AD815

REV. B

–12–

Other Power Considerations

There are additional power considerations applicable to the

AD815. First, as with many current feedback amplifiers, there is an

increase in supply current when delivering a large peak-to-peak

voltage to a resistive load at high frequencies. This behavior is

affected by the load present at the amplifier’s output. Figure 12

summarizes the full power response capabilities of the AD815.

These curves apply to the differential driver applications (e.g.,

Figure 49 or Figure 53). In Figure 12, maximum continuous

peak-to-peak output voltage is plotted vs. frequency for various

resistive loads. Exceeding this value on a continuous basis can

damage the AD815.

The AD815 is equipped with a thermal shutdown circuit. This

circuit ensures that the temperature of the AD815 die remains

below a safe level. In normal operation, the circuit shuts down

the AD815 at approximately 180

°C and allows the circuit to

turn back on at approximately 140

°C. This built-in hysteresis

means that a sustained thermal overload will cycle between

power-on and power-off conditions. The thermal cycling

typically occurs at a rate of 1 ms to several seconds, depending

on the power dissipation and the thermal time constants of the

package and heat sinking. Figures 46 and 47 illustrate the

thermal shutdown operation after driving OUT1 to the + rail,

and OUT2 to the – rail, and then short-circuiting to ground

each output of the AD815. The AD815 will not be damaged by

momentary operation in this state, but the overload condition

should be removed.

10

0%

100

90

OUT 1

200 s

5V

OUT 2

Figure 46. OUT2 Shorted to Ground, Square Wave Is

10

0%

100

90

OUT 1

5ms

5V

OUT 2

Figure 47. OUT1 Shorted to Ground, Square Wave Is

Parallel Operation

To increase the drive current to a load, both of the amplifiers

within the AD815 can be connected in parallel. Each amplifier

should be set for the same gain and driven with the same signal.

In order to ensure that the two amplifiers share current, a small

resistor should be placed in series with each output. See Figure

48. This circuit can deliver 800 mA into loads of up to 12.5

Ω.

6

4

5

8

+15V

499

499

1

10

7

–15V

499

499

1

9

11

50

0.1 F

10 F

0.1 F

10 F

1/2

AD815

1/2

AD815

100

100

Figure 48. Parallel Operation for High Current Output

Differential Operation

Various circuit configurations can be used for differential

operation of the AD815. If a differential drive signal is avail-

able, the two halves can be used in a classic instrumentation

configuration to provide a circuit with differential input and

output. The circuit in Figure 49 is an illustration of this. With

the resistors shown, the gain of the circuit is 11. The gain can

provides no common-mode rejection.

6

4

5

8

+15V

10

7

–15V

499

9

11

0.1 F

10 F

100

499

0.1 F

10 F

1/2

AD815

1/2

AD815

100

100

+IN

–IN

OUT 1

OUT 2

Figure 49. Fully-Differential Operation

Creating Differential Signals

If only a single ended signal is available to drive the AD815 and

a differential output signal is desired, several circuits can be

used to perform the single-ended-to-differential conversion.

One circuit to perform this is to use a dual op amp as a

predriver that is configured as a noninverter and inverter. The

circuit shown in Figure 50 performs this function. It uses an

AD826 dual op amp with the gain of one amplifier set at +1 and

the gain of the other at –1. The 1 k

Ω resistor across the input

terminals of the follower makes the noise gain (NG = 1) equal

to the inverter’s. The two outputs then differentially drive the

inputs to the AD815 with no common-mode signal to first order.