OPA2634 Datasheet, PDF (12/17 Pages) Burr-Brown (TI) – Dual, Wideband, Single-Supply OPERATIONAL AMPLIFIER

The noise gain can be calculated as follows:
G₁
=
1+
R_F
R_G
G₂
=1+
R_T
+ 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, therefore, its response will be similar to the
typical performance curves with G = +2. Decreasing R_C to
20.0Ω will increase the noise gain to 3, which typically gives
a flat frequency response, but with less bandwidth.
The circuit in Figure 2 can be redesigned to have less peaking
by increasing the noise gain to 3. This is accomplished by
adding R_C = 2.55kΩ between the op amp’s inputs.
DESIGN-IN TOOLS
DEMONSTRATION BOARDS
A PC board is available to assist in the initial evaluation of
circuit performance using the OPA2634U. It is available
free as an unpopulated PC board delivered with descriptive
documentation. The summary information for this board is
shown in Table II.
PRODUCT
PACKAGE
BOARD
PART
NUMBER
LITERATURE
REQUEST
NUMBER
OPA2634U
SO-8
DEM-OPA268xU
MKT-352
TABLE II. 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 OPA2634 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. 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 (approximately 0.2pF) across the feed-
back resistor may cause unintentional bandlimiting in the
amplifier response.
A good rule of thumb is to target the parallel combination of
R_F and R_G (Figure 6) 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 VERSUS 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 (in-
creased feedback factors), most amplifiers will exhibit a
more complex response with lower phase margin. The
OPA2634 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 150MHz, far exceeding that
predicted by dividing the 140MHz 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 16MHz bandwidth shown
in the Specifications is close to that predicted using the
simple formula and the typical GBP.
The OPA2634 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.
12
OPA2634
SBOS098A