OPA4820IPWR datasheet(21/38 Pages) TI2

21 / 38 page

OPA4820

SBOS317D − SEPTEMBER 2004 − REVISED AUGUST 2008

www.ti.com

Another type of filter, a high-Q bandpass filter, is shown in

Figure 16. The transfer function for this filter is:

OUT

) s 1

)

with

2R4R5C1C2

and

+ 1

1C1

For the values chosen in Figure 16:

O +

p ]

1MHz

and

Q = 100

See Figure 17 for the frequency response of the filter

shown in Figure 16.

OUT

500

Ω

500

Ω

158

Ω

1000pF

15.8k

Ω

158

Ω

1000pF

1/4

OPA4820

1/4

OPA4820

Figure 16. High-Q 1MHz Bandpass Filter

−6

−12

−18

−24

−30

−36

−42

−48

−54

−60

−66

−72

Frequency (Hz)

100k

10M

100M

Figure 17. High-Q 1MHz Bandpass Filter

Frequency Response

DESIGN-IN TOOLS

DEMONSTRATION FIXTURES

Two printed circuit boards (PCBs) are available to assist

in the initial evaluation of circuit performance using the

OPA4820 in its two package options. Both of these are

offered free of charge as unpopulated PCBs, delivered

with a user’s guide. The summary information for these

fixtures is shown in the table below.

PRODUCT

PACKAGE

ORDERING NUMBER

LITERATURE

NUMBER

OPA4820ID

SO-14

DEM-OPA-SO-4A

SBOU016

OPA4820IPW

TSSOP-14

DEM-OPA-TSSOP-4A

SBOU017

The demonstration fixtures can be requested at the Texas

Instruments web site (www.ti.com) through the OPA4820

product folder.

MACROMODELS AND APPLICATIONS

SUPPORT

Computer simulation of circuit performance using SPICE

is often a quick way to analyze the performance of the

OPA4820 and its circuit designs. This is particularly true for

video and RF amplifier circuits where parasitic capaci-

tance and inductance can play a major role on circuit

performance. A SPICE model for the OPA4820 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 OPA4820 is a unity-gain stable, voltage-feed-

back 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) and parasitic capacitance considerations. Usu-

ally, the feedback resistor value should be between 200

Ω

and 1k

Ω. Below 200Ω, the feedback network will present

additional output loading which can degrade the harmonic

distortion performance of the OPA4820. Above 1k

Ω, the

typical

parasitic

capacitance

(approximately 0.2pF)

across the feedback resistor may cause unintentional

band limiting in the amplifier response. A 25

Ω feedback

resistor is suggested for AV = +1V/V.

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 network, and thus a zero in the forward

(5)

(6)

(7)

(8)

OPA4820IPWR Datasheet(PDF) 21 Page - Texas Instruments

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