OPA686 Burr-Brown, OPA686 Datasheet - Page 10

• Manufacturer Part Number: OPA686
• Description: Wideband / Low Noise / Voltage Feedback OPERATIONAL AMPLIFIER
• Manufacturer: Burr-Brown

To choose the values for both C
only three equations need to be solved. The first parameter
is the target high frequency noise gain NG
greater than the minimum stable gain for the OPA686. Here,
a target NG
the desired low frequency signal gain, which also sets the
low frequency noise gain NG
we will target a maximally flat second-order low pass
Butterworth frequency response (Q = 0.707). The signal
gain of –2 shown in Figure 4 will set the low frequency noise
gain to NG
only these two gains and the GBP for the OPA686
(1600MHz), the key frequency in the compensation can be
determined as:
Physically, this Z
set by 1/(2 • R
rising portion of the noise gain would intersect unity gain if
projected back to 0dB gain. The actual zero in the noise gain
occurs at NG
NG
and use this to get C
Finally, since C
determine C
The resulting closed-loop bandwidth will be approximately
equal to:
For the values shown in Figure 4, the f
mately 130MHz. This is less than that predicted by simply
dividing the GBP product by NG
network controls the bandwidth to a lower value while
providing the full slew rate at the output and an excep-
tional distortion performance due to increased loop gain at
frequencies below NG
in Figure 4 are calculated for NG
no adjustment for parasitics.
Figure 5 shows the measured frequency response for the
circuit of Figure 4. This shows the expected gain of
–2 (6dB) with exceptional flatness through 70MHz and a
–3dB bandwidth of 170MHz. Measured distortion into a
100
the performance shown in the Typical Performance Curves.
Into a 500 load, the 5MHz, 2Vp-p, 2nd harmonic improves
from –90dBc to –96dBc.
2
• Z
load shows > 5dB improvement through 20MHz over
0
. Since GBP is expressed in Hz, multiply Z
Z
1
2
O
S
= 1 + R
f
of 10.5 will be used. The second parameter is
1
®
– dB
by [Using NG
• Z
C
C
F
S
OPA686
NG
(C
S
GBP
F
0
and C
0
(10.6MHz for the values shown above) is
F
and the pole in the noise gain occurs at
F
1
+ C
2
/R
F
NG
Z
by solving:
1
G
F
• R
S
O
1 –
)) and is the frequency at which the
2
• Z
set the high frequency noise gain,
(= 3 in this example). Then, using
GBP
– 1 C
F
0
NG
NG
Z
2
. The capacitor values shown
1
O
= 10.5]:
. To simplify this discussion,
S
NG
1
2
and C
F
1
= 3 and NG
– 1 – 2
1
. The compensation
F
(= 2.86pF)
(= 27.2pF)
(= 130MHz)
–3dB
, two parameters and
2
, which should be
will be approxi-
NG
NG
2
1
2
= 10.5 with
0
by 2
10
FIGURE 5. G = –2 Frequency Response with External
LOW NOISE FIGURE, HIGH DYNAMIC RANGE “IF”
AMPLIFIER
The low input noise voltage of the OPA686 and its high
two-tone intercept can be used to good advantage as a fixed
gain IF amplifier. While input noise figures in the 10dB
range (for a matched 50
just OPA686 alone, Figure 6 shows a technique which
reduces the noise figure even further while providing a
broadband, low gain IF amplifier stage using the OPA686.
FIGURE 6. Low Noise Figure IF Amplifier.
Bringing the signal in through a step-up transformer to the
inverting input gain resistor has several advantages for the
OPA686. First, grounding the non-inverting input elimi-
nates the contribution of the non-inverting input current
noise to the output noise. Secondly, the non-inverting
input voltage noise of the op amp is actually attenuated if
reflected to the input side of R
step up transformer reflects the 50
50 Source
–12
–15
–18
12
–3
–6
–9
9
6
3
0
1
1:2
Compensation.
200
R
G
10
Frequency (MHz)
20pF
input) are easily achieved with
+5V
–5V
OPA686
G
. Using the 1:2 (turns ratio)
1k
2pF
R
F
source impedance at
170MHz
100
V
O
50
50 Load
500