Datasheet

OPA695
24
SBOS293G
www.ti.com
FIGURE 16. Recommended Feedback Resistor vs Noise
Gain.
600
500
400
300
200
100
0
0 2 4 6 8 10 12 14 16 18
20
Noise Gain (V/V)
Feedback Resistor (Ω)
V
S
= +5V
V
S
= ±5V
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 the buffer gain α < 1.0, the CMRR =
–20 • log (1 – α).
R
I
, the buffer output impedance, is a critical portion of the
bandwidth control equation. For the OPA695, it is typically
about 28Ω for ±5V operation and 31Ω for single +5V opera-
tion.
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 Characteristic 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 18 gives Equation 9:
V
V
R
R
RR
R
R
Z
NG
RRNG
Z
O
I
F
G
FI
F
G
S
FI
S
=
+






+
++






=
•
+
+•
α
α
1
1
1
1
()
()
Where
NC
R
R
Noise Gain
F
G
=+ =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 9 would reduce to 1, and the ideal
desired signal gain shown in the numerator would be achieved.
The fraction in the denominator of Equation 9 determines the
frequency response. Equation 10 shows this as the loop gain
equation:
Z
RRNG
Loop Gain
S
FI
()
+•
=
If 20 • log (R
F
+ NG • R
I
) were superimposed on 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 10, at which
point the loop gain has reduced to 1 (and the curves have
intersected). This point of equality is where the amplifier
closed-loop frequency response given by Equation 9 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-feedback op amp. The difference here is that the
total impedance in the denominator of Equation 10 may be
controlled separately from the desired signal gain (or NG).
The OPA695 is internally compensated to give a maximally
flat frequency response for R
F
= 402Ω at NG = 8 on ±5V
supplies. Evaluating the denominator of Equation 7 (which is
the feedback transimpedance) gives an optimal target of
663Ω. As the signal gain changes, the contribution of the
NG • R
I
term in the feedback transimpedance will change, but
the total can be held constant by adjusting R
F
. Equation 11
gives an approximate equation for optimum R
F
over signal
gain:
RNGR
FI
=•663Ω –
As the desired signal gain increases, this equation will
eventually predict a negative R
F
. A somewhat subjective limit
to this adjustment can also be set by holding R
G
to a
minimum value of 10Ω. Lower values will load both the buffer
stage at the input and the output stage if R
F
gets too low,
actually decreasing the bandwidth. Figure 16 shows the
recommended R
F
versus NG for both ±5V and a single +5V
operation. The optimum target feedback impedance for +5V
operation used in Equation 8 is 663Ω, while the typical buffer
output impedance is 32Ω. The values for R
F
versus gain
shown here are approximately equal to the values used to
generate the Typical Characteristic curves. In some cases,
the values used differ slightly from that shown here, in that
the values used in the Typical Characteristics are also
correcting for board parasitics not considered in the simpli-
fied analysis leading to Equation 11. The values shown in
Figure 16 give a good starting point for designs where
bandwidth optimization is desired and a flat frequency re-
sponse is needed.
(9)
(10)
The total impedance presented to the inverting input may be
used to adjust the closed-loop signal bandwidth. Inserting a
series resistor between the inverting input and the summing
junction will increase the feedback impedance (denominator
of Equation 10), decreasing the bandwidth. The internal
buffer output impedance for the OPA695 is slightly influ-
enced by the source impedance looking out of the non-
inverting input terminal. High source resistors will have the
effect of increasing R
I
, decreasing the bandwidth. For those
single-supply applications which develop a midpoint bias at
the non-inverting input through high-valued resistors, the
decoupling capacitor is essential for power-supply ripple
rejection, non-inverting input noise current shunting, and
minimizing the high-frequency value for R
I
in Figure 15.
(11)