AD815 datasheet(11/16 Pages) AD

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AD815

REV. C

–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 45), 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

× RN) 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

× RF. 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 45. 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 appli-

cations. The thermal diagram of Figure 46 gives the basic

relationship between junction temperature (TJ) and various

components of

θJA.

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

θ JC = THERMAL RESISTANCE – JUNCTION TO CASE

θ CA = THERMAL RESISTANCE – CASE TO AMBIENT

Figure 46. A Breakdown of Various Package Thermal

Resistances

Figure 47 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.

VOUT – Volts p-p

RL = 50

RL = 100

RL = 200

f = 1kHz

SQUARE WAVE

SINE WAVE

Figure 47. Total Power Dissipation vs. Differential Output

Voltage

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