OPA2846 datasheet(18/24 Pages) TI

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OPA2846

SBOS274A −JUNE 2003 − REVISED MARCH 2004

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−80

−85

−90

−95

−100

−105

−110

Frequency (MHz)

110

O =2VPP Differential

3rd−Harmonic, 0V

3rd−Harmonic, +2.5V

2nd−Harmonic, 0V

2nd−Harmonic, +2.5V

Figure 11. Harmonic Distortion for Figure 10

Figure 11 shows 2nd- and 3rd-harmonic distortion for a

2VPP differential output swing at both 0V output

common-mode voltage and +2.5V common-mode volt-

age. Since there is no DC current required from the output

to level shift to +2.5V in this circuit, no pull-up resistors to

the power supply were used as in the circuit of Figure 8.

The 2nd harmonic remains the dominant distortion

mechanism, but shows little sensitivity to the common-

mode operating voltage (improved 2nd-harmonic distor-

tion results were achieved with this circuit using two

individual OPA846’s with an extremely symmetrical

layout). The 3rd harmonic is essentially unmeasureable

for the ground-centered output swing, but increases as the

output is shifted to a +2.5V DC output. Narrowband

systems, where a bandpass filter less than an octave wide

can be inserted between the amplifier and the converter,

will only be concerned about 2-tone, 3rd-order inter-

modulation distortion. Since this bandpass filter is also

AC-coupled, the outputs of Figure 10 may be operated

ground-centered, giving the extremely low 3rd-order

distortions of Figure 11.

DESIGN-IN TOOLS

DEMONSTRATION BOARDS

A PC board is available to assist in the initial evaluation of

circuit performance using the OPA2846. It is available free,

as an unpopulated PC board delivered with descriptive

documentation. The summary information for this board is

shown in Table 1.

Contact the Texas Instruments applications support line to

request this board or visit Texas Instruments’ web site at

www.ti.com.

PRODUCT

PACKAGE

BOARD PART

NUMBER

LITERATURE

REQUEST

NUMBER

OPA2846ID

SO-8 Surface-Mount

DEM-OPA268xU

SBOU003

Table 1. Evaluation Module Ordering Information

MACROMODELS AND APPLICATIONS

SUPPORT

Computer simulation of circuit performance using SPICE

is often useful when analyzing the performance of analog

circuits and systems. This is particularly true for video and

RF amplifier circuits where parasitic capacitance and

inductance can have a major effect on circuit performance.

A SPICE model for the OPA2846 is available through the

Texas Instruments web page (http://www.ti.com).

These models do a good job of predicting small-signal AC

and transient performance under a wide variety of

operating conditions. They do not do as well in predicting

the harmonic distortion characteristics.

OPERATING SUGGESTIONS

SETTING RESISTOR VALUES TO MINIMIZE

NOISE

The OPA2846 provides a very low input noise voltage

while requiring a low 12.6mA/channel quiescent current.

To take full advantage of this low input noise, careful

attention to the other possible noise contributors is

required. Figure 12 shows the op amp noise analysis

model with all the noise terms included. In this model, all

the noise terms are taken to be noise voltage or current

density terms in either nV/

√Hz or pA/√Hz.

4kT

1/2

OPA2846

IBN

4kT = 1.6E

− 20J

at 290

4kTR

√

4kTR

√

Figure 12. Op Amp Noise Analysis Model

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