7

®

OPA2607

DESIGN-IN TOOLS

DEMONSTRATION BOARDS

Several PC boards are available to assist in the initial

evaluation of circuit performance using the OPA2607 in its

3 package styles. All are available free as an unpopulated

PC board delivered with descriptive documentation. The

summary information for these boards 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 a buffer gain

α < 1.0, the

CMRR = –20 • log (1–

α) dB.

cally about 33

Ω.

A current-feedback op amp senses an error current in the

inverting node (as opposed to a differential input error

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

the output through an internal frequency dependent

transimpedance gain. The typical performance curves 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 2 gives Equation 1:

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

errors arising from a finite 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:

Contact the Burr-Brown applications support line to request

any of these boards.

MACROMODELS AND APPLICATIONS SUPPORT

Computer simulation of circuit performance using SPICE is

often useful when analyzing the performance of analog

circuits and systems. This is particularly true for video and

RF amplifier circuits where parasitic capacitance and induc-

tance can have a major effect on circuit performance. SPICE

models for some op amps are available through the Burr-

Brown web site (http://www.burr-brown.com). These mod-

els 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,

dG/dP, or temperature characteristics. These models do not

attempt to distinguish between the package types in their

small-signal AC performance, nor do they attempt to simu-

late channel-to-channel coupling.

OPERATING SUGGESTIONS

SETTING RESISTOR VALUES TO

OPTIMIZE BANDWIDTH

A current-feedback op amp like the OPA2607 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 Performance 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 2 shows the small-

signal frequency-response analysis circuit for the OPA2607.

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

α → Buffer Gain from the Non-inverting Input to the Inverting Input

Z(s)

→ Frequency Dependent Open Loop Transimpedance Gain

DEMO BOARD

ORDERING

PRODUCT

PACKAGE

NUMBER

NUMBER

OPA2607U

SO-8

DEM-OPA268xU

MKT-352

OPA2607N

SO-14 SO-Cool

DEM-OPA2607N

MKT-367

OPA2607H

SO-8 SO-Cool

DEM-OPA2607H

MKT-366

R

F

V

O

R

G

R

I

Z

I

ERR

α

V

I

FIGURE 2. Current-Feedback Transfer Function Analysis

Circuit.

V

O

V

I

=

α 1 +

R

F

R

G









1

+

R

1

+

R

F

R

G









Z

(S)

=

α NG

1

+

R

Z

(S)

NG

≡ 1 +

R

F

R

G

























(1)

Z

(S)

R

= Loop Gain

(2)

TABLE I.