OPA684IDBVT datasheet(14/25 Pages) TI

14 / 25 page

OPA684

SBOS219A

www.ti.com

INVERTING SUMMING APPLICATIONS

The OPA684 provides one of the most robust summing

operations available in a wideband op amp. Figure 6 shows

a typical inverting summing application where, in this case, 4

sources are summed through 500

Ω gain resistors while also

including an 88.9

Ω terminating impedance to present a 75Ω

input impedance to each source. The gain for each channel

is –2 to the output pin and –1 to the matched load. The

extremely low inverting input impedance of the CFBplus

architectures ensures noninteractive summing for all of the

channels. The amplifier bandwidth is largely independent of

variations in the gain setting elements, depending primarily

on the feedback resistor value instead. This type of circuit

may be used to sum numerous signals together or, where the

prior stages can be disabled, allow multiple channels to be

brought together with only the active channel passing on to

the output.

Ω

500

Ω

OPA684

+5V

DIS

–5V

500

Ω

500

Ω

500

Ω

88.9

Ω

88.9

Ω

88.9

Ω

88.9

Ω

Ω Load

Supply Decoupling not shown.

–2(IN

1 + IN2 + IN3 + IN4)

FIGURE 7. Single-Supply, High-Gain SAW Post-Amplifier.

SAW FILTER POST- AMPLIFIER

While SAW filters provide unmatched selectivity in a passive

filter element, this comes at the cost of significant insertion loss

for the desired signal. The OPA684 can provide a low-cost and

low-power post-amplifier over a wide range of gains due to the

CFBplus internal architecture. Figure 7 shows a 44MHz SAW

filter with 20dB insertion loss followed by an OPA684 operating

at a gain of 20V/V (26dB) providing a net gain of 1 (0dB) from

the input of the SAW filter to a matched 50

Ω load. The circuit

shown also includes a simple bandpass around 44MHz to

reduce out-of-band noise. The gain for the circuit of Figure 7

ranges from 0dB at DC to 28dB at 44MHz and then back to

< 0dB above 200MHz. The LC elements in the gain circuit peak

the response slightly above a gain of 26dB—removing those

would give a broadband gain of 26dB.

950

Ω

OPA684

DIS

Ω

Ω Load

20k

Ω

20k

Ω

1000pF

100pF

1000pF

160nH

Ω

SAW

Filter

44MHz

20dB

Insertion

Loss

+5V to +12V

= 0dB at 44MHz

FIGURE 6. Gain of –2 Video Summing Amplifier.

OPA684IDBVT Datasheet(PDF) 14 Page - Texas Instruments

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