OPA890IDBV BURR-BROWN [Burr-Brown Corporation], OPA890IDBV Datasheet - Page 21

• Manufacturer Part Number: OPA890IDBV
• Description: Low-Power, Wideband, Voltage-Feedback OPERATIONAL AMPLIFIER with Disable
• Manufacturer: BURR-BROWN [Burr-Brown Corporation]

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
external 50
driving impedance of 50
impedance is added in series with R
the noise gain (NG). The resulting NG is 3.14V/V for
Figure
eliminated as discussed previously. The bandwidth is
therefore slightly lower for the gain of –2V/V circuit of
Figure 51
Figure
The
amplifier
cancellation resistor on the noninverting input (R
this resistor is set equal to the total dc resistance
looking out of the inverting node, the output dc error
(because of the input bias currents) is reduced to
(Input Offset Current)
impedance is dc-coupled in
resistance to ground on the inverting input is 351 .
Combining this resistance in parallel with the
feedback resistor gives the value of R
in
high-frequency noise introduced by this resistor, it is
sometimes bypassed with a capacitor. As long as R
< 350 , a 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 OPA890 requires an R
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.
Source
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50W
this
Figure 51. Gain of –2V/V Example Circuit
third
51, the R
46.
51, as opposed to only 2 if R
0.1 F
example.
design
R
59W
than for the gain of +2V/V circuit of
R
240W
m
M
important
B
324W
source impedance, yielding an effective
R
G
M
value combines in parallel with the
0.1 F
is
m
To
setting
OPA890
consideration
+5V
-5V
R
B
reduce
F
value of 50
750W
. If the 50
R
Figure
F
59
the
0.1 F
0.1 F
m
m
DIS
G
the
B
= 27 . This
for calculating
bias
51, the total
= 240
50W
+
in
+
R
M
O
6.8 F
6.8 F
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additional
50
could be
m
m
to damp
inverting
W
current
source
Load
used
B
). If
B
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 OPA890 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 amplifier open-loop output resistance 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 reduce the phase lag from the
capacitive load pole, thus increasing the phase
margin and improving stability.
The Typical Characteristics show the recommended
R
frequency response at the load. Parasitic capacitive
loads greater than 2pF can begin to degrade the
performance of the OPA890. Long PCB traces,
unmatched cables, and connections to multiple
devices can easily exceed this value. Always
consider
recommended series resistor as close as possible to
the OPA890 output pin (see the
Guidelines
NOISE PERFORMANCE
The input-referred voltage noise, and the two
input-referred current noise terms, combine to give
low output noise under a wide variety of operating
conditions.
analysis model with all the noise terms included. In
this model, all noise terms are taken to be noise
voltage or current density terms in either nV/ Hz or
pA/ Hz.
S
versus
this
section).
Figure 52
capacitive
effect
shows the op amp noise
carefully,
load
and
and
SBOS369 – MAY 2007
Board Layout
the
OPA890
add
resulting
the
21