OPA643 Burr-Brown, OPA643 Datasheet - Page 10

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

required 0.8pF value shown in Figure 3 to get the required
feedback pole.
This will set the –3dB bandwidth according to:
The example of Figure 3 will give approximately 23MHz
flat bandwidth using the 0.8pF feedback compensation.
WIDEBAND INVERTING SUMMING AMPLIFIER
One common application for a wideband op amp like the
OPA643 is to sum a number of signal sources together.
Figure 4 shows the inverting summing configuration that is
most often used. This circuit offers the benefit that each
input sees an input impedance set only by its individual input
resistor, since the summing junction (inverting op amp
node) is a virtual ground. Each input is non-interactive with
every other. However, the bandwidth from any input to the
summed output is set by the op amp noise gain (NG), equal
to the non-inverting voltage gain. So, even though each
inverting channel may have a low gain to the output (like the
–1 shown in Figure 4), the overall noise gain will set the
frequency response and the loop stability. The non-inverting
gain for Figure 4 is equal to +5 which will give a 200MHz
bandwidth at a gain of –1 for each of the input signals.
FIGURE 4. Wideband Inverting Summing Amplifier.
OPERATING SUGGESTIONS
OPTIMIZING RESISTOR VALUES
Since the OPA643 is a voltage feedback op amp, a wide
range of resistor values may be used for the feedback and
gain setting resistors (R
limits to these values are set by dynamic range (noise and
distortion) and parasitic capacitive considerations. Usually,
the feedback resistor value should be between 200
1k . Below 200 , the feedback network will present
additional output loading which can degrade the harmonic
distortion performance of the OPA643. Above 1k , the
typical parasitic capacitance (approximately 0.2pF) across
the feedback resistor may cause unintentional band-limiting
in the amplifier response.
V
V
V
V
1
2
3
4
402
402
402
402
0.1µF
®
F
OPA643
–3dB
81.6
F
+5V
–5V
OPA643
(GBP/2 R
and R
Supply Decoupling
402
R
F
Not Shown
G
in Figure 1). The primary
F
C
S
) Hz
V
O
= – (V
1
+ V
2
+ V
3
+ V
and
4
)
10
A good rule of thumb is to target the parallel combination of
R
combined impedance R
input capacitance, placing an additional pole in the feedback
network and thus a zero in the forward response. Assuming
a 3pF total parasitic on the inverting node, holding R
< 200
constraint implies that the feedback resistor R
to several k at high gains. This is acceptable as long as the
pole formed by R
in parallel with it is kept out of the frequency range of
interest. The exception to this is in wideband transimpedance
applications as described earlier. There, a feedback pole is
used to compensate for the zero formed by the input
capacitance and the feedback resistor.
In the inverting configuration, an additional design contraint
must be considered. R
therefore the load impedance to the driving source. If
impedance matching is desired, R
required termination value. However, at low inverting gains,
the resulting feedback resistor value can present a significant
load to the amplifier output. For example, an inverting gain
of –4 (noise gain of 5) with a 50
(= R
increase output loading in parallel with the external load. To
decrease the added loading, it would be preferable to increase
both the R
matching impedance with a third resistor to ground at the
input. The total input impedance becomes the parallel
combination of R
BANDWIDTH VS GAIN
Voltage feedback op amps exhibit decreasing closed-loop
bandwidth as the signal gain is increased. In theory, this
relationship is described by the Gain Bandwidth Product
(GBP) shown in the Electrical Specifications. Ideally, dividing
GBP by the non-inverting signal gain (also called the noise
gain, or NG) will predict the closed-loop bandwidth. In
practice, this relationship only holds true when the phase
margin approaches 90 , as it does in high gain configurations.
At low signal gains, most high speed amplifiers will exhibit
a more complex response with lower phase margin. The
OPA643 is optimized to give a maximally flat frequency
response at a gain of +5. Dividing the typical 800MHz gain
bandwidth product by the noise gain of 5 would predict a
closed-loop bandwidth of 160MHz. However, the actual
bandwidth is extended to > 200MHz due to the reduced
(< 90 ) phase margin at this noise gain. Increasing the gain
will increase the phase margin moving the closed-loop
bandwidth closer to that predicted by the gain bandwidth
product. The 40MHz bandwidth at a gain of +20, shown in
the Electrical Specifications, agrees with that predicted using
the 800MHz GBP.
LOW GAIN OPERATION
Decreasing the operating gain for the OPA643 from the
nominal design point of +5 will decrease the phase margin.
F
and R
G
) would require a 200 feedback resistor, which would
will keep this pole above 250MHz. By itself, this
G
F
(Figure 1) to be less than about 200 . The
and R
F
G
and any parasitic capacitance appearing
and this additional shunt input resistor.
G
values, and then achieve the input
G
F
becomes the input resistor and
R
G
interacts with the inverting
G
may be set equal to the
input matching resistor
F
can increase
F
R
G