OPA2890_08 Datasheet, PDF (23/35 Pages) Texas Instruments – Low-Power, Wideband, Voltage-Feedback OPERATIONAL AMPLIFIER with Disable

OPA2890
www.ti.com.................................................................................................................................................... SBOS364B – DECEMBER 2007 – REVISED MAY 2008
discussed in the previous section. 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 56), 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
OPA2890
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
= -2V/V
0.1mF
6.8mF
+
-5V
Figure 59. 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 59, 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 resultant NG is 2.86V/V for
Figure 59, 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 59 than for the gain of +2V/V circuit
of Figure 49.
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 57, the total resistance to
ground on the inverting input is 402Ω.
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 is less than that of the
op amp input noise voltage. As a minimum, the
OPA2890 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
OPA2890 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 Typical Characteristics show the recommended
R_S versus capacitive load (see Figure 15 and
Figure 36) and the resulting frequency response at
the load. Parasitic capacitive loads greater than 2pF
can begin to degrade the performance of the
OPA2890. 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 OPA2890 output pin (see the Board
Layout Guidelines section).
DISTORTION PERFORMANCE
The OPA2890 provides good distortion performance
into a 100Ω load on ±5V supplies. Relative to
alternative solutions, it provides exceptional
performance into lighter loads and/or operating on a
single +5V supply. Generally, until the fundamental
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