AD546KN 查看數據表（PDF） - Analog Devices

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AD546KN

1 pA Monolithic Electrometer Operational Amplifier

Analog Devices 

input offset voltage drift with temperature is not affected. How-

ever, variation of the power supply voltages will cause offset

shifts.

AD546

Figure 32. Inverter Pulse Response with 1 MΩ Source and

Feedback Resistance

Figure 30. Alternate Offset Null Circuit for Inverter

AC RESPONSE WITH HIGH VALUE SOURCE AND

FEEDBACK RESISTANCE

Source and feedback resistances greater than 100 kΩ will

magnify the effect of input capacitances (stray and inherent to

the AD546) on the ac behavior of the circuit. The effects of

common-mode and differential-input capacitances should be

taken into account since the circuit’s bandwidth and stability

can be adversely affected.

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

mode capacitance, CS (including capacitance due to board and

capacitance inherent to the AD546), form a pole that limits cir-

cuit bandwidth to 1/2 π RSCS. Figure 31 shows the follower

pulse response from a 1 MΩ source resistance with the

amplifier’s input pin isolated from the board, only the effect of

the AD546’s input common-mode capacitance is seen.

Figure 33. 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 AD546 is

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

than the negative supply voltage. Exceeding this range will de-

grade the amplifier’s CMRR. Driving the common-mode volt-

age above the positive supply will cause the amplifier’s output to

saturate at the upper limit of 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 volt-

age within 1 V of the negative supply causes phase reversal of

the output signal. In this case, normal operation is typically

resumed within 0.5 ms of the input voltage returning within

range.

DIFFERENTIAL INPUT VOLTAGE OVERLOAD

A plot of the AD546’s input current versus differential input

voltage (defined as VIN+ –VIN–) appears in Figure 34. The

Figure 31. Follower Pulse Response from 1 MΩ Source

Resistance

In an inverting configuration, the differential input capacitance

forms a pole in the circuit’s loop transmission. 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, and the input pin

isolated from the board appears in Figure 32. Figure 33 shows

the response of the same circuit with a 1 pF feedback capaci-

tance. Typical differential input capacitance for the AD546

is 1 pF.

Figure 34. Input Current vs. Differential Input Voltage

REV. A

–9–

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