Operational Amplifiers
 

Selection of Operational Amplifiers:

Test Parameter

Typ. Unit

Typical Description

Input Bias Current

µA

Each input of an operational amplifier has a certain amount of current that flows in or out of it. This is basically the leakage current of the input transistor, i.e., the base leakage current if the input transistor is bipolar, or the gate leakage current if it is a FET. This current is known as the input bias current, and is ideally zero.

Example of an Actual Spec:

AD829: 3.3 µAtyp.; 7 µA max.

Input Offset Current

nA

This is simply the mismatch or difference between the input bias currents flowing through the inputs. This is ideally zero.

Example of an Actual Spec:

AD829: 50 nAtyp.; 500 nA max.

Input Offset Voltage

mV

An ideal operational amplifier will give an output of 0V if both of its inputs are shorted together. A real-world op amp will have a non-zero voltage output even if its inputs are shorted together. This is the effect of its input offset voltage, which is the slight voltage present across its inputs brought about by its non-zero input offset current. In essence, the input voltage offset is also the voltage that needs to be applied across the inputs of an op amp to make its output zero.

Example of an Actual Spec:

AD712C: 0.1 mV typ.; 0.3 mV max.

Open-Loop Gain

V/mV

This is the ratio of the op amp's output voltage to its differential input voltage without any external feedback.

Example of an Actual Spec:

AD712: 150 V/mVmin.; 400 V/mV typ.

Gain-Bandwidth Product

MHz

This is the product of the op amp's open-loop voltage gain and the frequency at which it was measured.

Example of an Actual Spec:

AD829: 750 MHz for Vs=+/-15V

Slew Rate

V/µsec

This is the rate of change of the op amp's voltage output over time when its gain is set to unity (Gain =1).

Example of an Actual Spec:

AD712: 16 V/µsecmin.; 20 V/µsec typ.

Settling Time

nsec

This is the length of time for the output voltage of an operational amplifier to approach, and remain within, a certain tolerance of its final value. This is usually specified for a fast full-scale input step.

Example of an Actual Spec:

Settling time to 0.1% for a 10V step with Vs=+/-15V: 90 nsec

Common Mode Rejection (CMR)

dB

This is the ability of an operational amplifier to cancel out or reject any signals that are common to both inputs, and amplify any signals that are differential between them. Common mode rejection is the logarithmic expression of CMRR, i.e., CMR=20logCMRR. CMRR is simply the ratio of the differential gain to the common-mode gain.

Example of an Actual Spec:

AD829: 100 dBmin.; 120 dB typ.

Power Supply Rejection (PSR)

dB

PSR is a measure of an op amp’s ability to prevent its output from being affected by noise or ripples at the power supply. It is computed as the ratio of the change in the power supply voltage to the change in the op amp's output voltage (caused by the power supply change). It is often expressed in dB.

Example of an Actual Spec:

AD829: 98 dBmin.; 120 dB typ. for Vs=+/-4.5V to +/-18V

Phase Margin

degrees

An op amp will tend to oscillate at a frequency wherein the loop phase shift exceeds -180°, if this frequency is below the closed-loop bandwidth. The closed-loop bandwidth of a voltage-feedback op amp circuit is equal to the op amp's bandwidth at unity gain, divided by the circuit's closed loop gain.

The phase margin of an op amp circuit is the amount of additional phase shift at the closed loop bandwidth required to make the circuit unstable (i.e., phase shift + phase margin = -180°). As phase margin approaches zero, the loop phase shift approaches -180° and the op amp circuit approaches instability.

Typically, values of phase margin much less than 45° can cause problems such as "peaking" in frequency response, and overshoot or "ringing" in step response. In order to maintain conservative phase margin, the pole generated by capacitive loading should be at least a decade above the circuit's closed loop bandwidth.

Reference: www.analog.com

Example of an Actual Spec:

AD847: 50 degrees

AD829: 60 degrees

Input Voltage Range, Common Mode

V

This is the maximum voltage (negative or positive) that can be applied at both inputs of an operational amplifier at the same time, with respect to the ground.

Examples of Actual Specs:

AD712: +14.5V, -11.5 V typ. for Vs=+/-15 V;

AD844: +/- 10V for Vs=+/-15 V

Input Voltage Range, Differential

V

This is the maximum voltage (negative or positive) that can be applied across the two inputs of an operational amplifier.

Example of an Actual Spec: AD712: +/-20V

Output Voltage Swing

+/-V

This is the maximum output voltage that the op amp can deliver without saturation or clipping for a given load and operating supply voltage.

Example of an Actual Spec:

+/-11V min.; +/-13V typ. for R=1K; Vs=+/-15V

Input Resistance or Impedance, Differential

MW

This is the small-signal resistance between the two inputs (both ungrounded) of an op amp.

Example of an Actual Spec:

OP27C: 0.7 MW min.; 4 MW typ.

Input Resistance or Impedance, Common Mode

GW

Each input of an op amp has a resistance with respect to ground. The common mode input resistance of an op amp is the equivalent resistance value of the op amp's two input resistances in parallel. This is the resistance of the two inputs shorted together with respect to ground.

Example of an Actual Spec:

OP27C: 2 GW typ.

Output Resistance or Impedance

W

This is the small-signal resistance or impedance between the output of an op amp and ground.

Example of an Actual Spec: AD844: 15 W typ., open loop

Power Supply Range

V

This refers to the minimum and maximum values of supply voltages that the negative and positive supplies of an operational amplifier can accept.

Example of an Actual Spec:

AD712: +/- 4.5V min.; +/-18V max.

Quiescent Current

mA

This is the non-signal power supply current that the op amp will consume within a specified power supply voltage operating range.

Example of an Actual Spec:

AD712: 5 mA typ.; 6.8 mA max. for Vs=+/-15V

Total Power Dissipation

mW

The total DC power supplied to the op amp minus the power delivered by the op amp to its load.

Example of an Actual Spec:

OP27: 90-100 mW typ.; 140-170 mW max.

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