AD815
REV. A
–12–
Other Power Considerations
T here 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. T his behavior is
affected by the load present at the amplifier’s output. Figure 12
summarizes the full power response capabilities of the AD815.
T hese 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.
T he AD815 is equipped with a thermal shutdown circuit. T his
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. T his built-in hysteresis
means that a sustained thermal overload will cycle between
power-on and power-off conditions. T he 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 OUT 1 to the + rail,
and OUT 2 to the – rail, and then short-circuiting to ground
each output of the AD815. T he AD815 will not be damaged by
momentary operation in this state, but the overload condition
should be removed.
10
0%
100
90
OUT 1
200μs
5V
OUT 2
Figure 46. OUT2 Shorted to Ground, Square Wave Is
OUT1, R
F
= 1 k
, R
G
= 222
10
0%
100
90
OUT 1
5ms
5V
OUT 2
Figure 47. OUT1 Shorted to Ground, Square Wave Is
OUT2, R
F
= 1 k
, R
G
= 222
Parallel Operation
T o 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. T his circuit can deliver 800 mA into loads of up to 12.5
.
6
4
5
8
+15V
499
499
1
10
7
–15V
499
499
1
R
L
9
11
50
0.1μF
10μF
0.1μF
10μF
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 available,
the two halves can be used in a classic instrumentation config-
uration to provide a circuit with differential input and output.
T he circuit in Figure 49 is an illustration of this. With the
resistors shown, the gain of the circuit is 11. T he gain can be
changed by changing the value of R
G
. T his circuit, however,
provides no common-mode rejection.
6
4
5
8
+15V
10
7
–15V
R
F
499
R
L
9
11
0.1μF
10μF
R
G
100
R
F
499
0.1μF
10μF
1/2
AD815
V
OUT
V
IN
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 pre-
driver that is configured as a noninverter and inverter. T he
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. T he 1 k
resistor across the input
terminals of the follower makes the noise gain (NG = 1) equal
to the inverter’s. T he two outputs then differentially drive the
inputs to the AD815 with no common-mode signal to first order.