AD8698

Rev. 0 | Page 14 of 20

APPLICATIONS

INPUT OVERVOLTAGE PROTECTION

The AD8698 has internal protective circuitry which allows

voltages at either input to exceed the supply voltage. However,

if voltages applied at either input exceed the supply voltage by

more than 2 V, it is recommended to use a resistor in series

with the inputs to limit the input current and prevent damaging

the device.

The value of the resistor can be calculated from the following

formula:

mA

5

500

≤

+

−

S

S

IN

R

V

V

DRIVING CAPACITIVE LOADS

The AD8698 is stable even when driving heavy capacitive

loads in any configuration. Although the AD8698 will safely

drive capacitive loads well over 10 nF, it is recommended to

use external compensation should the amplifier be subjected

to driving a load exceeding 50 nF. This is particularly

important in positive unity gain configurations, the worst

case for stability. Figure 49 shows the output of the AD8698

with a 68 nF load in response to a 400 mV signal at its

positive input; the overshoot is less than 25% without any

external compensation. Using a simple “snubber” network

reduces the overshoot to less than 10% as shown in

Figure

50.

TIME (10

µs/DIV)

Figure 49. Heavy Capacitive Load Drive without Compensation

TIME (10

µs/DIV)

Figure 50. Compensated Capacitive Load Drive with Snubber

The snubber network consists of a simple RC network

whose values are determined empirically.

V+

V–

400mV

+

–

Figure 51. Snubber Network

Table 5

provides a few starting values for optimum

compensation.

Table 5. Compensation Values

47

20

7

68

30

5

100

50

3

The use of the snubber network does not recover the loss of

bandwidth incurred by the load capacitance. The AD8698

maintains a unity gain bandwidth of 1 MHz with load

capacitances of up to 1 nF.