OPA684IDBVT datasheet(16/25 Pages) TI

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OPA684

SBOS219A

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the open-loop voltage gain curve for a voltage-feedback

op amp. Developing the transfer function for the circuit of

Figure 10 gives Equation 1:

(1)

R 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.

(2)

R 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 sepa-

rately from the desired signal gain (or NG).

The OPA684 is internally compensated to give a maximally

flat frequency response for RF = 1kΩ at NG = 2 on ±5V

supplies. That optimum value goes to 1.3k

Ω on a single +5V

supply. Normally, with a current-feedback amplifier, it is

possible to adjust the feedback resistor to hold this band-

width up as the gain is increased. The CFBplus architecture

has reduced the contribution of the inverting input impedance

to provide exceptional bandwidth to higher gains without

adjusting the feedback resistor value. The Typical Character-

istics show the small-signal bandwidth over gain with a fixed

feedback resistor.

At very high gains, 2nd-order effects in the inverting output

impedance cause the overall response to peak up. If desired,

it is possible to retain a flat frequency response at higher

gains by adjusting the feedback resistor to higher values as

the gain is increased. See Figure 11 for the empirically

determined feedback resistor and resulting –3dB bandwidth

from gains of +2 to +100 to hold a < 0.5dB peaked response.

See Figure 12 for the measured frequency response curves

with the adjusted feedback resistor value. While the band-

width for this low-power part does reduce at higher gains,

OPERATING SUGGESTIONS

SETTING RESISTOR VALUES TO OPTIMIZE BANDWIDTH

Any current-feedback op amp like the OPA684 can hold high

bandwidth over signal-gain settings with the proper adjust-

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

OPA684 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.

(S) iERR

ERR

FIGURE 10. Current Feedback Transfer Function Analysis

Circuit.

The key elements of this current-feedback op amp model

are:

α ⇒ Buffer gain from the non-inverting input to the inverting input.

RI ⇒ Buffer output impedance.

iERR ⇒ Feedback error current signal.

Z(s)

⇒ Frequency dependent open loop transimpedance gain from i

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 amplifier configura-

tion. For the buffer gain

α < 1.0, the CMRR = –20 • log(1 – α).

The closed-loop input stage buffer used in the OPA684 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.

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

bandwidth control equation. The OPA684 reduces this ele-

ment to approximately 2.5

Ω, using the loop gain of the local

input buffer stage. This significant reduction in output imped-

ance, on very low power, contributes significantly to extend-

ing the bandwidth at higher gains.

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

transimpedance gain. The Typical Characteristics show this

open-loop transimpedance response. This is analogous to

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