OPA2695 Datasheet, PDF (27/40 Pages) Texas Instruments – Dual, Ultra-Wideband, Current-Feedback OPERATIONAL AMPLIFIER with Disable

OPA2695
www.ti.com..................................................................................................................................................................................................... SBOS354 – APRIL 2008
Developing the transfer function for the circuit of
Figure 79 gives Equation 5:
a 1+ ^RF
V_O
=
R_G
aNG
=
V_I
R_F + R_I
1+
1+ ^RF
R_G
1+R^{F + RI ´ NG}
Z_(S)
Z_(S)
(5)
Where: NC = 1 + R_F/R_G = Noise Gain
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:
Z_(S)
= Loop Gain
R_F + R_I ´ NG
(6)
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
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 5 starts 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 6 may be controlled
separately from the desired signal gain (or NG).
The OPA2695 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 5 (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 changes, but the total can
be held constant by adjusting R_F. Equation 7 gives an
approximate equation for optimum R_F over signal
gain:
R_F = 663W - NG ´ R_I
(7)
As the desired signal gain increases, this equation
eventually predicts 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
load both the buffer stage at the input and the output
stage if R_F goes too low, actually decreasing the
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
operation used in Equation 5 is 604Ω, 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
simplified analysis leading to Equation 7. The values
shown in Figure 77 give a good starting point for
designs where bandwidth optimization is desired and
a flat frequency response is needed.
600
500
V_S = ±5V
400
300
V_S = +5V
200
100
0
0 2 4 6 8 10 12 14 16 18 20
Noise Gain (V/V)
Figure 77. Recommended Feedback Resistor
versus Noise Gain
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 increases
the feedback impedance (denominator of Equation 6)
and decreases the bandwidth. The internal buffer
output impedance for the OPA2695 is slightly
influenced by the source impedance looking out of
the noninverting input terminal. High source resistors
have the effect of increasing R_I and decreasing the
bandwidth. For those single-supply applications that
develop a midpoint bias at the noninverting input
through high-valued resistors, the decoupling
capacitor is essential for power-supply ripple
rejection, noninverting input noise current shunting,
and minimizing the high-frequency value for R_I in
Figure 76.
OUTPUT CURRENT AND VOLTAGE
The OPA2695 provides output voltage and current
capabilities that are consistent with driving
doubly-terminated 50Ω lines. For a 100Ω load at a
gain of +8V/V (see Figure 68), the total load is the
parallel combination of the 100Ω load and the 458Ω
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