Data Sheet

AD8603/AD8607/AD8609

APPLICATIONS

analog.com

Rev. D | 13 of 17

NO PHASE REVERSAL

The AD8603/AD8607/AD8609 do not exhibit phase inversion even

when the input voltage exceeds the maximum input common-mode

voltage. Phase reversal can cause permanent damage to the ampli-

fier, resulting in system lockups. The AD8603/AD8607/AD8609 can

handle voltages of up to 1 V over the supply.

Figure 41. No Phase Response

INPUT OVERVOLTAGE PROTECTION

If a voltage 1 V higher than the supplies is applied at either input,

the use of a limiting series resistor is recommended. If both inputs

are used, each one should be protected with a series resistor.

To ensure good protection, the current should be limited to a maxi-

mum of 5 mA. The value of the limiting resistor can be determined

from the following equation:

(VIN − VS)/(RS + 200 Ω) ≤ 5 mA

(1)

DRIVING CAPACITIVE LOADS

The AD8603/AD8607/AD8609 are capable of driving large capaci-

tive loads without oscillating. Figure 42 shows the output of the

AD8603/AD8607/AD8609 in response to a 100 mV input signal,

with a 2 nF capacitive load.

Although it is configured in positive unity gain (the worst case), the

AD8603 shows less than 20% overshoot. Simple additional circuitry

can eliminate ringing and overshoot.

One technique is the snubber network, which consists of a series

RC and a resistive load (see Figure 43). With the snubber in

place, the AD8603/AD8607/AD8609 are capable of driving capaci-

tive loads of 2 nF with no ringing and less than 3% overshoot.

The use of the snubber circuit is usually recommended for unity

gain configurations. Higher gain configurations help improve the

stability of the circuit. Figure 44 shows the same output response

with the snubber in place.

Figure 42. Output Response to a 2 nF Capacitive Load, Without Snubber

Figure 43. Snubber Network

Figure 44. Output Response to a 2 nF Capacitive Load with Snubber

Optimum values for RS and CS are determined empirically; Table 5

lists a few starting values.

Table 5. Optimum Values for the Snubber Network

CL (pF)

RS (Ω)

CS (pF)

100 to ~500

500

680

1500

100

330

1600 to ~2000

400

100