Datasheet

V
O
V
I
=
=
R R+
F I
1+
Z
(S)
1+
R
F
R
G
R R NG+ ´
F I
Z
(S)
1+
aNG
a
1+
R
F
R
G
(5)
= LoopGain
Z
(S)
R +R ´
F I
NG
(6)
600
500
400
300
200
100
0
0 2 4 6 8 10 12 14 16 18
20
NoiseGain(V/V)
FeedbackResistor( )W
V =+5V
S
V = 5V
S
±
R = 663 NG RW - ´
F I
(7)
OUTPUT CURRENT AND VOLTAGE
OPA2695
www.ti.com ...................................................................................................................................................... SBOS354A – APRIL 2008 – REVISED AUGUST 2008
Developing the transfer function for the circuit of operation used in Equation 5 is 604 Ω , while the
Figure 79 gives Equation 5 : typical buffer output impedance is 32 Ω . 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
simplified analysis leading to Equation 7 . The values
shown in Figure 77 give a good starting point for
Where: NC = 1 + R
F
/R
G
= Noise Gain
designs where bandwidth optimization is desired and
a flat frequency response is needed.
This formula 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 5 would reduce to 1, and the ideal desired
signal gain shown in the numerator would be
achieved. The fraction in the denominator of
Equation 5 determines the frequency response and
also gives an expression for the loop gain:
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 6 , at which point the loop
Figure 77. Recommended Feedback Resistor
gain has reduced to 1 (and the curves have
versus Noise Gain
intersected). This point of equality is where the
amplifier closed-loop frequency response given by
Equation 5 starts to roll off, and is exactly analogous The total impedance presented to the inverting input
to the frequency at which the noise gain equals the may be used to adjust the closed-loop signal
open-loop voltage gain for a voltage-feedback op bandwidth. Inserting a series resistor between the
amp. The difference here is that the total impedance inverting input and the summing junction increases
in the denominator of Equation 6 may be controlled the feedback impedance (denominator of Equation 6 )
separately from the desired signal gain (or NG). and decreases the bandwidth. The internal buffer
output impedance for the OPA2695 is slightly
The OPA2695 is internally compensated to give a
influenced by the source impedance looking out of
maximally flat frequency response for R
F
= 402 Ω at
the noninverting input terminal. High source resistors
NG = 8 on ± 5V supplies. Evaluating the denominator
have the effect of increasing R
I
and decreasing the
of Equation 5 (the feedback transimpedance) gives
bandwidth. For those single-supply applications that
an optimal target of 663 Ω . As the signal gain
develop a midpoint bias at the noninverting input
changes, the contribution of the NG × R
I
term in the
through high-valued resistors, the decoupling
feedback transimpedance changes, but the total can
capacitor is essential for power-supply ripple
be held constant by adjusting R
F
. Equation 7 gives an
rejection, noninverting input noise current shunting,
approximate equation for optimum R
F
over signal
and minimizing the high-frequency value for R
I
in
gain:
Figure 76 .
As the desired signal gain increases, this equation
eventually predicts a negative R
F
. A somewhat
The OPA2695 provides output voltage and current
subjective limit to this adjustment can also be set by
capabilities that are consistent with driving
holding R
G
to a minimum value of 10 Ω . Lower values
doubly-terminated 50 Ω lines. For a 100 Ω load at a
load both the buffer stage at the input and the output
gain of +8V/V (see Figure 68 ), the total load is the
stage if R
F
goes too low, actually decreasing the
parallel combination of the 100 Ω load and the 458 Ω
bandwidth. Figure 77 shows the recommended R
F
versus NG for both ± 5V and a single +5V operation.
The optimum target feedback impedance for +5V
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