OPA842IDBVT datasheet(13/19 Pages) TI

13 / 19 page

OPA842

SBOS267A

www.ti.com

DESIGN-IN TOOLS

DEMONSTRATION BOARDS

Two PC boards are available to assist in the initial evaluation

of circuit performance using the OPA842 in its two package

styles. Both of these are available, free, as an unpopulated PC

board delivered with descriptive documentation. The summary

information for these boards is shown in the table below.

and parasitic capacitance considerations. For a noninverting

unity-gain follower application, the feedback connection should

be made with a 25

Ω resistor—not a direct short. This will

isolate the inverting input capacitance from the output pin

and improve the frequency response flatness. Usually, the

feedback resistor value should be between 200

Ω and 1kΩ.

Below 200

Ω, the feedback network will present additional

output loading which can degrade the harmonic distortion

performance of the OPA842. Above 1k

Ω, the typical parasitic

capacitance (approximately 0.2pF) across the feedback re-

sistor may cause unintentional band limiting in the amplifier

response.

A good rule of thumb is to target the parallel combination of RF

and RG (see Figure 1) to be less than about 200Ω. The

combined impedance RF || RG interacts with the inverting input

capacitance, placing an additional pole in the feedback net-

work, and thus a zero in the forward response. Assuming a 2pF

total parasitic on the inverting node, holding RF || RG < 200Ω

will keep this pole above 400MHz. By itself, this constraint

implies that the feedback resistor RF can increase to several

Ω at high gains. This is acceptable as long as the pole formed

by RF and any parasitic capacitance appearing in parallel is

kept out of the frequency range of interest.

In the inverting configuration, an additional design consider-

ation must be noted. RG becomes the input resistor and

therefore the load impedance to the driving source. If imped-

ance matching is desired, RG may be set equal to the

required termination value. However, at low inverting gains,

the resultant feedback resistor value can present a signifi-

cant load to the amplifier output. For example, an inverting

gain of 2 with a 50

Ω input matching resistor (= R

G) would

require a 100

Ω feedback resistor, which would contribute to

output loading in parallel with the external load. In such a

case, it would be preferable to increase both the RF and RG

values, and then achieve the input matching impedance with

a third resistor to ground (see Figure 2). The total input

impedance becomes the parallel combination of RG and the

additional shunt resistor.

BANDWIDTH vs GAIN

Voltage-feedback op amps exhibit decreasing closed-loop

bandwidth as the signal gain is increased. In theory, this

relationship is described by the GBP shown in the specifica-

tions. Ideally, dividing GBP by the noninverting signal gain

(also called the Noise Gain, or NG) will predict the closed-

loop bandwidth. In practice, this only holds true when the

phase margin approaches 90

°, as it does in high-gain con-

figurations. At low signal gains, most amplifiers will exhibit a

more complex response with lower phase margin. The

OPA842 is optimized to give a maximally flat 2nd-order

Butterworth response in a gain of 2. In this configuration, the

OPA842 has approximately 60

° of phase margin and will

show a typical –3dB bandwidth of 150MHz. When the phase

margin is 60

°, the closed-loop bandwidth is approximately √2

greater than the value predicted by dividing GBP by the noise

gain. Increasing the gain will cause the phase margin to

approach 90

° and the bandwidth to more closely approach

the predicted value of (GBP/NG). At a gain of +10, the

FIGURE 6. 5MHz Butterwoth Low-Pass Active Filter.

OPA842

+5V

–5V

505

Ω

150pF

124

Ω

402

Ω

402

Ω

100pF

Power-supply

decoupling not shown.

Go to the TI web site (www.ti.com) to request evaluation

boards in the OPA842 product folder.

MACROMODELS AND APPLICATIONS SUPPORT

Computer simulation of circuit performance using SPICE is

often a quick way to analyze the performance of the OPA842

and its circuit designs. This is particularly true for video and RF

amplifier circuits where parasitic capacitance and inductance

can play a major role on circuit performance. A SPICE model

for the OPA842 is available through the TI web page

(www.ti.com). The applications department is also available

for design assistance. These models predict typical small-

signal AC, transient steps, DC performance, and noise under

a wide variety of operating conditions. The models include the

noise terms found in the electrical specifications of the data

sheet. These models do not attempt to distinguish between

the package types in their small-signal AC performance.

OPERATING SUGGESTIONS

OPTIMIZING RESISTOR VALUES

Since the OPA842 is a unity-gain stable, voltage-feedback

op amp, a wide range of resistor values may be used for the

feedback and gain setting resistors. The primary limits on

these values are set by dynamic range (noise and distortion)

LITERATURE

BOARD

REQUEST

PRODUCT

PACKAGE

PART NUMBER

NUMBER

OPA842ID

SO-8

DEM-OPA68xU

SBOU010

OPA842IDBV

SOT23-5

DEM-OPA6xxN

SBOU009

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