AD795

REV. A

–12–

INPUT PROTECTION

The AD795 safely handles any input voltage within the supply

voltage range. Some applications may subject the input

terminals to voltages beyond the supply voltages—in these

cases, the following guidelines should be used to maintain the

AD795’s functionality and performance.

If the inputs are driven more than a 0.5 V below the minus sup-

ply, milliamp level currents can be produced through the input

terminals. That current should be limited to 10 mA for “tran-

sient” overloads (less than 1 second) and 1 mA for continuous

overloads, this can be accomplished with a protection resistor in

the input terminal (as shown in Figures 40 and 41). The pro-

tection resistor’s Johnson noise will add to the amplifier’s input

voltage noise and impact the frequency response.

Driving the input terminals above the positive supply will cause

the input current to increase and limit at 40

µA. This condition

is maintained until 15 volts above the positive supply—any

input voltage within this range does not harm the amplifier.

Input voltage above this range causes destructive breakdown

and should be avoided.

6

3

2

SOURCE

AD795

R

F

C

F

R

P

Figure 40. Inverter with Input Current Limit

6

2

3

SOURCE

AD795

Figure 41. Follower with Input Current Limit

Figure 42 is a schematic of the AD795 as an inverter with an

input voltage clamp. Bootstrapping the clamp diodes at the

inverting input minimizes the voltage across the clamps and

keeps the leakage due to the diodes low. Low leakage diodes

(less than 1 pA), such as the FD333s should be used, and

should be shielded from light to keep photocurrents from being

generated. Even with these precautions, the diodes will mea-

surably increase the input current and capacitance.

In order to achieve the low input bias currents of the AD795, it

is not possible to use the same on-chip protection as used in

other Analog Devices op amps. This makes the AD795

sensitive to handling and precautions should be taken to

minimize ESD exposure whenever possible.

6

3

2

SOURCE

AD795

R

F

PROTECT DIODES

(LOW LEAKAGE)

Figure 42. Input Voltage Clamp with Diodes