OPA2889 Datasheet, PDF (23/35 Pages) Burr-Brown (TI) – Dual, Low-Power, Wideband, Voltage Feedback OPERATIONAL AMPLIFIER with Disable

OPA2889
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considered in Figure 60 The amplifier output,
however, now sees the 100Ω feedback resistor in
parallel with the external load. In general, the
feedback resistor should be limited to the 200Ω to
1.5kΩ range. In this case, it is preferable to increase
both the R_F and R_G values (see Figure 60), and then
achieve the input matching impedance with a third
resistor (R_M) to ground. The total input impedance
becomes the parallel combination of R_G and R_M.
+5V
0.1mF
R_B
261W
+
0.1mF
6.8mF
1/2
OPA2889
R_O
V_O 50W
50W Load
50W
Source
V_I
R_G
375W
R_M
57.6W
R_F
750W
V_O
V_I
= -2
0.1mF
6.8mF
+
-5V
Figure 60. Gain of –2V/V Example Circuit
The second major consideration, touched on in the
previous paragraph, is that the signal source
impedance becomes part of the noise gain equation
and influences the bandwidth. For the example in
Figure 60, the R_M value combined in parallel with the
external 50Ω source impedance yields an effective
driving impedance of 50Ω || 57.6Ω = 26.7Ω. This
impedance is added in series with R_G for calculating
the noise gain (NG). The resulting NG is 2.86V/V for
Figure 60, as opposed to only 2V/V if R_M could be
eliminated as discussed above. Therefore, the
bandwidth is slightly lower for the gain of –2V/V
circuit of Figure 60 than for the gain of +2V/V circuit
of Figure 50.
The third important consideration in inverting
amplifier design is setting the bias current
cancellation resistor on the noninverting input (R_B). If
this resistor is set equal to the total dc resistance
looking out of the inverting node, the output dc error,
as a result of the input bias currents, is reduced to
(Input Offset Current) × R_F. If the 50Ω source
impedance is dc-coupled in Figure 60, the total
resistance to ground on the inverting input is 402Ω.
SBOS373 – JUNE 2007
Combining this resistance in parallel with the
feedback resistor gives the R_B = 261Ω used in this
example. To reduce the additional high-frequency
noise introduced by this resistor, it is sometimes
bypassed with a capacitor. As long as R_B < 350Ω,
the capacitor is not required because the total noise
contribution of all other terms will be less than that of
the op amp input noise voltage. As a minimum, the
OPA2889 requires an R_B value of 50Ω to damp out
parasitic-induced peaking—a direct short to ground
on the noninverting input runs the risk of a very
high-frequency instability in the input stage.
DRIVING CAPACITIVE LOADS
One of the most demanding and yet very common
load conditions for an op amp is capacitive loading.
Often, the capacitive load is the input of an
ADC—including additional external capacitance that
may be recommended to improve ADC linearity. A
high-speed, high open-loop gain amplifier such as
the OPA2889 can be very susceptible to decreased
stability and closed-loop response peaking when a
capacitive load is placed directly on the output pin.
When the open-loop output resistance of the
amplifier is considered, this capacitive load
introduces an additional pole in the signal path that
can decrease the phase margin. Several external
solutions to this problem have been suggested.
When the primary considerations are frequency
response flatness, pulse response fidelity, and/or
distortion, the simplest and most effective solution is
to isolate the capacitive load from the feedback loop
by inserting a series-isolation resistor between the
amplifier output and the capacitive load. This solution
does not eliminate the pole from the loop response,
but rather shifts it and adds a zero at a higher
frequency. The additional zero acts to cancel the
phase lag from the capacitive load pole, thus
increasing the phase margin and improving stability.
The 5 Typical Chararacteristics show the
recommended R_S versus capacitive load (see
Figure 15 and Figure 16) and the resulting frequency
response at the load. Parasitic capacitive loads
greater than 2pF can begin to degrade the
performance of the OPA2889. Long PCB traces,
unmatched cables, and connections to multiple
devices can easily exceed this value. Always
consider this effect carefully, and add the
recommended series resistor as close as possible to
the OPA2889 output pin (see the Board Layout
Guidelines section).
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