AD8390ACP データシートの表示（PDF） - Analog Devices

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AD8390ACP

Low Power, High Output Current Differential Amplifier

Analog Devices 

THEORY OF OPERATION

RF

+IN

RADJ

VEE

RG

IADJ

PWDN0

PWDN1

DGND

–IN

RG

A

VCC

50kΩ

C

56kΩ

B 50kΩ

56kΩ

AD8390 VEE

–OUT

VOCM

BYP

+OUT

RF

Figure 22. Functional Block Diagram

The AD8390 is a true differential operational amplifier with

common-mode feedback. The AD8390 is functionally equivalent

to three op amps, as shown in Figure 22. Amplifiers A and B act

like a standard dual op amp in an inverting configuration that

requires four resistors to set the desired gain.

The third amplifier (C) maintains the common-mode voltage

(VOCM) at the output of the AD8390. VOCM is internally

generated, as shown in Figure 22. The common-mode feedback

amplifier (C) drives the noninverting terminals of A and B such

that the difference between the output common-mode voltage

and VOCM is always zero. This functionality forces the outputs to

sit at midsupply, which results in differential outputs of identical

amplitude and 180 degrees out of phase. The user also has the

option to externally drive the VOCM pin as an input to set the dc

output common-mode voltage. For details, see the Setting the

Output Common-Mode Voltage section.

AD8390

APPLICATIONS

CIRCUIT DEFINITIONS

Differential voltage refers to the difference between two node

voltages. For example, the output differential voltage (or output

differential-mode voltage) is defined as

( ) VOUT,DM = V+OUT −V−OUT

(1)

V+OUT and V–OUT refer to the voltages at the +OUT and –OUT

terminals with respect to a common reference.

Common-mode voltage refers to the average of the two node

voltages. The output common-mode voltage is defined as

( ) VOUT ,CM

=

V +OUT

+ V −OUT

2

(2)

ANALYZING A BASIC APPLICATION CIRCUIT

The AD8390 uses high open-loop gain and negative feedback to

force its differential and common-mode output voltages in such

a way as to minimize the differential and common-mode error

voltages. The differential error voltage is defined as the voltage

between the differential inputs +IN and –IN, as shown in

Figure 23. For most purposes, this voltage can be assumed to be

zero. Similarly, the difference between the actual output

common-mode voltage and the voltage applied to VOCM can also

be assumed to be zero. Starting from these two assumptions, any

application circuit can be analyzed.

RF

+

VIN,DM

–

RG +IN

VOCM

RG

–IN

–OUT

–

+OUT

RL,DM VOUT,DM

+

RF

Figure 23. Basic Applications Circuit

(IADJ Pin Not Connected, and PWDN0 and PWDN1 Held High)

SETTING THE CLOSED-LOOP GAIN

The differential-mode gain of the circuit in Figure 23 can be

described by

VOUT ,DM = RF

(3)

VIN,DM RG

CALCULATING INPUT IMPEDANCE

The input impedance of the circuit in Figure 23 between the

inputs (V+IN and V−IN) is simply

RIN,DM = 2 × RG

(4)

Rev. C | Page 9 of 16

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