OPA2631 Datasheet, PDF (13/18 Pages) Burr-Brown (TI) – Dual, Low Power, Single-Supply OPERATIONAL AMPLIFIER

Make sure that V_IN and V_OUT stay within the specified input
and output voltage ranges.
The front page circuit is a good example of this type of
application. It was designed to take V_IN between 0V and 0.5V,
and produce V_OUT between 1V and 2V, when using a +3V
supply. This means G = 2.00, and ∆V_OUT = 1.50V – G • 0.25V
= 1.00V. Plugging into the above equations (with R₄ = 750Ω)
gives: NG = 2.33, R₁ = 375Ω, R₂ = 2.25kΩ, and R₃ = 563Ω.
The resistors were adjusted to the nearest standard values.
NON-INVERTING AMPLIFIER WITH
REDUCED PEAKING
Figure 6 shows a non-inverting amplifier that reduces peak-
ing at low gains. The resistor R_C compensates the OPA2631
to have higher Noise Gain (NG), which reduces the AC
response peaking (typically 5dB at G = +1 without R_C)
without changing the DC gain. V_IN needs to be a low
impedance source, such as an op amp. The resistor values
are low to reduce noise. Using both R_T and R_F helps
minimize the impact of parasitic impedances.
R_T
V_IN
1/2
R_{C OPA2631}
V_OUT
R_G
R_F
FIGURE 6. Compensated Non-Inverting Amplifier.
The Noise Gain can be calculated as follows:
G₁
=
1+
R_F
R_G
G₂
=
1+
R_T
+ R_F
R_C
/
G₁
NG = G₁G₂
A unity gain buffer can be designed by selecting
R_T = R_F = 20.0Ω and R_C = 40.2Ω (do not use R_G). This gives
a Noise Gain of 2, so its response will be similar to the
typical performance curves with G = +2 which typically
gives a flat frequency response, but with less bandwidth.
DESIGN-IN TOOLS
DEMONSTRATION BOARDS
A single PC board is available to assist in the initial evalu-
ation of circuit performance using the OPA2631U. It is
available free as an unpopulated PC board delivered with
descriptive documentation. The summary information for
this board is shown in Table I.
PRODUCT
PACKAGE
BOARD
PART
NUMBER
LITERATURE
REQUEST
NUMBER
OPA2631U
SO-8
DEM-OPA268xU
MKT-352
TABLE I. Demo Board Summary Information.
Contact the Texas Instruments Technical Applications Sup-
port Line at 1-972-644-5580 to request this board.
OPERATING SUGGESTIONS
OPTIMIZING RESISTOR VALUES
Since the OPA2631 is a voltage-feedback op amp, 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 20Ω resistor, not a direct short (see Figure 6).
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 200Ω and 1.5kΩ. Below 200Ω, the
feedback network will present additional output loading
which can degrade the harmonic-distortion performance.
Above 1.5kΩ, 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_F and R_G (Figure 1) to be less than approximately 400Ω.
The combined impedance (R_F || R_G) interacts with the invert-
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_F || R_G < 400Ω will keep this pole above 130MHz. By
itself, this constraint implies that the feedback resistor R_F
can increase to several kΩ at high gains. This is acceptable
as long as the pole formed by R_F, and any parasitic capaci-
tance appearing in parallel, is kept out of the frequency
range of interest.
OPA2631
13
SBOS067A