OPA690 datasheet(11/22 Pages) TI

11 / 22 page

OPA690

SBOS223A

www.ti.com

FIGURE 1. DC-Coupled, G = +2, Bipolar Supply, Specifica-

tion and Test Circuit.

FIGURE 2. AC-Coupled, G = +2, Single-Supply, Specifica-

tion and Test Circuit.

APPLICATIONS INFORMATION

WIDEBAND VOLTAGE FEEDBACK OPERATION

The OPA690 provides an exceptional combination of high

output power capability with a wideband, unity-gain stable

voltage feedback op amp using a new high slew rate input

stage. Typical differential input stages used for voltage feed-

back op amps are designed to steer a fixed-bias current to

the compensation capacitor, setting a limit to the achievable

slew rate. The OPA690 uses a new input stage which places

the transconductance element between two input buffers,

using their output currents as the forward signal. As the error

voltage increases across the two inputs, an increasing cur-

rent is delivered to the compensation capacitor. This pro-

vides very high slew rate (1800V/

µs) while consuming

relatively low quiescent current (5.5mA). This exceptional

full-power performance comes at the price of a slightly higher

input noise voltage than alternative architectures. The

5.5nV/

√Hzinput voltage noise for the OPA690 is exception-

ally low for this type of input stage.

Figure 1 shows the DC-coupled, gain of +2, dual power-

supply circuit configuration used as the basis of the

±5V

Electrical Characteristics and Typical Characteristics. For test

purposes, the input impedance is set to 50

Ω with a resistor to

ground and the output impedance is set to 50

Ω with a series

output resistor. Voltage swings reported in the specifications

are taken directly at the input and output pins, while output

powers (dBm) are at the matched 50

Ω load. For the circuit of

Figure 1, the total effective load will be 100

Ω || 804Ω. The

disable control line is typically left open to ensure normal

amplifier operation. Two optional components are included in

Figure 1. An additional resistor (175

Ω) is included in series

with the noninverting input. Combined with the 25

Ω DC

source resistance looking back towards the signal generator,

this gives an input bias current cancelling resistance that

matches the 200

Ω source resistance seen at the inverting

input (see the DC Accuracy and Offset Control section). In

addition to the usual power-supply decoupling capacitors to

ground, a 0.1

µF capacitor is included between the two power-

supply pins. In practical PC board layouts, this optional-added

capacitor will typically improve the 2nd-harmonic distortion

performance by 3dB to 6dB.

Figure 2 shows the AC-coupled, gain of +2, single-supply

circuit configuration which is the basis of the +5V Specifica-

tions and Typical Characteristics. Though not a “rail-to-rail”

design, the OPA690 requires minimal input and output volt-

age headroom compared to other very wideband voltage

feedback op amps. It will deliver a 3Vp-p output swing on

a single +5V supply with > 150MHz bandwidth. The key

requirement of broadband single-supply operation is to main-

tain input and output signal swings within the useable voltage

ranges at both the input and the output. The circuit of Figure 2

establishes an input midpoint bias using a simple resistive

divider from the +5V supply (two 698

Ω resistors). The input

signal is then AC-coupled into the midpoint voltage bias. The

input voltage can swing to within 1.5V of either supply pin,

giving a 2Vp-p input signal range centered between the

supply pins. The input impedance matching resistor (59

Ω)

used for testing is adjusted to give a 50

Ω input load when the

parallel combination of the biasing divider network is in-

cluded. Again, an additional resistor (50

Ω in this case) is

included directly in series with the noninverting input. This

minimum recommended value provides part of the DC source

resistance matching for the noninverting input bias current. It

is also used to form a simple parasitic pole to roll off the

frequency response at very high frequencies (> 500MHz)

using the input parasitic capacitance to form a bandlimiting

pole. The gain resistor (RG) is AC-coupled, giving the circuit

a DC gain of +1, which puts the input DC bias voltage (2.5V)

at the output as well. The output voltage can swing to within

OPA690

+5V

DIS

–5V

Ω Load

Ω

Ω Source

402

Ω

402

Ω

6.8

µF

0.1

µF

6.8

µF

0.1

µF

0.1

µF

175

Ω

OPA690

+5V

DIS

S/2

698

Ω

100

Ω

698

Ω

0.1

µF

0.1

µF

6.8

µF

0.1

µF

402

Ω

402

Ω

Ω Source

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