AD815 datasheet(11/16 Pages) AD

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AD815

REV. B

–11–

DC ERRORS AND NOISE

There are three major noise and offset terms to consider in

a current feedback amplifier. For offset errors refer to the

equation below. For noise error the terms are root-sum-squared

to give a net output error. In the circuit below (Figure 42), they

are input offset (VIO) which appears at the output multiplied by

the noise gain of the circuit (1 + RF/RG), noninverting input

current (IBN

× R

N) also multiplied by the noise gain, and the

inverting input current, which when divided between RF and RG

and subsequently multiplied by the noise gain always appear at

the output as IBI

× R

F. The input voltage noise of the AD815 is

less than 2 nV/

√Hz. At low gains though, the inverting input

current noise times RF is the dominant noise source. Careful

layout and device matching contribute to better offset and

drift specifications for the AD815 compared to many other

current feedback amplifiers. The typical performance curves

in conjunction with the equations below can be used to predict

the performance of the AD815 in any application.

OUT = VIO ×









± I

BN × RN ×









± I

BI × RF

I BI

I BN

VOUT

Figure 42. Output Offset Voltage

POWER CONSIDERATIONS

The 500 mA drive capability of the AD815 enables it to drive

a 50

Ω load at 40 V p-p when it is configured as a differential

driver. This implies a power dissipation, PIN, of nearly 5 watts.

To ensure reliability, the junction temperature of the AD815

should be maintained at less than 175

°C. For this reason,

the AD815 will require some form of heat sinking in most

applications. The thermal diagram of Figure 43 gives the

basic relationship between junction temperature (TJ) and

various components of

JA.

AIN

Equation 1

θA (JUNCTION TO

DIE MOUNT)

B (DIE MOUNT

TO CASE)

A + θ B = θ JC

CASE

WHERE:

PIN = DEVICE DISSIPATION

TA = AMBIENT TEMPERATURE

TJ = JUNCTION TEMPERATURE

JC = THERMAL RESISTANCE – JUNCTION TO CASE

CA = THERMAL RESISTANCE – CASE TO AMBIENT

Figure 43. A Breakdown of Various Package Thermal

Resistances

Figure 44 gives the relationship between output voltage swing

into various loads and the power dissipated by the AD815 (PIN).

This data is given for both sine wave and square wave (worst

case) conditions. It should be noted that these graphs are for

mostly resistive (phase <

±10°) loads. When the power dissipation

requirements are known, Equation 1 and the graph on Figure 45

can be used to choose an appropriate heat sinking configuration.

VOUT – Volts p-p

RL = 50

RL = 100

RL = 200

f = 1kHz

SQUARE WAVE

SINE WAVE

Figure 44. Total Power Dissipation vs. Differential Output

Voltage

Normally, the AD815 will be soldered directly to a copper pad.

Figure 45 plots

JA against size of copper pad. This data pertains

to copper pads on both sides of G10 epoxy glass board connected

together with a grid of feedthroughs on 5 mm centers.

This data shows that loads of 100 ohms or less will usually not

require any more than this. This is a feature of the AD815’s

15-lead power SIP package.

An important component of

JA is the thermal resistance of the

package to heatsink. The data given is for a direct soldered

connection of package to copper pad. The use of heatsink

grease either with or without an insulating washer will increase

this number. Several options now exist for dry thermal connec-

tions. These are available from Bergquist as part # SP600-90.

Consult with the manufacturer of these products for details of

their application.

COPPER HEAT SINK AREA (TOP AND BOTTOM) – mm2

2.5k

0.5k

1.5k

AD815AVR, AY

(θ

JC = 2 C/W)

Figure 45. Power Package Thermal Resistance vs. Heat

Sink Area

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