OPA2631 Datasheet, PDF (13/17 Pages) Burr-Brown (TI) – Dual, Low Power, Single-Supply OPERATIONAL AMPLIFIER

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 + ^RT
+ R_F/G₁
R_C
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 Characteris-
tics Plots 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 OPA2631. It is avail-
able free as an unpopulated PC board delivered with descrip-
tive documentation. The summary information for this board
is shown below:
PRODUCT
OPA2631U
PACKAGE
8-Pin SO-8
BOARD
PART
NUMBER
DEM-OPA268xU
LITERATURE
REQUEST
NUMBER
MKT-352
Contact the Burr-Brown Applications support line 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 25Ω 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.
BANDWIDTH VS GAIN: NON-INVERTING OPERATION
Voltage feedback op amps exhibit decreasing closed-loop
bandwidth as the signal gain is increased. In theory, this
relationship is described by the Gain Bandwidth Product
(GBP) shown in the specifications. Ideally, dividing GBP by
the non-inverting signal gain (also called the Noise Gain, or
NG) will predict the closed-loop bandwidth. In practice, this
only holds true when the phase margin approaches 90°, as it
does in high gain configurations. At low gains (increased
feedback factors), most amplifiers will exhibit a more com-
plex response with lower phase margin. The OPA2631 is
compensated to give a slightly peaked response in a non-
inverting gain of 2 (Figure 1). This results in a typical gain
of +2 bandwidth of 75MHz, far exceeding that predicted by
dividing the 68MHz GBP by 2. Increasing the gain will
cause the phase margin to approach 90° and the bandwidth
to more closely approach the predicted value of (GBP/NG).
At a gain of +10, the 7.6MHz bandwidth shown in the
Typical Specifications is close to that predicted using the
simple formula and the typical GBP.
The OPA2631 exhibits minimal bandwidth reduction going
to +3V single supply operation as compared with +5V
supply. This is because the internal bias control circuitry
retains nearly constant quiescent current as the total supply
voltage between the supply pins is changed.
®
13
OPA2631