OPA690 datasheet(14/23 Pages) TI

14 / 23 page

OPA691

SBOS226A

www.ti.com

Each channel includes a bandpass filter. Each bandpass

filter is set for a different frequency band. This allows the

channelizing part of this circuit. The role of the channelizers

OPA691s is to provide impedance isolation. This is done

through the use of four matching resistances (59

Ω in this

case). These matching resistors ensure that the signals will

combine during the transition between channels. They have

been used to get a gain of +1 at the load.

This circuit may be used with a different number of channels.

Its limitation comes from the drive requirement for each

channel as well as the minimum acceptable return loss.

The output resistor value (RO) to keep a gain of +1 at the load

depends on the number of channels. For the OPA691 with a

gain of 2 using RF = 402Ω and RG = 402Ω, Equation 1 is:

(1)

O =

•

(

) +

[]

•+

•

(

) +

[]













804

241200

804

ΩΩ

Ω

ΩΩ

–

SINGLE-SUPPLY “IF” AMPLIFIER

The high bandwidth provided by the OPA691 while operating

on a single +5V supply lends itself well to IF amplifier

applications. One of the advantages of using an op amp like

the OPA691 as an IF amplifier is that precise signal gain is

achieved along with much lower 3rd-order intermodulation

versus quiescent power dissipation. In addition, the OPA691

in the SOT23-6 package offers a very small package with a

power shutdown feature for portable applications. One con-

cern with using op amps for an IF amplifier is their relatively

high noise figures. It is sometimes suggested that an opti-

mum source resistance can be used to minimize op amp

noise figures. Adding a resistor to reach this optimum value

may improve the noise figure, but will actually decrease the

signal-to-noise ratio. A more effective way to move towards

an optimum source impedance is to bring the signal in

through an input transformer. Figure 6 shows an example

that is particularly useful for the OPA691.

Bringing the signal in through a step-up transformer to the

inverting input gain resistor has several advantages for the

OPA691. First, the decoupling capacitor on the noninverting

input eliminates the contribution of the noninverting input current

noise to the output noise. Secondly, the noninverting input noise

voltage of the op amp is actually attenuated if reflected to the

input side of RG. Using the 1:2 (turns ratio) step-up transformer

reflects the 50

Ω source impedance at the primary through to the

secondary as a 200

Ω source impedance (and the 200Ω R

resistor is reflected through to the transformer primary as a 50

Ω

input matching impedance). The noise gain to the amplifier

output is then 1 + 600/400 = 2.5V/V. Taking the op amp’s

2.2nV/

√Hz input voltage noise times this noise gain to the

output, then reflecting this noise term to the input side of the RG

resistor, divides it by 3. This gives a net gain of 0.833 for the

noninverting input voltage noise when reflected to the input

point for the op amp circuit. This is further reduced when

referred back to the transformer primary.

The relatively low-gain IF amplifier circuit of Figure 6 gives a

12dB noise figure at the input of the transformer. Increasing

the RF resistor to 600Ω (once RG is set to 200Ω for input

impedance matching) will slightly reduce the bandwidth.

Measured results show 150MHz small-signal bandwidth for

the circuit of Figure 6 with exceptional flatness through

30MHz. Although the OPA691 does not show an intercept

characteristic for the 2-tone, 3rd-order intermodulation distor-

tion, it does hold a very high Spurious-Free Dynamic Range

(SFDR) through high output powers and frequencies. The

maximum single-tone power at the matched load for the

single-supply circuit of Figure 6 is 1dBm (this requires a

2.8Vp-p swing at the output pin of the OPA691 for the 2-tone

envelope). Measured 2-tone SFDR at this maximum load

power for the circuit of Figure 6 exceeds 55dBc for frequen-

cies to 20MHz.

DESIGN-IN TOOLS

DEMONSTRATION BOARDS

Several PC boards are available to assist in the initial

evaluation of circuit performance using the OPA691 in its two

package styles. All of these are available free as an

unpopulated PC board delivered with descriptive documen-

tation. The summary information for these boards is shown

in the table below.

BOARD

LITERATURE

PART

REQUEST

PRODUCT

PACKAGE

NUMBER

OPA691ID

SO-8

DEM-OPA68xU

SBOU009

OPA691IDBV

SOT23-6

DEM-OPA6xxN

SBOU010

600

Ω

= 3V/V (9.54dB)

200

Ω

OPA691

+5V

DIS

Power-supply

decoupling not shown.

Ω Load

Ω

Ω Source

1:2

Ω

µF

0.1

µF

FIGURE 6. Low-Noise, Single-Supply IF Amplifier.

To request any of these boards, check the Texas Instru-

ments web site at www.ti.com.

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

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