OPA694 datasheet(13/27 Pages) TI2

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OPA2694

SBOS320D − SEPTEMBER 2004 − REVISED APRIL 2013

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SAW FILTER BUFFER

One common requirement in an IF strip is to buffer the

output of a mixer with enough gain to recover the insertion

loss of a narrowband SAW filter. Figure 5 shows one

possible configuration driving a SAW filter. The 2-Tone,

3rd-Order Intermodulation Intercept plot is shown in the

Typical Characteritics curves. Operating in the inverting

mode at a voltage gain of –8V/V, this circuit provides a 50Ω

input match using the gain set resistor, has the feedback

optimized for maximum bandwidth (250MHz in this case),

and drives through a 50Ω output resistor into the matching

network at the input of the SAW filter. If the SAW filter gives

a 12dB insertion loss, a net gain of 0dB to the 50Ω load at

the output of the SAW (which could be the input

impedance of the next IF amplifier or mixer) will be

delivered in the passband of the SAW filter. Using the

OPA2694 in this application will isolate the first mixer from

the impedance of the SAW filter and provide very low

two-tone, 3rd-order spurious levels in the SAW filter

bandwidth.

SAW

Filter

+12V

Matching

Network

=12dB

− (SAW Loss)

Source

50Ω

400

0.1

μF

1000pF

5kΩ

1/2

OPA2694

Figure 5. IF Amplifier Driving SAW Filter

DIFFERENTIAL INTERFACE APPLICATIONS

Dual op amps are particularly suitable to differential input

to differential output applications. Typically, these fall into

either ADC input interface or line driver applications. Two

basic approaches to differential I/O are noninverting or

inverting configurations. Since the output is differential, the

signal

polarity

somewhat

meaningless—the

noninverting and inverting terminology applies here to

where the input is brought into the OPA2694. Each has its

advantages and disadvantages. Figure 6 shows a basic

starting point for noninverting differential I/O applications.

1/2

OPA2694

+VCC

−VCC

1/2

OPA2694

Figure 6. Noninverting Differential I/O Amplifier

This approach provides for a source termination

impedance that is independent of the signal gain. For

instance, simple differential filters may be included in the

signal path right up to the noninverting inputs without

interacting with the gain setting. The differential signal gain

for the circuit of Figure 6 is shown in Equation (1):

AD + 1 ) 2

The differential gain, however, may be adjusted with

considerable freedom using just the RG resistor. In fact, RG

may be a reactive network providing a very isolated

shaping to the differential frequency response. Since the

inverting inputs of the OPA2694 are low-impedance

closed-loop buffer outputs, the RG element does not

interact with the amplifier bandwidth. Wide ranges of

resistor values and/or filter elements may be inserted here

with minimal amplifier bandwidth interaction.

Various combinations of single-supply or AC-coupled gain

can also be delivered using the basic circuit of Figure 6.

Common-mode bias voltages on the two noninverting

inputs pass on to the output with a gain of 1, since an equal

DC voltage at each inverting node creates no current

through RG. This circuit does show a common-mode gain

of 1 from input to output. The source connection should

either remove this common-mode signal if undesired

(using an input transformer can provide this function), or

the common-mode voltage at the inputs can be used to set

the output common-mode bias. If the low common-mode

rejection of this circuit is a problem, the output interface

may also be used to reject that common-mode. For

instance, most modern differential input ADCs reject

(1)

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