Datasheet

= =
V
V
O
I
a 1+
R
R
F
G
(
(
R +R
F I
Z
(s)
R
R
F
G
(
(
1+
1+
aNG
R +R
F
I
·NG
Z
(s)
NG=1+
R
R
F
G
(
(
Z
R +R NG
(s)
F I
·
=LoopGain
R
F
V
O
R
G
R
I
Z i
(S) ERR
i
ERR
a
V
I
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
R
I
→ Buffer output impedance
Computer simulation of circuit performance using
i
ERR
→ Feedback error current signal
SPICE is often useful when analyzing the
Z
(s)
→ Frequency-dependent, open-loop
performance of analog circuits and systems. This is
transimpedance gain from i
ERR
to V
O
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
R
I
, the buffer output impedance, is a critical portion of
dG/df characteristics. These models do not attempt
the bandwidth control equation. R
I
for the OPA694 is
to distinguish between package types in their
typically about 30Ω.
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 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
(1)
op amp can be treated as frequency response
compensation elements while their ratios set the
where:
signal gain. Figure 38 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(R
F
+ NG × R
I
) 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
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Product Folder Link(s): OPA694