OPA684IDBVT datasheet(15/25 Pages) TI

15 / 25 page

OPA684

SBOS219A

www.ti.com

Figure 8 shows the small-signal frequency response for the

amplifier portion of this circuit, from the non-inverting input to

the output pin. This particular example is configured as a

single-supply circuit that will operate over a supply range from

+5V to +12V. Where higher frequencies or 3rd-order intercepts

are required, consider the OPA685, a very high bandwidth

(> 500MHz) current-feedback op amp.

input to be used for the CM (Common-Mode) voltage from

the ADS825 converter. This midpoint reference biases both

the non-inverting converter input and the amplifier non-

inverting input. With an AC-coupled gain path, this +2.5V has

a gain of +1 to the output, putting the output at the DC

midpoint for the converter. The output then drives through an

isolating resistor (50

Ω) to the inverting input of the converter,

which is further de-coupled by a 10pF external capacitance

to add to its 5pF input capacitance. This coupling network

provides a high frequency cutoff, while also giving a low

source impedance at high frequencies for the converter. The

gain for this circuit is set by adjusting RG to the desired value.

For a 2Vp-p maximum output driving the light load of Figure

9, the OPA684 will provide > 75dB SFDR through 5MHz (see

the Typical Characteristics).

DESIGN-IN TOOLS

DEMONSTRATION BOARDS

Two PC boards are available to assist in the initial evaluation

of circuit performance using the OPA684 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 Table I.

Frequency (MHz)

100

BANDPASS SAW FILTER POST AMPLIFIER

44MHz

LOW POWER, ADC DRIVER

Where a low-power, single-supply, interface to a single-

ended input +5V ADC is required, the circuit of Figure 9 can

provide a very flexible, higher performance solution. Running

in an AC-coupled inverting mode allows the non-inverting

FIGURE 8. Bandpass SAW Filter Post-Amplifier.

FIGURE 9. Low-Power, Single-Supply, ADC Driver.

OPA684

+5V

DIS

Ω

1.3k

Ω

10pF

1.3k

Ω

0.1

µF

O = –

ADS825

10-Bit

40MSPS

2.5V

+2.5V

2Vp-p

Max

BOARD

LITERATURE

PART

REQUEST

PRODUCT

PACKAGE

NUMBER

OPA684ID

SO-8

DEM-OPA68xU

SBOU009

OPA684IDBQ

SOT23-6

DEM-OPA6xxN

SBOU010

TABLE I.

OPA684IDBVT Datasheet(PDF) 15 Page - Texas Instruments

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