Datasheet

OPA843
15
SBOS268C
www.ti.com
occurs at NG
1
• Z
0
and the pole in the noise gain occurs at
NG
2
• Z
0
. Since GBP is expressed in Hz, multiply Z
0
by 2π and
use this to get C
F
by solving:
C
RZNG
F
F
=
1
2
02
π
(12)
Finally, since C
S
and C
F
set the high-frequency noise gain,
determine C
S
by:
C
S
= (NG
2
– 1)C
F
(13)
The resulting closed-loop bandwidth will be approximately
equal to:
f Z GBP
dB−
≅
30
(14)
For the values shown in Figure 10, the f
–3dB
will be approxi-
mately 105MHz. This is less than that predicted by simply
dividing the GBP product by NG
1
. The compensation network
controls the bandwidth to a lower value while providing full
slew rate and exceptional distortion performance due to in-
creased loop gain at frequencies below NG
1
• Z
0
. The capaci-
tor values shown in Figure 10 are calculated for NG
1
= 3 and
NG
2
= 7.5 with no adjustment for parasitics.
OUTPUT DRIVE CAPABILITY
The OPA843 has been optimized to drive the demanding load
of a doubly-terminated transmission line. When a 50Ω line is
driven, a series 50Ω into the cable and a terminating 50Ω load
at the end of the cable are used. Under these conditions, the
impedance of the cable appears resistive over a wide fre-
quency range and the total effective load on the OPA843 is
100Ω in parallel with the resistance of the feedback network.
The Electrical Characteristics show a 6.1V
PP
swing into a
100Ω load—which is then reduced to a 3V
PP
swing at the
termination resistor. The ±85mA output drive over tempera-
ture provides adequate current drive margin for this load.
A common IF amplifier specification, which describes avail-
able output power is the –1dB compression point. This is
usually defined at a matched 50Ω 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 OPA843, the –1dB
compression at a matched 50Ω load will be > 13dBm based
on the minimum available 3Vp-p 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 OPA843. The high
open-loop gain and Class AB output stage of the OPA843
produce a much higher intercept than the –1dB compression
would predict, as shown in the Typical Characteristics.
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 OPA843 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 Characteristics show the recommended
R
S
vs
Capacitive Load
and the resulting frequency response at the
load. The criterion for setting the recommended resistor is
maximum bandwidth and 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 roll off 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.
Parasitic capacitive loads greater than 2pF can begin to
degrade the performance of the OPA843. 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 OPA843 output pin
(see Board Layout section).
DISTORTION PERFORMANCE
The OPA843 is capable of delivering an exceptionally low
distortion signal at high frequencies and medium gains. The
distortion plots in the Typical Characteristics show the typical
distortion under a wide variety of conditions. Most of these
plots are limited to 100dB dynamic range. The OPA843’s
distortion does not rise above –100dBc until either the signal
level exceeds 0.5Vp-p 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 2nd-harmonic will dominate the
distortion with a negligible 3rd-harmonic component. Focus-
ing then on the 2nd-harmonic, increasing the load imped-
ance improves distortion directly. Remember that the total
load includes the feedback network—in the noninverting
configuration this is the sum of R
F
+ R
G
, whereas in the
inverting configuration this is just R
F
(see Figure 1). Increas-
ing output voltage swing increases harmonic distortion di-
rectly. A 6dB increase in output swing will generally increase