OPA830-EP Datasheet, PDF (22/36 Pages) National Semiconductor (TI) – Low-Power, Single-Supply, Wideband Operational Amplifier

OPA830-EP
SBOS655 – MARCH 2014
www.ti.com
8.2.1.5.3 Operating Suggestions
8.2.1.5.3.1 Optimizing Resistor Values
Since the OPA830 is a unity-gain stable, 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 direct short.
Below 200Ω, the feedback network will present additional output loading which can degrade the harmonic
distortion performance of the OPA830. Above 1kΩ, the typical parasitic capacitance (approximately 0.2pF)
across the feedback resistor may cause unintentional band limiting in the amplifier response.
A good rule of thumb is to target the parallel combination of R_F and R_G (see Figure 52) to be less than about
400Ω. The combined impedance R_F || R_G interacts with the inverting input capacitance, placing an additional pole
in the feedback network, and thus a zero in the forward response. Assuming a 2pF total parasitic on the inverting
node, holding R_F || R_G < 400Ω will keep this pole above 200MHz. By itself, this constraint implies that the
feedback resistor RF can increase to several kΩ at high gains. This is acceptable as long as the pole formed by
R_F and any parasitic capacitance appearing in parallel is kept out of the frequency range of interest.
In the inverting configuration, an additional design consideration must be noted. R_G becomes the input resistor
and therefore the load impedance to the driving source. If impedance matching is desired, R_G may be set equal
to the required termination value. However, at low inverting gains, the resultant feedback resistor value can
present a significant load to the amplifier output. For example, an inverting gain of 2 with a 50Ω input matching
resistor (R_G) would require a 100Ω feedback resistor, which would contribute to output loading in parallel with the
external load. In such a case, it would be preferable to increase both the R_F and R_G values, and then achieve the
input matching impedance with a third resistor to ground (see Figure 56). The total input impedance becomes the
parallel combination of R_G and the additional shunt resistor.
8.2.1.5.3.2 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 complex response with lower phase
margin. The OPA830 is compensated to give a slightly peaked response in a non-inverting gain of 2 (see
Figure 52). This results in a typical gain of +2 bandwidth of 110MHz, far exceeding that predicted by dividing the
110MHz 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 11MHz bandwidth shown in the
Electrical Characteristics agrees with that predicted using the simple formula and the typical GBP of 110MHz.
Frequency response in a gain of +2 may be modified to achieve exceptional flatness simply by increasing the
noise gain to 3. One way to do this, without affecting the +2 signal gain, is to add an 2.55kΩ resistor across the
two inputs, as shown in Figure 59. A similar technique may be used to reduce peaking in unity-gain (voltage
follower) applications. For example, by using a 750Ω feedback resistor along with a 750Ω resistor across the two
op amp inputs, the voltage follower response will be similar to the gain of +2 response of . Further reducing the
value of the resistor across the op amp inputs will further dampen the frequency response due to increased noise
gain. The OPA830 exhibits minimal bandwidth reduction going to single-supply (+5V) operation as compared
with ±5V. This minimal reduction is because the internal bias control circuitry retains nearly constant quiescent
current as the total supply voltage between the supply terminals is changed.
8.2.1.5.3.3 Inverting Amplifier Operation
All of the familiar op amp application circuits are available with the OPA830 to the designer. See Figure 56 for a
typical inverting configuration where the I/O impedances and signal gain from Figure 51 are retained in an
inverting circuit configuration. Inverting operation is one of the more common requirements and offers several
performance benefits. It also allows the input to be biased at V_S/2 without any headroom issues. The output
voltage can be independently moved to be within the output voltage range with coupling capacitors, or bias
adjustment resistors.
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