®

OPA642

11

In the inverting configuration, an additional design consid-

eration must be noted. R

therefore the load impedance to the driving source. If imped-

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 50

Ω input matching resistor (= R

a 100

Ω feedback resistor, which would contribute to output

loading in parallel with the external load. In such a case, it

and then achieve the input matching impedance with a third

resistor to ground. The total input impedance becomes the

BANDWIDTH VS GAIN

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 signal gains, most

amplifiers will exhibit a more complex response with lower

phase margin. The OPA642 is optimized to give a maxi-

mally flat second order Butterworth response in a gain of 2.

In this configuration, the OPA642 has approximately 60

° of

phase margin and will show a typical –3dB bandwidth of

150MHz. When the phase margin is 60

°, the closed-loop

bandwidth is approximately

√2 greater than the value pre-

dicted 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 21MHz bandwidth shown

in the Typical Specifications agrees with that predicted

using the simple formula and the typical GBP of 210MHz.

OUTPUT DRIVE CAPABILITY

The OPA642 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’s impedance will appear resistive over

a wide frequency range, and the total effective load on the

OPA642 is 100

Ω in parallel with the resistance of the

feedback network. The Specifications show a guaranteed

±2.5V swing into a such a load—which will then be reduced

to a

±1.25V swing at the termination resistor. The guaran-

teed

±35mA output drive over temperature provides ad-

equate 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 OPA642

provides adequate voltage and current drive to support up to

3 parallel video loads (50

Ω total load) for an NTSC signal.

With only one load, the OPA642 achieves an exceptionally

low 0.007%/0.008

° dG/dP error.

DRIVING CAPACITIVE LOADS

One of the most demanding, and yet very common, load

conditions for an op amp is capacitive loading. A high speed,

high open-loop gain, amplifier like the OPA642 can be very

susceptible to decreased stability and closed-loop response

peaking when a capacitive load is placed directly on the

output pin. In simple terms, the capacitive load reacts with

the open-loop output resistance of the amplifier to introduce

an additional pole into the loop and thereby decrease the

phase margin. This issue has become a popular topic of

application notes and articles, and several external solutions

to this problem have been suggested. When the primary

considerations are frequency response flatness, pulse re-

sponse fidelity and/or distortion, the simplest and most

effective solution is to isolate the capacitive load from the

feedback loop by inserting a series isolation resistor between

the amplifier output and the capacitive load. This does not

eliminate the pole from the loop response, but rather shifts

it and adds a zero at a higher frequency. The additional zero

acts to cancel the phase lag from the capacitive load pole,

thus increasing the phase margin and improving stability.

The Typical Performance Curves show the recommended

R

at the load. The criterion for setting the recommended

resistor is maximum bandwidth, flat frequency response at

the load. Since there is now a passive low pass filter between

the output pin and the load capacitance, the response at the

output pin itself is typically somewhat peaked, and becomes

flat after the rolloff action of the RC network. This is not a

concern in most applications, but can cause clipping if the

desired signal swing at the load is very close to the amplifier’s

swing limit. Such clipping would be most likely to occur in

pulse response applications where the frequency peaking is

manifested as an overshoot in the step response.

Parasitic capacitive loads greater than 2pF can begin to

degrade the performance of the OPA642. Long PC board

traces, unmatched cables, and connections to multiple de-

vices can easily cause this value to be exceeded. Always

consider this effect carefully, and add the recommended

series resistor as close as possible to the OPA642 output pin

(see Board Layout Guidelines).

DISTORTION PERFORMANCE

The OPA642 is capable of delivering an exceptionally low

distortion signal at high frequencies and low gains. The

distortion plots in the Typical Performance Curves show the

typical distortion under a wide variety of conditions. Most of

these plots are limited to 100dB dynamic range. The

OPA642’s distortion does not rise above –100dBc until

either the signal level exceeds 0.5V and/or the fundamental

frequency exceeds 500kHz. Distortion in the audio band is

Generally, until the fundamental signal reaches very high

frequencies or powers, the second harmonic will dominate the

distortion with negligible third harmonic component. Focus-

ing then on the second harmonic, increasing the load imped-

ance improves distortion directly. Remember that the total

load includes the feedback network—in the non-inverting