ADA4938-2ACPZ-R2 datasheet(19/26 Pages) AD

19 / 26 page

Data Sheet

ADA4938-1/ADA4938-2

Rev. B | Page 19 of 26

THEORY OF OPERATION

The ADA4938-1/ADA4938-2 differ from conventional op amps

in that they have two outputs whose voltages move in opposite

directions. Like an op amp, it relies on open-loop gain and

negative feedback to force these outputs to the desired voltages.

The ADA4938-1/ADA4938-2 behave much like a standard

voltage feedback op amp and makes it easier to perform single-

ended-to-differential conversions, common-mode level shifting,

and amplifications of differential signals. Also like an op amp,

the ADA4938-1/ADA4938-2 have high input impedance and

low output impedance.

Two feedback loops are employed to control the differential and

common-mode output voltages. The differential feedback, set

with external resistors, controls only the differential output

voltage. The common-mode feedback controls only the common-

mode output voltage. This architecture makes it easy to set the

output common-mode level to any arbitrary value. It is forced,

by internal common-mode feedback, to be equal to the voltage

applied to the VOCM input, without affecting the differential

output voltage.

The ADA4938-1/ADA4938-2 architecture results in outputs

that are highly balanced over a wide frequency range without

requiring tightly matched external components. The common-

mode feedback loop forces the signal component of the output

common-mode voltage to zero, which results in nearly perfectly

balanced differential outputs that are identical in amplitude and

are exactly 180° apart in phase.

ANALYZING AN APPLICATION CIRCUIT

The ADA4938-1/ADA4938-2 use 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 labeled

+IN and −IN (see Figure 57). 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.

SETTING THE CLOSED-LOOP GAIN

The differential-mode gain of the circuit in Figure 57 can be

determined by

OUT

This assumes the input resistors (RG) and feedback resistors (RF)

on each side are equal.

ESTIMATING THE OUTPUT NOISE VOLTAGE

The differential output noise of the ADA4938-1/ADA4938-2

can be estimated using the noise model in Figure 58. The input-

referred noise voltage density, vnIN, is modeled as a differential

input, and the noise currents, inIN− and inIN+, appear between

each input and ground. The noise currents are assumed to be

equal and produce a voltage across the parallel combination of

the gain and feedback resistances. vn,cm is the noise voltage

density at the VOCM pin. Each of the four resistors contributes

(4kTR)1/2. Table 9 summarizes the input noise sources, the

multiplication factors, and the output-referred noise density terms.

ADA4938

RF2

VnOD

VnCM

VOCM

VnIN

RF1

RG2

RG1

VnRF1

VnRF2

VnRG1

VnRG2

inIN+

inIN–

Figure 58. ADA4938-1/ADA4938-2 Noise Model

Table 9. Output Noise Voltage Density Calculations

Input Noise Contribution

Input Noise Term

Input Noise

Voltage Density

Output

Multiplication Factor

Output Noise

Voltage Density Term

Differential Input

vnIN

vnO1 = GN(vnIN)

Inverting Input

inIN−

inIN− × (RG2||RF2)

vnO2 = GN[inIN− × (RG2||RF2)]

Noninverting Input

inIN+

inIN+ × (RG1||RF1)

vnO3 = GN[inIN+ × (RG1||RF1)]

VOCM Input

vn, cm

GN(β1 − β2)

vnO4 = GN(β1 − β2)(vnCM)

Gain Resistor, RG1

vnRG1

(4kTRG1)1/2

GN(1 − β1)

vnO5 = GN(1 − β1)(4kTRG1)1/2

Gain Resistor, RG2

vnRG2

(4kTRG2)1/2

GN(1 − β2)

vnO6 = GN(1 − β2)(4kTRG2)1/2

Feedback Resistor, RF1

vnRF1

(4kTRF1)1/2

vnO7 = (4kTRF1)1/2

Feedback Resistor, RF2

vnRF2

(4kTRF2)1/2

vnO8 = (4kTRF2)1/2

ADA4938-2ACPZ-R2 数据表(PDF) 19 Page - Analog Devices

ADA4938-2ACPZ-R2 数据表(HTML) 19 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26

数据表下载