Datasheet

1/2
OPA2890
50W
R
F
750W
R
G
375W
R
B
261W
R
M
57.6W
Source
+5V
-5V
R
O
50W
0.1 Fm 6.8 Fm
+
0.1 Fm
0.1 Fm
6.8 Fm
+
50 LoadW
V
O
V
I
= 2V/V-
V
O
V
I
OPA2890
www.ti.com SBOS364C –DECEMBER 2007–REVISED DECEMBER 2009
discussed in the previous section. The amplifier Combining this resistance in parallel with the
output, however, now sees the 100Ω feedback feedback resistor gives the R
B
= 261Ω used in this
resistor in parallel with the external load. In general, example. To reduce the additional high-frequency
the feedback resistor should be limited to the 200Ω to noise introduced by this resistor, it is sometimes
1.5kΩ range. In this case, it is preferable to increase bypassed with a capacitor. As long as R
B
< 350Ω, the
both the R
F
and R
G
values (see Figure 56), and then capacitor is not required because the total noise
achieve the input matching impedance with a third contribution of all other terms is less than that of the
resistor (R
M
) to ground. The total input impedance op amp input noise voltage. As a minimum, the
becomes the parallel combination of R
G
and R
M
. 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
Figure 59. Gain of –2V/V Example Circuit
load from the feedback loop by inserting a
series-isolation resistor between the amplifier output
and the capacitive load. This solution does not
The second major consideration, touched on in the
eliminate the pole from the loop response, but rather
previous paragraph, is that the signal source
shifts it and adds a zero at a higher frequency. The
impedance becomes part of the noise gain equation
additional zero acts to cancel the phase lag from the
and influences the bandwidth. For the example in
capacitive load pole, thus increasing the phase
Figure 59, the R
M
value combined in parallel with the
margin and improving stability.
external 50Ω source impedance yields an effective
driving impedance of 50Ω || 57.6Ω = 26.7Ω. This
The Typical Characteristics show the recommended
impedance is added in series with R
G
for calculating
R
S
versus capacitive load (see Figure 15 and
the noise gain (NG). The resultant NG is 2.86V/V for
Figure 36) and the resulting frequency response at
Figure 59, as opposed to only 2V/V if R
M
could be
the load. Parasitic capacitive loads greater than 2pF
eliminated as discussed above. Therefore, the
can begin to degrade the performance of the
bandwidth is slightly lower for the gain of –2V/V
OPA2890. Long PCB traces, unmatched cables, and
circuit of Figure 59 than for the gain of +2V/V circuit
connections to multiple devices can easily exceed
of Figure 49.
this value. Always consider this effect carefully, and
add the recommended series resistor as close as
The third important consideration in inverting amplifier
possible to the OPA2890 output pin (see the Board
design is setting the bias current cancellation resistor
Layout Guidelines section).
on the noninverting input (R
B
). If this resistor is set
equal to the total dc resistance looking out of the
DISTORTION PERFORMANCE
inverting node, the output dc error (as a result of the
input bias currents) is reduced to [(Input Offset
The OPA2890 provides good distortion performance
Current) × R
F
]. If the 50Ω source impedance is
into a 100Ω load on ±5V supplies. Relative to
DC-coupled in Figure 57, the total resistance to
alternative solutions, it provides exceptional
ground on the inverting input is 402Ω.
performance into lighter loads and/or operating on a
single +5V supply. Generally, until the fundamental
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