OPA631N BURR-BROWN [Burr-Brown Corporation], OPA631N Datasheet - Page 12

• Manufacturer Part Number: OPA631N
• Description: Low Power, Single-Supply OPERATIONAL AMPLIFIERS TM
• Manufacturer: BURR-BROWN [Burr-Brown Corporation]

FIGURE 3. DC Level-Shifting Circuit.
The front page circuit is a good example of this type of
application. It was designed to take V
0.5V, and produce V
+3V supply. This means G = 2.00, and V
• 0.25V = 1.00V. Plugging into the above equations gives:
NG = 2.33, R
resistors were changed to the nearest standard values.
NON-INVERTING AMPLIFIER WITH
REDUCED PEAKING
Figure 4 shows a non-inverting amplifier that reduces peak-
ing at low gains. The resistor R
or OPA632 to have higher Noise Gain (NG), which reduces
the AC response peaking (typically 5dB at G = +1 without
R
impedance source, such as an op amp. The resistor values
are low to reduce noise. Using both R
minimize the impact of parasitic impedances.
FIGURE 4. Compensated Non-Inverting Amplifier.
The Noise Gain can be calculated as follows:
C
) without changing the DC gain. V
V
IN
V
IN
®
1
= 375 , R
R
OPA631, OPA632
1
R
G
G
NG
T
R
1
2
OUT
G
R
1
3
1
G G
between 1V and 2V, when using a
2
1
R
R
R
R
R
= 2.25k , and R
2
C
G
2
F
T
C
OPA63x
R
OPA63x
+V
R
compensates the OPA631
C
F
S
R
R
/
4
F
G
IN
IN
1
needs to be a low
between 0V and
OUT
T
3
and R
= 563 . The
= 1.50V – G
V
V
OUT
OUT
F
helps
12
A unity gain buffer can be designed by selecting R
20.0
Gain of 2, so its response will be similar to the Characteris-
tics Plots with G = +2 which typically gives a flat frequency
response, but with less bandwidth.
DESIGN-IN TOOLS
DEMONSTRATION BOARDS
Two PC boards are available to assist in the initial evaluation
of circuit performance using the OPA631 and OPA632 in
their three package styles. These are available free as an
unpopulated PC board delivered with descriptive documen-
tation. The summary information for these boards is shown
below:
Contact the Burr-Brown Applications support line to request
any of these boards.
OPERATING SUGGESTIONS
OPTIMIZING RESISTOR VALUES
Since the OPA631 and OPA632 are voltage feedback op
amps, a wide range of resistor values may be used for the
feedback and gain setting resistors. The primary limits on
these values are set by dynamic range (noise and distortion)
and parasitic capacitance considerations. For a non-inverting
unity gain follower application, the feedback connection
should be made with a 25
Figure 4). This will isolate the inverting input capacitance
from the output pin and improve the frequency response
flatness. Usually, for G > 1 application, the feedback resistor
value should be between 200 and 1.5k . Below 200 , the
feedback network will present additional output loading
which can degrade the harmonic distortion performance.
Above 1.5k , the typical parasitic capacitance (approxi-
mately 0.2pF) across the feedback resistor may cause unin-
tentional band-limiting in the amplifier response.
A good rule of thumb is to target the parallel combination of
R
The combined impedance R
ing input capacitance, placing an additional pole in the
feedback network and thus, a zero in the forward response.
Assuming a 3pF total parasitic on the inverting node, hold-
ing R
itself, this constraint implies that the feedback resistor R
can increase to several k at high gains. This is acceptable
as long as the pole formed by R
tance appearing in parallel is kept out of the frequency range
of interest.
PRODUCT
OPA63xU
OPA63xN
F
and R
F
and R
|| R
G
G
(Figure 1) to be less than approximately 400 .
<400
C
= 40.2
5-Pin SOT23-5
6-Pin SOT23-6
PACKAGE
8-Pin SO-8
will keep this pole above 130MHz. By
(do not use R
F
resistor, not a direct short (see
|| R
DEM-OPA68xU
DEM-OPA6xxN
F
G
NUMBER
BOARD
and any parasitic capaci-
PART
interacts with the invert-
G
). This gives a Noise
LITERATURE
REQUEST
NUMBER
MKT-351
MKT-348
T
= R
F
=
F