OPA694IDBVT datasheet(13/31 Pages) TI2

13 / 31 page

(

R +R

(s)

(

R +R

(s)

NG=1+

(

R +R NG

(s)

I·

=LoopGain

(S) ERR

ERR

OPA694

www.ti.com

SBOS319G – SEPTEMBER 2004 – REVISED JANUARY 2010

The demonstration fixtures can be requested at the

The key elements of this current-feedback op amp

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

model are:

OPA694 product folder.

a → Buffer gain from the noninverting input to the

inverting input

MACROMODELS AND APPLICATIONS SUPPORT

RI → Buffer output impedance

Computer simulation of circuit performance using

iERR → Feedback error current signal

SPICE

often

useful

when

analyzing

the

Z(s)

→

Frequency-dependent,

open-loop

performance of analog circuits and systems. This is

transimpedance gain from iERR to VO

particularly true for video and RF amplifier circuits

The buffer gain is typically very close to 1.00 and is

where parasitic capacitance and inductance can have

normally neglected from signal gain considerations. It

a major effect on circuit performance. A SPICE model

will, however, set the CMRR for a single op amp

for the OPA694 is available through the TI web site

differential amplifier configuration.

(www.ti.com). These models do a good job of

predicting small-signal AC and transient performance

For a buffer gain a < 1.0, the CMRR = –20 × log (1–

under a wide variety of operating conditions. They do

a) dB.

not do as well in predicting the harmonic distortion or

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

dG/df characteristics. These models do not attempt

the bandwidth control equation. RI for the OPA694 is

distinguish

between

package

types

their

typically about 30

Ω.

small-signal AC performance.

A current-feedback op amp senses an error current in

OPERATING SUGGESTIONS

the inverting node (as opposed to a differential input

error voltage for a voltage-feedback op amp) and

space

passes this on to the output through an internal

frequency

dependent

transimpedance

gain.

The

SETTING RESISTOR VALUES TO OPTIMIZE

Typical

Characteristics

show

this

open-loop

BANDWIDTH

transimpedance response. This is analogous to the

A current-feedback op amp like the OPA694 can hold

open-loop voltage gain curve for a voltage-feedback

an almost constant bandwidth over signal gain

op amp. Developing the transfer function for the

settings with the proper adjustment of the external

circuit of Figure 38 gives Equation 1:

resistor

values.

This

shown

the

Typical

Characteristic curves; the small-signal bandwidth

decreases only slightly with increasing gain. Those

curves also show that the feedback resistor has been

changed for each gain setting. The resistor values on

the inverting side of the circuit for a current-feedback

(1)

op amp can be treated as frequency response

compensation elements while their ratios set the

where:

signal

gain.

Figure

shows

the

small-signal

frequency response analysis circuit for the OPA694.

This is written in a loop-gain analysis format, where

the errors arising from a noninfinite open-loop gain

are shown in the denominator. If Z(s) were infinite

over all frequencies, the denominator of Equation 1

would reduce to 1 and the ideal desired signal gain

shown in the numerator would be achieved. The

fraction in the denominator of Equation 1 determines

the frequency response. Equation 2 shows this as the

loop-gain equation:

(2)

If 20 × log(RF + NG × RI) were drawn on top of the

open-loop

transimpedance

plot,

the

difference

Figure 38. Recommended Feedback Resistor

between the two would be the loop gain at a given

Versus Noise Gain

frequency. Eventually, Z(s) rolls off to equal the

denominator of Equation 2, at which point the loop

Product Folder Link(s): OPA694

OPA694IDBVT Datasheet(PDF) 13 Page - Texas Instruments

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