OPA690 datasheet(12/27 Pages) TI

12 / 27 page

OPA3692

SBOS228E

www.ti.com

HIGH-SPEED INSTRUMENTATION AMPLIFIER

Figure 3 shows an instrumentation amplifier based on the

OPA3692. The offset matching between inputs makes this an

attractive input stage for this application. The differential-to-

single-ended gain for this circuit is 2.0V/V. The inputs are high

impedance, with only 1pF to ground at each input. The loads

on the OPA3692 outputs are equal for the best harmonic

distortion possible.

1/3

OPA3692

1/3

OPA3692

1/3

OPA3692

402

Ω

402

Ω

402

Ω

402

Ω

402

Ω

200

Ω

200

Ω

402

Ω

OUT

1/3

OPA3692

402

Ω

402

Ω

100

Ω

1/3

OPA3692

402

Ω

402

Ω

100

Ω

1/3

OPA3692

402

Ω

402

Ω

100

Ω

Selection

Logic

ADS826

10-Bit

60MSPS

0.1

µF

0.1

µF

4.99k

Ω

0.1

µF

4.99k

Ω

0.1

µF

100pF

REFT

+3.5V

REFB

+1.5V

+In

–In

FIGURE 3. High-Speed Instrumentation Amplifier.

FIGURE 5. Multiplexed Converter Driver.

As shown in Figure 4, the OPA3692 used as an instrumenta-

tion amplifier has a 240MHz, –3dB bandwidth. This plot has

been made for a 1Vp-p output signal using a low-impedance

differential input source.

FIGURE 4. High-Speed Instrumentation Amplifier Response.

–1

–2

–3

Frequency (MHz)

1400

100

20log

OUT

1 – V2|

MULTIPLEXED CONVERTER DRIVER

The converter driver in Figure 5 multiplexes among the three

input signals. The OPA3692s enable and disable times

support multiplexing among video signals. The make-before-

break disable characteristic of the OPA3692 ensures that the

output is always under control. To avoid large switching

glitches, switch during the sync or retrace portions of the

video signal—the two inputs should be almost equal at these

times. The output is always under control, so the switching

glitches for two 0V inputs are < 20mV. With standard

video signals levels at the inputs, the maximum differential

voltage across the disabled inputs will not exceed the

±1.2V

maximum rating.

The output resistors isolate the outputs from each other

when switching between channels. The feedback network of

the disabled channels forms part of the load seen by the

enabled amplifier, attenuating the signal slightly.

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