OPA4872MDREP datasheet(14/24 Pages) TI

14 / 24 page

OPERATING SUGGESTIONS

SETTING RESISTOR VALUES TO OPTIMIZE

R =

1)+804

[75

1)+804]

241200

- 1

DESIGN-IN TOOLS

DEMONSTRATION FIXTURE

(S) ERR

ERR

MACROMODELS AND APPLICATIONS SUPPORT

OPA4872-EP

SBOS444 – DECEMBER 2008........................................................................................................................................................................................... www.ti.com

Solving for RO, with n devices connected together,

results in Equation 3:

BANDWIDTH

The

output

stage

the

OPA4872

current-feedback op amp, meaning it can hold an

almost constant bandwidth over signal gain settings

(3)

with the proper adjustment of the external resistor

Results for n varying from 2 to 6 are given in Table 2.

values. This performance is shown in the Typical

Characteristic curves; the small-signal bandwidth

Table 2. Series Resistance vs

decreases only slightly with increasing gain. These

Number of Parallel Outputs

curves also show that the feedback resistor has been

changed for each gain setting. The resistor values on

NUMBER OF OPA4872s

RO (Ω)

the feedback path can be treated as frequency

response compensation elements while the ratio sets

63.94

the signal gain of the feedback resistor divided by the

59.49

gain resistor. Figure 32 shows the small-signal

55.59

frequency response analysis circuit for a current

feedback amplifier.

52.15

The two major limitations of this circuit are the device

requirements for each OPA4872 and the acceptable

return loss resulting from the mismatch between the

load and the matching resistor.

A printed circuit board (PCB) is available to assist in

the initial evaluation of circuit performance using the

OPA4872. The fixture is offered free of charge as an

unpopulated PCB, delivered with a user's guide. The

summary information for this fixture is shown in

Table 3.

Figure 32. Recommended Feedback Resistor vs

Noise Gain

Table 3. OPA4872 Demonstration Fixture

LITERATURE

PACKAGE

ORDERING NUMBER

NUMBER

The key elements of this current-feedback op amp

model are:

SO-14

DEM-OPA-SO-1E

SBOU045

α → Buffer gain from the noninverting input to the

The demonstration fixture can be requested at the

inverting input

Texas Instruments web site at (www.ti.com) through

the OPA4872 product folder.

RI → Buffer output impedance

iERR → Feedback error current signal

Z(s)

→

Frequency-dependent

open-loop

Computer simulation of circuit performance using

transimpedance gain from iERR to VO

SPICE

often

useful

when

analyzing

the

performance of analog circuits and systems. This

The buffer gain is typically very close to 1.00 and is

practice is particularly true for video and RF amplifier

normally neglected from signal gain considerations. It

circuits, where parasitic capacitance and inductance

will, however, set the CMRR for a single op amp

can have a major effect on circuit performance. A

differential amplifier configuration. For a buffer gain

SPICE model for the OPA4872 is available through

α < 1.0, the CMRR = –20 × log (1 – α) dB.

the Texas Instruments web site at www.ti.com. This

model does a good job of predicting small-signal ac

and transient performance under a wide variety of

operating conditions. It does not do as well in

predicting

the

harmonic

distortion

dG/dP

characteristics.

Submit Documentation Feedback

Product Folder Link(s): OPA4872-EP

OPA4872MDREP Datasheet(PDF) 14 Page - Texas Instruments

OPA4872MDREP Datasheet(HTML) 14 Page - Texas Instruments

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

Datasheet Download

Link URL