Datasheet

OPA695
16
SBOS293G
www.ti.com
curves. The inverting configuration holds almost constant
bandwidth (with correctly selected external resistor values)
until R
G
reduces to equal 50Ω, and remains at that value to
satisfy the input impedance matching requirement, with fur-
ther increases in gain achieved by increasing R
F
in Figure 2.
The bandwidth then decreases rapidly as shown by the gain
of –16V/V plot in the Typical Characteristic curves.
REVERSE ISOLATION (S
12
)
Reverse isolation is a measure of how much power injected
into the output pin makes it back to the source. This is rarely
specified for an op amp because it is so good. Op amps are
very nearly uni-directional signal devices. Below 300MHz,
the noninverting configuration of Figure 1 gives much better
isolation than the inverting of Figure 2. Both are well below
40dB isolation through 350MHz.
LIMITS TO DYNAMIC RANGE
The next set of considerations for RF amplifier applications
are the defined limits to dynamic range. Typical fixed-gain RF
amplifiers include:
•–1dB compression (a measure of maximum output power)
• Two-tone, 3rd-order, output intermodulation intercept (a
measure of achievable spurious-free dynamic range)
• Noise figure (a measure of degradation in signal to noise
ratio in passing through the amplifier)
–1dB COMPRESSION
The definition for –1dB compression power is that output
power where the actual power is 1dB less than the input
power plus the log gain. In classic RF amplifiers, this is
typically 10dB less than the 3rd-order intercept. That relation-
ship does not hold for op amps since their intercept is
considerably improved by loop gain to be far more than 10dB
higher than the –1dB compression. A simple estimate for
–1dB compression for the OPA695 is the maximum non-slew
limited output voltage swing available at the matched load
converted into a power with 1dB added to satisfy the defini-
tion. For the OPA695 on ±5V supplies, its output will deliver
approximately ±4.0V at the output pin or ±2.0V at the matched
load. The conversion from V
PP
to power (for a sine wave) is:
P dBm
V
O
PP
(
)
=






(
)














10
22
0 001 50
2
log
. Ω
Converting this 4.0V
PP
swing at the load to dBm gives
16dBm; adding 1dB to this (to satisfy the definition) gives a
–1dB compression of 17dBm for the OPA695 operating on
±5V supplies. This will be a good estimate for frequencies
that require less than the full slew rate of the OPA695.
The maximum frequency of operation given an available
slew rate and desired peak output swing (at the output pin for
a sine wave) is:
F
Slew Rate
V
MAX
p
=
2 0 707π (. )
Putting in the 4600V/µs slew rate available in the inverting
mode of operation and the 4.0V peak output swing at the
output pin gives a maximum frequency of 259MHz. This is
the maximum frequency where the –1dB compression would
be 17dBm at the matched load. Higher useable bandwidths
are possible at lower output powers, as shown in the Large
Signal Bandwidth curves. As those graphs show, 7V
PP
out-
puts are possible with almost perfect frequency response
flatness through 100MHz for both non-inverting or inverting
operation.
TWO-TONE 3rd-ORDER OUTPUT
INTERMODULATION INTERCEPT (OP
3
)
In narrowband IF strips, each amplifier typically feeds into a
bandpass filter that attenuates most harmonic distortion
terms. The most troublesome remaining distortion is the 3rd-
order, two-tone intermodulations that can fall very close (in
frequency) to the desired signals and cannot be filtered out.
If two test frequencies are defined at F
O
+ ∆F and F
O
– ∆F,
the 3rd-order intermodulation distortion products will fall at
F
O
+ 3∆F and F
O
– 3∆F. If the two test power levels (P
T
) are
equal, the OPA695 will produce 3rd-order spurious terms
(P
S
) that are at these frequencies and at a power level below
the test power levels given by:
PP OPP
T
S
T
––=
( )
2
3
The 3rd-order intercept plot shown in the Typical Character-
istic curves shows a very high intercept at low frequencies
that decreases with increasing frequency. This intercept is
defined at the matched load to allow direct comparison with
fixed-gain RF amplifiers. To produce a 2V
PP
total two-tone
envelope at the matched load, each power level must be
4dBm at the matched load (1V
PP
). Using Equation 5, and the
performance curve for inverting operation, at 50MHz (41.5dBm
intercept) the 3rd-order spurious will be 2 • (41.5 – 4) = 75dB
below these 4dBm test tones. This is an exceptionally low
distortion for an amplifier that only uses 13mA supply current.
Considerable improvement from this level of performance is
also possible if the output drives directly into the lighter load
of an ADC input (see
High SFDR Differential ADC driver
section).
This very high intercept versus quiescent power is achieved
by the high loop gain of the OPA695. This loop gain does,
however, decrease with frequency, giving the decreasing
OP3 performance shown in the Typical Characteristics. Ap-
plication as an IF amplifier through 200MHz is possible with
output intercepts exceeding 21dBm at 200MHz. Intercept
performance will vary slightly with gain setting decreasing at
higher gains (that is, gains greater than the 8V/V, or 12dB,
gain used in the Typical Characteristic curves) and increas-
ing at lower gains.
(3)
(4)
(5)