OPA643 Burr-Brown, OPA643 Datasheet - Page 11

• Manufacturer Part Number: OPA643
• Description: Wideband Low Distortion / High Gain OPERATIONAL AMPLIFIER
• Manufacturer: Burr-Brown

This will increase the Q for the closed-loop poles, peaking
up the frequency response and extending the bandwidth. A
peaked frequency response will show overshoot and ringing
in the pulse response as well as a higher integrated output
noise. Operating at a noise gain less than +3 runs the risk of
sustained oscillation (loop instability). However, operation
at low gains would be desirable to take advantage of the
much higher slew rate and lower input noise voltage available
in the OPA643, as compared to performance offered by
unity gain stable op amps. Numerous external compensation
techniques have been suggested for operating a high gain op
amp at low gains. Most of these give zero/pole pairs in the
closed-loop response that cause long term settling tails in the
pulse response and/or phase non-linearity in the frequency
response. Figure 5 shows an external compensation method
for the non-inverting configuration that does not suffer from
these drawbacks.
FIGURE 5. Broadband Low Gain Non-Inverting External
The R
gain (i.e. decrease the loop gain) without changing the signal
gain. This approach will retain the full slew rate to the output
but will give up some of the low noise benefit of the
OPA643. Assuming a low source impedance, set R
1+R
Where a low gain is desired, and inverting operation is
acceptable, a new external compensation technique may be
used to retain the full slew rate and noise benefits of the
OPA643 while maintaining the increased loop gain and the
associated improvement in distortion offered by the
decompensated architecture. This technique shapes the loop
gain for good stability while giving an easily controlled
second-order low pass frequency response. Figure 6 shows
this circuit (the same amplifier circuit as shown on the front
page). Considering only the noise gain for the circuit of
Figure 6, the low frequency noise gain, (NG
the resistor ratios while the high frequency noise gain (NG
will be set by the capacitor ratios. The capacitor values set
both the transition frequencies and the high frequency noise
gain. If this noise gain, determined by NG
to a value greater than the recommended minimum stable
F
50 Source
/(R
I
resistor across the two inputs will increase the noise
G
|| R
Compensation.
I
) is
50
R
T
+3.
133
402
R
R
G
I
OPA643
+5V
–5V
402
R
F
2
= 1+ C
1
V
) will be set by
O
S
50
/C
I
F
so that
, is set
2
)
11
gain for the op amp and the noise gain pole, set by 1/R
is placed correctly, a very well controlled second-order low
pass frequency response will result.
FIGURE 6. Broadband Low Gain Inverting External
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 OPA643. 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 6 will set the low frequency noise
gain to NG
only these two gains and the Gain Bandwidth Product (GBP)
for the OPA643 (800MHz), the key frequency in the
compensation can be determined as:
Physically, this Z
6) is set by 1/(2 • R
which the 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
gain occurs at NG
multiply Z
Finally, since C
determine C
The resulting closed-loop bandwidth will be approximately
equal to:
V
I
Z
0
1
0.1µF
O
2
by 2 and use this to get C
S
= 1 + R
402
of 7.5 will be used. The second parameter is
Compensation.
by:
R
G
NG
S
GBP
0
and C
(13.6MHz for the values shown in Figure
C
2
1
280
F
2
F
F
C
R
• Z
/R
–3dB
T
S
F
G
(C
F
12.6pF
1 –
0
2
. Since GBP is expressed in Hz,
set the high frequency noise gain,
C
(= 3 in this example). Then, using
F
S
NG
NG
NG
+ C
1
OPA643
1
R
• Z
+5V
–5V
. To simplify this discussion,
Z
2
S
F
1
2
S
1
O
OPA643
and C
– 1 C
Z
0
)) and is the frequency at
O
GBP
and the pole in the noise
– 1 – 2
NG
806
1.9pF
F
R
C
F
, two parameters and
2
F
F
F
2
by solving:
, which should be
NG
NG
1
2
V
O
F
C
F
,
®