OPA861IDBVT datasheet(23/28 Pages) TI

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DESIGN-IN TOOLS

DEMONSTRATION BOARDS

) RSibn

) 4kTRS

)

) RGibi

) 4kTRG

THERMAL ANALYSIS

MACROMODELS AND APPLICATIONS

NOISE PERFORMANCE

BOARD LAYOUT GUIDELINES

√4kTR

OPA861

SBOS338 – AUGUST 2005

The total output spot noise voltage can be computed

as the square root of the sum of all squared output

noise voltage contributors. Equation 8 shows the

general form for the output noise voltage using the

terms shown in Figure 48.

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

the initial evaluation of circuit performance using the

OPA861. This module is available free, as an

unpopulated PCB delivered with descriptive docu-

mentation. The summary information for the board is

(8)

shown below:

LITERATURE

BOARD PART

REQUEST

PRODUCT

PACKAGE

NUMBER

Maximum desired junction temperature will set the

OPA861ID

SO-8

DEM-OPA86xD

SBOU035

maximum allowed internal power dissipation as de-

scribed below. In no case should the maximum

The board can be requested on Texas Instruments

junction temperature be allowed to exceed 150

°C.

web site (www.ti.com).

Operating junction temperature (TJ) is given by

TA + PD × θJA. The total internal power dissipation

(PD) is the sum of quiescent power (PDQ) and

SUPPORT

additional power dissipated in the output stage (PDL)

Computer simulation of circuit performance using

to deliver output current. Quiescent power is simply

SPICE is often useful when analyzing the perform-

the specified no-load supply current times the total

ance of analog circuits and systems. This principle is

supply voltage across the part. PDL will depend on the

particularly true for Video and RF amplifier circuits

required output signal and load but would, for the

where parasitic capacitance and inductance can have

OPA861 be at a maximum when the maximum IO is

a major effect on circuit performance. A SPICE model

being driven into a voltage source that puts the

for the OPA861 is available through the Texas

maximum voltage across the output stage. Maximum

Instruments web page (www.ti.com). These models

IO is 15mA times a 9V maximum across the output.

do a good job of predicting small-signal AC and

Note that it is the power in the output stage and not

transient

performance

under

wide

variety

into the load that determines internal power dissi-

operating conditions. They do not do as well in

pation.

predicting the harmonic distortion. These models do

not attempt to distinguish between the package types

As a worst-case example, compute the maximum TJ

in their small-signal AC performance.

using an OPA861IDBV in the circuit of Figure 29b

operating at the maximum specified ambient tem-

perature of +85

°C and driving a –1V voltage refer-

ence.

The OTA noise model consists of three elements: a

voltage noise on the B-input; a current noise on the

PD = 10V × 5.4mA + (15mA × 9V) = 185mW

B-input; and a current noise on the E-input. Figure 48

Maximum TJ = +85°C + (0.19W × 150°C/W) = 114°C.

shows the OTA noise analysis model with all the

noise terms included. In this model, all noise terms

Although this is still well below the specified maxi-

are taken to be noise voltage or current density terms

mum junction temperature, system reliability con-

in either nV/

√Hz or pA/√Hz.

siderations may require lower tested junction tem-

peratures. The highest possible internal dissipation

will occur if the load requires current to be forced into

the output for positive output voltages or sourced

from the output for negative output voltages. This

puts a high current through a large internal voltage

drop in the output transistors.

Achieving

optimum

performance

with

high-frequency amplifier like the OPA861 requires

careful attention to board layout parasitics and exter-

Figure 48. OTA Noise Analysis Model

nal component types. Recommendations that will

optimize performance include:

OPA861IDBVT Datasheet(PDF) 23 Page - Texas Instruments

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