AD8615/AD8616/AD8618

Rev. E | Page 11 of 20

APPLICATIONS INFORMATION

INPUT OVERVOLTAGE PROTECTION

The AD8615/AD8616/AD8618 have internal protective circuitry

that allows voltages exceeding the supply to be applied at the input.

It is recommended, however, not to apply voltages that exceed

the supplies by more than 1.5 V at either input of the amplifier.

If a higher input voltage is applied, series resistors should be

used to limit the current flowing into the inputs.

The input current should be limited to <5 mA. The extremely

low input bias current allows the use of larger resistors, which

allows the user to apply higher voltages at the inputs. The use of

these resistors adds thermal noise, which contributes to the overall

output voltage noise of the amplifier.

For example, a 10 kΩ resistor has less than 13 nV/√Hz of

thermal noise and less than 10 nV of error voltage at room

temperature.

OUTPUT PHASE REVERSAL

The AD8615/AD8616/AD8618 are immune to phase inversion,

a phenomenon that occurs when the voltage applied at the input of

the amplifier exceeds the maximum input common mode.

Phase reversal can cause permanent damage to the amplifier

and can create lock ups in systems with feedback loops.

TIME (2ms/DIV)

Figure 36. No Phase Reversal

DRIVING CAPACITIVE LOADS

Although the AD8615/AD8616/AD8618 are capable of driving

capacitive loads of up to 500 pF without oscillating, a large amount

of overshoot is present when operating at frequencies above

100 kHz. This is especially true when the amplifier is configured

in positive unity gain (worst case). When such large capacitive

loads are required, the use of external compensation is highly

recommended.

This reduces the overshoot and minimizes ringing, which in

turn improves the frequency response of the AD8615/AD8616/

AD8618. One simple technique for compensation is the snubber,

which consists of a simple RC network. With this circuit in place,

output swing is maintained and the amplifier is stable at all gains.

Figure 38 shows the implementation of the snubber, which

reduces overshoot by more than 30% and eliminates ringing

that can cause instability. Using the snubber does not recover

the loss of bandwidth incurred from a heavy capacitive load.

TIME (2µs/DIV)

Figure 37. Driving Heavy Capacitive Loads Without Compensation

V+

200Ω

500pF

500pF

V–

200mV

–

+

–

Figure 38. Snubber Network

TIME (10µs/DIV)

Figure 39. Driving Heavy Capacitive Loads Using the Snubber Network