OPA690 datasheet(15/23 Pages) TI

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OPA691

SBOS226A

www.ti.com

amplifier circuits where parasitic capacitance and inductance

can have a major effect on circuit performance. A SPICE

model for the OPA691 is available through the TI web site

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

small-signal AC and transient performance under a wide

variety of operating conditions. They do not do as well in

predicting the harmonic distortion or dG/d

φ characteristics.

These models do not attempt to distinguish between the

package types in their small-signal AC performance.

OPERATING SUGGESTIONS

SETTING RESISTOR VALUES TO

OPTIMIZE BANDWIDTH

A current feedback op amp like the OPA691 can hold an

almost constant bandwidth over signal gain settings with the

proper adjustment of the external resistor values. This is

shown in 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 op amp can be

treated as frequency response compensation elements while

their “ratios” set the signal gain. Figure 7 shows the small-

signal frequency response analysis circuit for the OPA691.

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

bandwidth control equation. The OPA691 is typically about 35

Ω.

A current feedback op amp senses an error current in the

inverting node (as opposed to a differential input error volt-

age for a voltage feedback op amp) and passes this on to the

output through an internal frequency dependent transimped-

ance gain. The Typical Characteristics show this open-loop

transimpedance response. This is analogous to the open-

loop voltage gain curve for a voltage feedback op amp.

Developing the transfer function for the circuit of Figure 7

gives Equation 1:

RR NG





































()

This is written in a loop-gain analysis format where the errors

arising from a non-infinite 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:

RR NG

Loop Gain

()

If 20 • log (RF + NG • RI) were drawn on top of the open-loop

transimpedance plot, the difference between the two would

be the loop gain at a given frequency. Eventually, Z(S) rolls off

to equal the denominator of Equation 2 at which point the

loop gain has reduced to 1 (and the curves have intersected).

This point of equality is where the amplifier’s closed-loop

frequency response given by Equation 1 will start to roll off,

and is exactly analogous to the frequency at which the noise

gain equals the open-loop voltage gain for a voltage feed-

back op amp. The difference here is that the total impedance

in the denominator of Equation 2 may be controlled some-

what separately from the desired signal gain (or NG).

The OPA691 is internally compensated to give a maxi-

mally flat frequency response for RF = 402Ω at NG = 2 on

±5V supplies. Evaluating the denominator of Equation 2

(which is the feedback transimpedance) gives an optimal

target of 472

Ω. As the signal gain changes, the contribu-

tion of the NG • RI term in the feedback transimpedance

will change, but the total can be held constant by adjust-

ing RF. Equation 4 gives an approximate equation for

optimum RF over signal gain:

RNG R

= 472Ω –

FIGURE 7. Recommended Feedback Resistor versus Noise Gain.

(S) iERR

ERR

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)

→Frequencydependentopen-looptransimpedancegainfromi

ERR to VO

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 ampli-

fier configuration. For a buffer gain

α < 1.0, the CMRR =

–20 • log (1–

α) dB.

(2)

(3)

(4)

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