OPA820-HT

SBOS587

– DECEMBER 2011

www.ti.com

OPERATING SUGGESTIONS

Optimizing Resistor Values

Since the OPA820 is a unity-gain stable, voltage-feedback operational amplifier, 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. Usually, the feedback resistor value should

be between 200

Ω and 1 kΩ. Below 200 Ω, the feedback network will present additional output loading which can

degrade the harmonic distortion performance of the OPA820. Above 1 k

Ω, the typical parasitic capacitance

(approximately 0.2 pF) across the feedback resistor may cause unintentional band limiting in the amplifier

200

in the feedback network, and thus a zero in the forward response. Assuming a 2-pF total parasitic on the

to the required termination value. However, at low inverting gains, the resulting feedback resistor value can

present a significant load to the amplifier output. For example, an inverting gain of 2 with a 50-

Ω input matching

the input matching impedance with a third resistor to ground (see Figure 3). The total input impedance becomes

Bandwidth vs Gain

Voltage-feedback operational amplifiers exhibit decreasing closed-loop bandwidth as the signal gain is increased.

In theory, this relationship is described by the GBP shown in the specifications. Ideally, dividing GBP by the

noninverting 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 signal

gains, most amplifiers will exhibit a more complex response with lower phase margin. The OPA820 is optimized

to give a maximally-flat, 2nd-order Butterworth response in a gain of 2. In this configuration, the OPA820 has

approximately 64

° of phase margin and will show a typical −3-dB bandwidth of 240 MHz. When the phase

margin is 64

°, the closed-loop bandwidth is approximately √2 greater than the value predicted by dividing GBP

by the noise gain.

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 30-MHz bandwidth shown in the Electrical Characteristics

agrees with that predicted using the simple formula and the typical GBP of 280 MHz.

Output Drive Capability

The OPA820 has been optimized to drive the demanding load of a doubly-terminated transmission line. When a

50-

Ω line is driven, a series 50 Ω into the cable and a terminating 50-Ω load at the end of the cable are used.

Under these conditions, the cable impedance will appear resistive over a wide frequency range, and the total

effective load on the OPA820 is 100

Ω in parallel with the resistance of the feedback network. The electrical

characteristics show a

±3.6-V swing into this load—which will then be reduced to a ±1.8-V swing at the

termination resistor. The

±75-mA output drive over temperature provides adequate current drive margin for this

load. Higher voltage swings (and lower distortion) are achievable when driving higher impedance loads.

A single video load typically appears as a 150-

Ω load (using standard 75-Ω cables) to the driving amplifier. The

OPA820 provides adequate voltage and current drive to support up to three parallel video loads (50-

Ω total load)

for an NTSC signal. With only one load, the OPA820 achieves an exceptionally low 0.01%/0.03

° dG/dP error.

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© 2011, Texas Instruments Incorporated

Product Folder Link(s): OPA820-HT