AD795 datasheet(15/20 Pages) AD

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Data Sheet

AD795

Rev. D | Page 15 of 20

OVERLOAD ISSUES

Driving the amplifier output beyond its linear region causes

some sticking; recovery to normal operation is within 2 µs of

the input voltage returning within the linear range.

If either input is driven below the negative supply, the amplifier’s

output is driven high, causing a phenomenon called phase

reversal. Normal operation is resumed within 30 µs of the input

voltage returning within the linear range.

Figure 41 shows the AD795’s input bias currents vs. differential

input voltage. Picoamp level input current is maintained for

differential voltages up to several hundred millivolts. This

behavior is only important if the AD795 is in an open-loop

application where substantial differential voltages are produced.

10–4

10–5

10–6

10–7

10–8

10–9

10–10

10–11

10–12

10–13

10–14

–6

–5

–4

–3

–2

–1

DIFFERENTIAL INPUT VOLTAGE (±V)

+IIN

–IIN

Figure 41. Input Bias Current vs. Differential Input Voltage

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

supply, milliamp level currents can be produced through the

input terminals. That current should be limited to 10 mA for

transient 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 Figure 42 and Figure 43).

The protection resistor’s Johnson noise adds to the amplifier’s

input voltage noise and impacts the frequency response.

Driving the input terminals above the positive supply causes the

input current to increase and limit at 40 µA. This condition is

maintained until 15 V 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.

AD795

SOURCE

Figure 42. Inverter with Input Current Limit

AD795

SOURCE

Figure 43. Follower with Input Current Limit

Figure 44 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 measurably

increase the input current and capacitance.

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, Inc., op amps. This makes the AD795 sensitive

to handling and precautions should be taken to minimize ESD

exposure whenever possible.

AD795

PROTECTED DIODES

(LOW LEAKAGE)

SOURCE

Figure 44. Input Voltage Clamp with Diodes

AD795

10pF

OUTPUT

1GΩ

GUARD

PHOTODIODE

FILTERED

OUTPUT

OPTIONAL 26Hz

FILTER

Figure 45. AD795 Used as a Photodiode Preamplifier

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