FREQUENCY RESPONSE CONTROL
The criterion for setting the RS resistor for maximum band-
width, flat frequency response at the load is a simple proce-
dure. For the OPA846 operating in a gain of +10V/V, the
frequency response at the output pin is very flat to begin with,
allowing relatively small values of RS to be used for low
capacitive loads. As the signal gain increases, the unloaded
phase margin also increases. Driving capacitive loads at
higher gain settings require lower RS values than those
shown for a gain of +10V/V.
Voltage-feedback op amps exhibit decreasing closed-loop
bandwidth as the signal gain is increased. In theory, this
relationship is described by the GBP shown in the Electrical
Characteristics. Ideally, dividing GBP by the noninverting
signal gain (also called the noise gain, or NG) predicts 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 exhibit a more complex response
with lower phase margin. The OPA846 is compensated to
give a maximally flat 2nd-order Butterworth closed-loop re-
sponse at a noninverting gain of +10 (see Figure 1). This
results in a typical gain of +10 bandwidth of 400MHz, far
exceeding that predicted by dividing the 1750MHz GBP by
10. Increasing the gain causes the phase margin to approach
90° and the bandwidth to more closely approach the pre-
dicted value of (GBP/NG). At a gain of +50, the OPA846
shows the 35MHz bandwidth predicted using the simple
formula F–3dB = GBP/NG.
DISTORTION PERFORMANCE
The OPA846 is capable of delivering an exceptionally low
distortion signal at high frequencies over a wide range of
gains. The distortion plots found in the Typical Characteristic
curves show the typical distortion under a wide variety of
conditions. Most of these plots are limited to 110dB dynamic
range. The OPA846 distortion, while driving a 500Ω load,
does not rise above –90dBc until either the signal level
exceeds 2.0VPP and/or the fundamental frequency exceeds
5MHz. Distortion in the audio band is < –120dBc.
Inverting operation offers some interesting opportunities to
increase the available GBP. When the source impedance is
matched by the gain resistor (see Figure 2), the signal gain
is (– RF/RG), while the noise gain for bandwidth purposes is
(1 + RF/2RG). This cuts the noise gain almost in half,
increasing the minimum stable gain for inverting operation
under these conditions to –12V/V and increases the equiva-
lent GBP to > 3.5GHz.
Generally, until the fundamental signal reaches very high
frequencies or power, the 2nd-harmonic dominates the dis-
tortion with negligible 3rd-harmonic component. Focusing
then on the 2nd-harmonic, increasing the load impedance
improves distortion directly. Remember that the total load
includes the feedback network: in the noninverting configura-
tion, this is the sum of RF + RG, while in the inverting
configuration it is just RF (see Figures 1 and 2). Increasing
output voltage swing increases harmonic distortion directly.
A 6dB increase in output swing generally increases the 2nd-
harmonic to 12dB and the 3rd-harmonic to 18dB. Increasing
the signal gain also increases the 2nd-harmonic distortion.
Again, a 6dB increase in gain increases the 2nd- and 3rd-
harmonic by approximately 6dB, even with constant output
power and frequency. Finally, the distortion increases as the
fundamental frequency increases, due to the roll-off in the
loop gain with frequency. Conversely, the distortion improves
going to lower frequencies down to the dominant open-loop
pole at approximately 100kHz. Starting from the –86dBc 2nd-
harmonic for a 5MHz, 2VPP fundamental into a 200Ω load at
a gain = +10V/V (from the Typical Characteristic curves), the
2nd-harmonic distortion for frequencies lower than 100KHz
is approximately –86dBc – 20 log(5MHz/100kHz) = –120dBc.
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 that may be recommended to
improve A/D linearity. A high-speed, high open-loop gain
amplifier like the OPA846 is susceptible to decreasing stabil-
ity with capacitive loads and results in closed-loop response
peaking when a capacitive load is placed directly on the
amplifier output pin. If the primary considerations are fre-
quency response flatness, pulse 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 ca-
pacitive 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 OPA846 has extremely low 3rd-order distortion. This
also gives a high 2-tone, 3rd-order intermodulation intercept,
as shown in the Typical Characteristic curves. This intercept
curve is defined at the 50Ω load when driven through a 50Ω-
matching resistor to allow direct comparisons to RF devices.
This matching network attenuates the voltage swing from the
output pin to the load by 6dB. If the OPA846 drives directly
into the input of a high-impedance device, such as an A/D
converter, the 6dB attenuation is not present. Under these
conditions, the intercept increases by a minimum of 6dBm.
The intercept is used to predict the intermodulation spurious
for two closely-spaced frequencies. If the two test frequen-
cies f1 and f2 are specified in terms of average and delta
frequency, fO = (f1 + f2)/2 and ∆f = f2 – f1 /2, the two 3rd-
order, close-in spurious tones appear at fO ±3 • ∆f. The
The Typical Characteristic curves help the designer pick a
recommended RS versus Capacitive Load. The resulting fre-
quency response curves show the flat response for a given
capacitive load. Parasitic capacitive loads greater than 2pF
can begin to degrade the performance of the OPA846. Long
PC board traces, unmatched cables, and connections to
multiple devices can easily add additional capacitance to the
existing circuit. Always consider these effects carefully and
add the recommended series resistor as close to the output
pin of the OPA846 as possible (see the Board Layout section).
OPA846
16
SBOS250C
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