參數(shù)資料
型號(hào)： OPA685
英文描述： Ultra-Wideband, Current-Feedback OPERATIONAL AMPLIFIER With Disable TM
中文描述： 超寬帶，電流反饋運(yùn)算放大器商標(biāo)使用禁用
文件頁(yè)數(shù)： 16/28頁(yè)
文件大小： 283K
代理商： OPA685
16
OPA685
REVERSE ISOLATION (S
12
)
This is a measure of how much power injected into the
output matching resistor appears at the input. This is rarely
specified for an op amp because it is so good. Op amps are
very nearly uni-directional signal devices. The Typical Per-
formance Curves show this performance in the “Isolation
Characteristics vs Frequency” curve. Below 300MHz, the
non-inverting configuration of Figure 1 gives much better
isolation than the inverting of Figure 2. However, both are
well below 40dB isolation through 350MHz. Shown also on
this plot is the forward isolation for S
21
when the OPA685
is disabled. This also stays under –40dB up to 700MHz. This
specification is not shown for the inverting mode since the
signal will couple directly through the external resistors
when the amplifier is disabled for the circuit of Figure 2. If
off-isolation is a concern, the non-inverting configuration
would be preferred.
DYNAMIC RANGE LIMITS
The next consideration for RF amplifier applications are
what limits to dynamic range may be defined. Typical fixed-
gain RF amplifiers include:
–1dB compression (a measure of maximum output power)
2-tone, 3rd order, output intermodulation intercept (a mea-
sure of achievable Spurious Free Dynamic Range, SFDR)
Noise Figure (NF, a measure of degradation in signal-to-
noise ratio in passing through the amplifier)
–1dB Compression
The –1dB compression power is defined as the output power
at which 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. This does not hold for
op amps since their intercepts are considerably improved by
loop gain and exceed the –1dB compression by much more
than 10dB. A simple estimate for –1dB compression for the
OPA685 is the maximum non-slew limited output voltage
swing available at the matched load converted into power
with 1dB added to satisfy the definition. For the OPA685 on
±
5V supplies, the output will deliver
±
3.6V at the output pin,
or
±
1.80V at the matched load. The conversion from Vp-p
to power (for a sine wave) is:
(3)
Converting this 3.6Vp-p swing at the load to dBm gives
15.1dBm. Adding 1dB to this (to satisfy the definition) gives
a –1dB compression of 16.1dBm for the OPA685 operating
on
±
5V supplies. This will be a good estimate for frequen-
cies that require less than the full slew rate of the OPA685.
The maximum frequency of operation given an available
slew rate and desired peak output swing (at the output pin)
for a sine wave is:
Slew Rate
MAX
=
( 4)
Using the 4200V/
μ
s slew rate available in the inverting
mode of operation and the 3.6V peak output swing at the
output pin, gives a maximum frequency of 186MHz. This is
the maximum frequency where the –1dB compression would
be 16.1dBm at the matched load. Higher useable bandwidths
are possible at lower output power, as shown in the large-
signal bandwidth curves. As those curves show, 7Vp-p
outputs are possible with almost perfect frequency response
flatness through 100MHz for both non-inverting or inverting
operation.
Two-Tone, 3rd-Order Output Intermodulation Intercept (OP3)
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, 2-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 (P
T
) levels are
equal, the OPA685 will produce 3rd-order products (P
S
) that
are at these frequencies and at a power level below the test
power levels given by:
(5)
The “Two-Tone, 3rd-Order Intermodulation Intercept” curve
shown in the Typical Performance Curves shows a very
high intercept at low frequencies, that decreases with in-
creasing frequency. This intercept is defined at the matched
load to allow direct comparison with fixed-gain RF ampli-
fiers. To produce a 2Vp-p total, 2-tone envelope at the
matched load, each power level must be 4dBm at the
matched load (1Vp-p). Using Equation 5 and the perfor-
mance 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 exceptionally
low distortion for an amplifier that only uses 12mA supply
current. Considerable improvement from this level of per-
formance is also possible if the output drives directly into
the lighter load of an ADC input (see Differential ADC
Driver section of this data sheet).
This very high intercept versus quiescent power is achieved
by the high loop gain of the OPA685. This loop gain does,
however, decrease with frequency giving the decreasing
output intercept performance shown in the Typical Perfor-
mance Curves. Application 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 (than the 8V/V or
12dB gain used in the Typical Performance Curves) and
increasing at lower gains.
NOISE FIGURE
All fixed-gain RF amplifiers show good Noise Figure (typi-
cally < 5dB). For broadband RF amplifiers, this is achieved
by a low noise input transistor and an input match set by
feedback. This feedback greatly reduces the Noise Figure
P
P
OP
P
T
S
T
–
–
=
(
)
2
3
f
V
p
2
π
P
dBm
V
2 2
O
(
)
log
.
=
)
10
0 001 50
2
p-p
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