OPA2634 Datasheet, PDF (12/16 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
characteristics plots 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 OPA2634. It is available free
as an unpopulated PC board delivered with descriptive
documentation. The summary information for this board is
shown below:
PRODUCT
OPA2634U
PACKAGE
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 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 25Ω 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 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 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
Typical 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.
INVERTING AMPLIFIER OPERATION
Since the OPA2634 is a general purpose, wideband voltage
feedback op amp, all of the familiar op amp application
circuits are available to the designer. Figure 7 shows a
typical inverting configuration where the I/O impedances
and signal gain from Figure 1 are retained in an inverting
circuit configuration. Inverting operation is one of the more
common requirements and offers several performance ben-
efits. The inverting configuration shows improved slew rate
and distortion. It also biases the input at V_S/2 for the best
headroom. The output voltage can be independently moved
with bias-adjustment resistors connected to the input.
In the inverting configuration, three key design consider-
ation must be noted. The first is that the gain resistor (R_G)
becomes part of the signal channel input impedance. If input
impedance matching is desired (which is beneficial when-
ever the signal is coupled through a cable, twisted pair, long
®
OPA2634
12