OPA643P datasheet(12/18 Pages) TI1

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OPA643

For the values shown in Figure 6, the F–3dB will be

approximately 105MHz. This is less than that predicted by

simply dividing the GBP product by NG1. The compensation

network controls the bandwidth to a lower value while

providing full slew rate and exceptional distortion

performance due to increased loop gain at frequencies below

NG1 • Z0. The capacitor values shown in Figure 6 are

calculated for NG

1 = 3 and NG2 = 7.5 with no adjustment for

parasitics. These differ slightly from the application circuit

on the front page, since those have been adjusted for parasitics

and to account for the capacitive load (through RS) at the

ADC input.

OUTPUT VOLTAGE AND CURRENT DRIVE

The OPA643 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 source resistance 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 OPA643 is 100

Ω in parallel with the resistance

of the feedback network. The specifications show a

guaranteed

±2.5V swing over the full temperature range into

this 100

Ω load—which will then be reduced to a ±1.25V

swing at the termination resistor. The guaranteed

±35mA

output current over temperature provides adequate current

drive margin for this load. Higher voltage swings (and lower

distortion) are achievable when driving higher impedance

loads.

A common IF amplifier specification which describes

available output power is the –1dB compression point. This

is usually defined at a matched 50

Ω load to be the sinusoidal

power where the gain has compressed by –1dB vs the gain

seen at very low power levels. This compression level is

frequency dependent for an op amp, due to both bandwidth

and slew rate limitations. For frequencies well within the

bandwidth and slew rate limit of the OPA643, the –1dB

compression at a matched 50

Ω load will be > 13dBm based

on the minimum available

±1.25V swing at the load. One

common use for the –1dB compression is to predict

intermodulation intercept. This is normally 10dB greater

than the –1dB compression power for a standard RF amplifier.

This simple rule of thumb does NOT apply to the OPA643.

The high open loop gain and Class AB output stage of the

OPA643 produce a much higher intercept than the –1dB

compression would predict, as shown in the Typical

Performance Curves.

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 OPA643, 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 response

fidelity and/or distortion, the simplest and most effective

solution is to isolate this capacitive load from the feedback

loop by inserting a series isolation resistor between the

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, increasing the

phase margin and improving stability.

The Typical Performance Curves show the recommended

series RS vs Capacitive Load and the resulting frequency

response at the load. The criterion for setting this resistor is

a maximum bandwidth, flat frequency response at the load.

Since there is now a passive low pass filter from the output

pin to the load capacitor, 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 for a large

signal pulse response where this slight peaking causes an

overshoot in the step response at the output pin.

Parasitic capacitive loads greater than 2pF can begin to

degrade the performance of the OPA643. Long PC board

traces, unmatched cables, and connections to multiple devices

can easily exceed 2pF. Always take care to consider this, and

add the recommended series resistor as close as possible to

the OPA643 output pin (see Board Layout Guidelines).

DISTORTION PERFORMANCE

The OPA643 is capable of delivering an exceptionally low

distortion signal at high frequencies over a wide range of

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

OPA643’s distortion does not rise above –90dBc until either

the signal level exceeds 0.5V and/or the fundamental

frequency exceeds 500kHz. Distortion in the audio band is

< –120dBc.

Generally, until the fundamental signal reaches very high

frequencies or powers, the second harmonic will dominate

the distortion with negligible third harmonic component.

Focusing then on the second harmonic, increasing the load

impedance improves distortion directly. Remember that the

total load includes the feedback network—in the non-

inverting configuration this is sum of RF + RG, while in the

inverting configuration it is only R

F (Figure 1). Larger

output voltage swings lead directly to increased harmonic

distortion. A 6dB increase in output voltage swing will

generally increase the second harmonic by 12dB and the

third harmonic by 18dB. Higher signal gain settings will also

increase the second harmonic distortion. A 6dB increase in

voltage gain will raise the second and third harmonics by

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