MT-040| Application Note

MT-040 TUTORIAL
Op Amp Input Impedance

VOLTAGE FEEDBACK (VFB) OP AMP INPUT IMPEDANCE Voltage feedback (VFB) op amps normally have both differential and common-mode input impedances specified. Current feedback (CFB) op amps normally specify the impedance to ground at each input. Different models may be used for different voltage feedback op amps, but in the absence of other information, it is usually safe to use the model in Figure 1 below. In this model the bias currents flow into the inputs from infinite impedance current sources.
IB+ Zdiff + INPUT IB INPUT

Zcm+

Zcm

Zcm+ and Zcm are the common-mode input impedance. The figure on the data sheet is for one, not both, but they are approximately equal. Zdiff is the differential input impedance. They are high resistance (105 - 1012) in parallel with a small shunt capacitance (sometimes as high as 25pF). In most practical circuits, Zcm is swamped by negative feedback.

Figure 1: Input Impedance (Voltage Feedback Op Amp) The common-mode input impedance data sheet specification (Zcm+ and Zcm ) is the impedance from either input to ground (NOT from both to ground). The differential input impedance (Zdiff) is the impedance between the two inputs. These impedances are usually resistive and high (1051012 ) with some shunt capacitance (generally a few pF, but sometimes as high as 20-25 pF). In most op amp circuits, the inverting input impedance is reduced to a very low value by negative feedback, and only Zcm+ and Zdiff are of importance.

Rev.0, 10/08, WK

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