.

Dynamic Range (Intermods)

For RF applications, the CLC449 specifies a third

order intercept of 30dBm at 70MHz and P

o

= 10dBm.

6

Z

0

R

6

CLC449

-

V

o

R

7

R

f

Figure 6: Transmission Line Matching

In non-inverting gain applications, R

g

is connected directly

to ground.

The resistors R

1

, R

2

, R

6

, and R

7

are equal to the characteristic impedance, Z

o

, of the

transmission line or cable.

In inverting gain applications, R

3

is connected directly to

ground. The resistors R

4

, R

6

, and R

7

are equal

to Z

o

. The parallel combination of R

5

and R

g

is also equal

to Z

o

.

The input and output matching resistors attenuate the

signal by a factor of 2, therefore additional gain is needed.

http://www.national.com

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