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OPA686

Inserting high resistor values into Eq. 2 can quickly domi-

nate the total equivalent input referred noise. A 105

Ω source

impedance on the non-inverting input will add a Johnson

voltage noise term equal to that of the amplifier itself. As a

to ground on the non-inverting input). This results in Eq. 3,

where NG > 10 has been assumed to further simplify the

expression.

Equation 3

equivalent input noise of 1.7nV/

√Hz. Note that the NG has

dropped out of this expression. This is valid only for NG > 10

as will typically be required by stability considerations.

FREQUENCY RESPONSE CONTROL

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 factor), most high speed amplifiers will exhibit a

more complex response with lower phase margin. The

OPA686 is compensated to give a maximally flat 2nd-order

Butterworth closed-loop response at a non-inverting gain of

+10 (Figure 1). This results in a typical gain of +10 band-

width of 250MHz, far exceeding that predicted by dividing

the 1600MHz GBP by 10. 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 +40, the OPA686 will show the 40MHz bandwidth

predicted using the simple formula and the typical GBP of

1600MHz.

Inverting operation offers some interesting opportunities to

increase the available GBP. When the source impedance is

matched by the gain resistor (Figure 2), the signal gain is

ing the minimum stable gain for inverting operation under

these condition to –12 and the equivalent GBP to 3.2GHz.

DRIVING CAPACITIVE LOADS

One of the most demanding and yet very common load

conditions for an op amp is capacitive loading. Often, the

capacitive load is the input of an A/D converter, including

additional external capacitance which may be recommended

to improve A/D linearity. A high speed, high open-loop gain

amplifier like the OPA686 can be very susceptible to de-

creased stability and closed-loop response peaking when a

capacitive load is placed directly on the output pin. When

the amplifier’s open-loop output resistance is considered,

this capacitive load introduces an additional pole in the

signal path that can decrease the phase margin. Several

external solutions to this problem have been suggested.

When the primary considerations are frequency response

flatness, pulse response fidelity and/or distortion, the sim-

plest 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 re-

sponse, 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

at the load. Parasitic capacitive loads greater than 2pF can

begin to degrade the performance of the OPA686. Long PC

board traces, unmatched cables, and connections to multiple

devices 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 OPA686 output pin

(see Board Layout Guidelines).

bandwidth, flat frequency response at the load. For the

OPA686 operating in a gain of +10, the frequency response

at the output pin is very flat to begin with, allowing relatively

signal gain is increased, the unloaded phase margin will also

increase. Driving capacitive loads at higher gains will re-

DISTORTION PERFORMANCE

The OPA686 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 condi-

tions. Most of these plots are limited to 110dB dynamic

range. The OPA686’s distortion, driving a 500

Ω, load does

not rise above –90dBc until either the signal level exceeds

2.0Vp-p and/or the fundamental frequency exceeds 5MHz.

Distortion in the audio band is < –120dBc.

Generally, until the fundamental signal reaches very high

frequencies or powers, the 2nd harmonic will dominate the

distortion with negligible a 3rd harmonic component. Focus-

ing then on the 2nd harmonic, increasing the load impedance

improves distortion directly. Remember that the total load

includes the feedback network; in the non-inverting configu-

output voltage swing increases harmonic distortion directly.

A 6dB increase in output swing will generally increase the

2nd harmonic 12dB and the 3rd harmonic 18dB. Increasing

the signal gain will also increase the 2nd harmonic distor-

tion. Again, a 6dB increase in gain will increase the 2nd and

3rd harmonic by approximately 6dB even with constant

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