Datasheet

OPA691
15
SBOS226D
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/dφ 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.
R
I
, the buffer output impedance, is a critical portion of the
bandwidth control equation. The OPA691 is typically about 35Ω.
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:
V
V
R
R
RR
R
R
Z
NG
RRNG
Z
NG
R
R
O
I
F
G
FI
F
G
S
FI
S
F
G
=
+






+
++






=
+
+
=+














α
α
1
1
1
1
1
()
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:
Z
RRNG
Loop Gain
S
FI
()
+
=
If 20 • log (R
F
+ NG • R
I
) 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 R
F
= 402Ω at NG = 2 on
±5V supplies. Evaluating the denominator of Equation 2
(which is the feedback transimpedance) gives an optimal
target of 472Ω. As the signal gain changes, the contribu-
tion of the NG • R
I
term in the feedback transimpedance
will change, but the total can be held constant by adjust-
ing R
F
. Equation 4 gives an approximate equation for
optimum R
F
over signal gain:
RNGR
FI
= 472Ω –
FIGURE 7. Recommended Feedback Resistor versus Noise Gain.
R
F
V
O
R
G
R
I
Z
(S)
i
ERR
i
ERR
α
V
I
The key elements of this current feedback op amp model are:
α → Buffer gain from the noninverting input to the inverting input
R
I
→ Buffer output impedance
i
ERR
→ Feedback error current signal
Z(s) → Frequency dependent open-loop transimpedance gain from i
ERR
to V
O
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)