OPA695IDBVT datasheet(21/32 Pages) TI

21 / 32 page

OPA695

SBOS293B

www.ti.com

FIGURE 12. High Power, Wideband AC-Coupled Arbitrary Waveform Driver.

OPA695

+5V

–5V

+5V

–5V

464

Ω

66.5

Ω

0.01

µF

200

Ω

OPA695

464

Ω

66.5

Ω

0.01

µF

200

Ω

DAC

20mA Peak Output

Differential

Filter

Ω Source

1.4:1

±3.5V

Power supply decoupling not shown.

Ω

DIS

noninverting input), then inverts the complementary output

current to produce a signal that ranges from this 1.4V at zero

output current down to 0V at maximum output current level

(assuming a 20mA maximum output current). This will give a

very wideband (> 800MHz) video signal capability.

ARBITRARY WAVEFORM DRIVER

The OPA695 may be used as the output stage for moderate

output power Arbitrary Waveform Driver applications. Driving

out through a series 50

Ω matching resistor into a 50Ω

matched load will allow up to a 4.0VPP swing at the matched

load (15dBm) when operating the OPA695 on a

±5V power

supply. This level of power is available for gains of either

±8

with a flat response through 100MHz. When interfacing

directly from a complementary current output DAC, consider

the circuit of Figure 11, modified for the peak output currents

of the particular DAC being considered. Where purely

AC-coupled output signals are required from a complemen-

tary current output DAC, consider a push-pull output stage

using the circuit of Figure 12. The resistor values here have

been calculated for a 20mA peak output current DAC which

produces up to a 5VPP swing at the matched load (18dBm).

This approach will give higher power at the load with much

lower 2nd-harmonic distortion.

For a 20mA peak output current DAC, the mid-scale current

of 10mA will give a 2V DC output common-mode operating

voltage due to the 200

Ω resistor to ground at the outputs.

The total AC impedance at each output is 50

Ω, giving a

±0.5V swing around this 2V common-mode voltage for the

DAC. These resistors also act as a current divider, sending

75% of the DAC output current through the feedback resistor

(464

Ω). The blocking capacitor references the OPA695 out-

put voltage to ground, and turns the unipolar DAC output

current into a bipolar swing of 0.75 • 20mA • 464

Ω = 7V

PP at

each amplifier output. Each output is exactly 180

° out-of-

phase from the other, producing double 7VPP into the match-

ing resistors. To limit the peak output current and improve

distortion, the circuit of Figure 12 is set up with a 1.4:1 step-

down transformer. This reflects the 50

Ω load to be 100Ω at

the primary side of the transformer. For the maximum 14VPP

swing across the outputs of the two amplifiers, the matching

resistors will drop this to 7VPP at the input of the transformer,

then down to 5VPP maximum at the 50Ω load at the output of

the transformer. This step-down approach reduces the peak

output current to 14VP/(200Ω) = 70mA.

OPA695IDBVT Datasheet(PDF) 21 Page - Texas Instruments

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