OPA683IDBVT datasheet(15/31 Pages) TI

15 / 31 page

OPA683

SBOS221E

www.ti.com

DESIGN-IN TOOLS

DEMONSTRATION FIXTURES

Two printed circuit boards (PCBs) are available to assist in

the initial evaluation of circuit performance using the OPA683

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

Table I.

The buffer gain is typically very close to 1.00 and is normally

neglected from signal gain considerations. It will, however

set the CMRR for a single op amp differential amplifier

configuration. For the buffer gain

α < 1.0, the CMRR =

–20 • log(1 –

α). The closed loop input stage buffer used in

the OPA683 gives a buffer gain more closely approaching

1.00 and this shows up in a slightly higher CMRR than any

previous current feedback op amp. The 60dB typical CMRR

shown in the Electrical Characteristics implies a buffer gain

of 0.9990.

RI, the buffer output impedance, is a critical portion of the

bandwidth control equation. The OPA683 reduces this ele-

ment to approximately 4.5

Ω using the loop-gain of the input

buffer stage. This significant reduction in buffer output im-

pedance, on very low power, contributes significantly to

extending the bandwidth at higher gains.

ORDERING

LITERATURE

PRODUCT

PACKAGE

NUMBER

OPA683ID

SO-8

DEM-OPA-SO-1A

SBOU009

OPA683IDBQ

SOT23-6

DEM-OPA-SOT-1A

SBOU010

TABLE I. Demonstration Fixtures by Package.

The demonstration fixtures can be requested at the Texas

Instruments web site (www.ti.com) through the OPA683

product folder.

OPERATING SUGGESTIONS

SETTING RESISTOR VALUES TO OPTIMIZE

BANDWIDTH

Any current-feedback op amp like the OPA683 can hold high

bandwidth over signal gain settings with the proper adjust-

ment of the external resistor values. A low-power part like the

OPA683 typically shows a larger change in bandwidth due to

the significant contribution of the inverting input impedance

to loop-gain changes as the signal gain is changed. Figure

10 shows a simplified analysis circuit for any current feed-

back amplifier.

The key elements of this current feedback op amp model are:

α ⇒ Buffer gain from the noninverting input to the inverting input

RI ⇒ Buffer output impedance

iERR ⇒ Feedback error current signal

Z(s)

⇒ Frequency dependent open loop transimpedance gain

from iERR to VO

OPA683

+5V

DIS

Ω

1.4k

Ω

22pF

1.4k

Ω

0.1

µF

O = –

ADS820

10-Bit

20MSPS

2.5V

+2.5V

Max

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

(S) iERR

ERR

FIGURE 10. Current Feedback Transfer Function Analysis

Circuit.

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