AD815-EB Analog Devices, AD815-EB Datasheet - Page 12

• Manufacturer Part Number: AD815-EB
• Description: High Output Current Differential Driver
• Manufacturer: Analog Devices

AD815
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.
Figure 46. OUT2 Shorted to Ground, Square Wave Is
OUT1, R
Figure 47. OUT1 Shorted to Ground, Square Wave Is
OUT2, R
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
F
F
= 1 k , R
= 1 k , R
100
100
0%
90
0%
10
90
10
OUT 2
G
G
5V
5V
OUT 2
= 222
= 222
OUT 1
OUT 1
200 s
5ms
–12–
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 .
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
be changed by changing the value of R
provides no common-mode rejection.
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 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.
Figure 48. Parallel Operation for High Current Output
50
+IN
–IN
Figure 49. Fully-Differential Operation
V
IN
100
R
100
100
499
100
499
100
G
10
11
4
5
1/2
AD815
1/2
AD815
10
11
5
4
+15V
–15V
1/2
AD815
1/2
AD815
8
7
+15V
–15V
499
8
7
499
R
R
6
9
499
F
F
0.1 F
0.1 F
499
0.1 F
9
6
0.1 F
G
OUT 2
OUT 1
. This circuit, however,
10 F
10 F
10 F
10 F
R
L
1
1
V
OUT
REV. B
R
L