OPA643 Burr-Brown, OPA643 Datasheet - Page 9

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

pin. This capacitor provides a low source impedance for the
transient currents produced by the sampling process.
Improved SFDR is obtained by adding the capacitor, whose
value is often recommended in the converter data sheet. The
external capacitor, in combination with the built-in
capacitance of the A/D input, presents a significant capacitive
load to the OPA643. Without a series isolation resistor, the
result can be peaking and possibly oscillation in the amplifier.
Refer to the plot of “R
Performance Curves to obtain a good starting value for the
series resistor. The values shown in this curve will ensure a
flat frequency response at the input of the ADC. Increasing
the external capacitor value will allow either the series
resistor to be reduced, or, keeping this resistor fixed, will
bandlimit the signal and reduce high frequency noise to the
input of the converter.
WIDE DYNAMIC RANGE IF AMPLIFIER
The OPA643 offers an attractive alternative to standard
fixed gain IF amplifier stages. Narrowband systems will
benefit from the exceptionally high two tone third-order
intermodulation intercept as shown in the Typical
Performance Curves. Op amps with high open-loop gain,
like the OPA643, provide an intercept that decreases with
frequency along with the loop gain. The OPA643’s intercept
is > 25dBm up to 50MHz but improves to > 50dBm as the
operating frequency is reduced below 10MHz. Broadband
systems will also benefit from the very low even order
harmonics and intermodulation components produced by the
OPA643. Compared to standard fixed gain IF amplifiers, the
OPA643 operating at IF’s below 50MHz provides much
higher intercepts for its quiescent power dissipation (200mW),
superior gain accuracy, higher reverse isolation, and lower
I/O return loss. Noise figure for the OPA643 will be higher
than alternative fixed gain stages. If the application comes
late in the amplifier chain with significant gain in prior
stages, this higher noise figure will be acceptable. Figure 2
shows an example non-inverting configuration for the
OPA643 used as an IF amplifier.
FIGURE 2. Wide Dynamic Range IF Amplifier.
50 Source
P
I
52.3
0.1µF
1k
S
vs Capacitive Load” in the Typical
144
0.1µF
OPA643
R
+5V
–5V
G
Supply Decoupling
1k
Gain =
R
Not Shown
F
P
P
O
I
= 20log 1/2 1+
= 12dB with values shown
50
50 Load
(
P
O
R
R
F
G
)
dB
9
The input signal and the gain resistor are AC coupled
through the 0.1 F blocking capacitors. This holds the DC
input and output operating point at ground independent of
source impedance and gain setting. The R
Figure 2 (144 ) sets the gain to the matched load at 12dB.
Using standard 1% tolerance resistors for R
hold the gain to a 0.2dB tolerance. This example will give
a –3dB bandwidth of approximately 100MHz while
maintaining gain flatness within 1dB through 50MHz. For
narrowband IF’s in the 21.4MHz region, this configuration
of the OPA643 will show a third-order intercept of 40dBm
while dissipating only 200mW (23dBm) power from 5V
supplies.
PHOTODIODE TRANSIMPEDANCE AMPLIFIER
High Gain Bandwidth Product (GBP) and low input voltage
and current noise make the OPA643 an ideal wideband
transimpedance amplifier for low to moderate gains. Note
that unity gain stability is not required for application as a
transimpedance amplifier. Figure 3 shows an example
photodiode amplifier circuit. The key parameters of this
design are the estimated diode capacitance (C
applied DC reverse bias voltage (–V
transimpedance gain (R
(800MHz). With these three variables set (and adding the
OPA643’s parasitic input capacitance to the value of C
get C
control the transimpedance frequency response.
To achieve a maximally flat second-order Butterworth
frequency response, the feedback pole should be set to:
Adding the OPA643’s common-mode and differential mode
input capacitances (1.3 + 2.5)pF to the 20pF diode source
capacitance of Figure 3, and targeting a 10k transimpedance
gain using the 800MHz GBP for the OPA643, the required
feedback pole frequency is 16.4MHz. This will require a
total feedback capacitance of 1.0pF. Typical surface mount
resistors have a parasitic capacitance of 0.2pF, leaving the
FIGURE 3. Wideband, Low Noise, Transimpedance
S
), the feedback capacitor value (C
–V
B
I
D
1/(2 R
Amplifier.
20pF
F
C
C
D
F
) = (GBP/(4 R
F
), and the GBP for the OPA643
OPA643
+5V
–5V
Supply Decoupling
OPA643
Not Shown
10k
0.8pF
R
C
F
F
F
) may be chosen to
F
C
G
B
S
), the desired
value shown in
))
F
and R
V
D
O
) at the
= I
G
D
R
will
D
F
to
®