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AD549S

Ultralow Input Bias Current Operational Amplifier

Analog Devices 

AD549

AC RESPONSE WITH HIGH VALUE SOURCE AND

FEEDBACK RESISTANCE

Source and feedback resistances greater than 100 kΩ magnify

the effect of the input capacitances (stray and inherent to

the AD549) on the ac behavior of the circuit. The effects of

common-mode and differential input capacitances should be

taken into account because the circuit bandwidth and stability

can be adversely affected.

10mV

5µs

In an inverting configuration, the differential input capacitance

forms a pole in the loop transmission of the circuit. This can

create peaking in the ac response and possible instability. A

feedback capacitance can be used to stabilize the circuit. The

inverter pulse response with RF and RS equal to 1 MΩ appears

in Figure 34. Figure 35 shows the response of the same circuit

with a 1 pF feedback capacitance. Typical differential input

capacitance for the AD549 is 1 pF.

10mV

5µs

Figure 32. Follower Pulse Response from 1 MΩ Source Resistance,

Case Not Bootstrapped

10mV

5µs

Figure 34. Inverter Pulse Response with 1 MΩ Source

and Feedback Resistance

10mV

5µs

Figure 33. Follower Pulse Response from 1 MΩ Source Resistance,

Case Bootstrapped

In a follower, the source resistance and input common-mode

capacitance form a pole that limits the bandwidth to ½πRSCS.

Bootstrapping the metal case by connecting Pin 8 to the output

minimizes capacitance due to the package. Figure 32 and Figure 33

show the follower pulse response from a 1 MΩ source resistance

with and without the package connected to the output. Typical

common-mode input capacitance for the AD549 is 0.8 pF.

Figure 35. Inverter Pulse Response with 1 MΩ Source

and Feedback Resistance, 1 pF Feedback Capacitance

COMMON-MODE INPUT VOLTAGE OVERLOAD

The rated common-mode input voltage range of the AD549 is

from 3 V less than the positive supply voltage to 5 V greater than

the negative supply voltage. Exceeding this range degrades the

CMRR of the amplifier. Driving the common-mode voltage above

the positive supply causes the amplifier output to saturate at the

upper limit of the output voltage. Recovery time is typically 2 μs

after the input has been returned to within the normal operating

range. Driving the input common-mode voltage within 1 V of the

negative supply causes phase reversal of the output signal. In this

case, normal operation typically resumes within 0.5 μs of the

input voltage returning within range.

Rev. H | Page 12 of 20

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