OPA643 Burr-Brown, OPA643 Datasheet - Page 8

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

APPLICATIONS INFORMATION
TYPICAL APPLICATION AND
CHARACTERIZATION CIRCUIT
The OPA643’s combination of speed and dynamic range is
easily achieved in a wide variety of application circuits,
providing that simple guidelines common to all high speed
amplifiers are observed. For example, good power supply
decoupling, as shown in Figure 1, is essential to achieve the
lowest possible harmonic distortion and smooth frequency
response. Careful PC board layout and component selection
will maximize the performance of the OPA643 in all
applications, as discussed in the remaining sections of this
data sheet.
Figure 1 shows the gain of +5 configuration used as the basis
for most of the Typical Performance Curves. Most of the
curves were characterized using signal sources with 50
driving impedance, and with measurement equipment
presenting 50 load impedance. In Figure 1, the 50 shunt
resistor at the V
the test generator, while the 50
terminal provides a matching resistor for the measurement
equipment load. Generally, data sheet specifications refer to
the voltage swing at the output pin (V
total 100 load from the series and shunt matching resistors,
combined with the 502 total feedback network load, presents
the OPA643 with an effective output load of approximately
83 .
FIGURE 1. Gain of +5, High Frequency Application and
50 Source
R
T
Characterization Circuit (P or U Package).
V
®
I
50
100
I
OPA643
R
terminal matches the source impedance of
G
3
2
+5V
OPA643
7
4
+V
–V
S
S
2.2µF
0.1µF
0.1µF
2.2µF
–5V
+
402
+
R
8
5
0.1µF
0.1µF
F
6
V
series resistor at the V
O
Gain,
50
O
in Figure 1). The
V
V
O
I
= 1 +
50 Load
R
R
F
G
O
8
BUFFERING HIGH PERFORMANCE ADC’S
To achieve full performance from a high dynamic range
A/D converter, considerable care must be exercised in the
design of the input amplifier interface circuit. The example
circuit on the front page shows a typical AC-coupled interface
to a very high dynamic range converter. This circuit uses a
new external compensation technique which stabilizes the
OPA643 for low signal gain, while maintaining the high
gain bandwidth, fast slew rate and improved distortion
performance of the decompensated architecture. Testing
shows that a high loop gain and flat response are maintained
through the Nyquist frequency on this circuit using the
ADS805 giving very high SFDR
Nyquist, the loop gain is rolled off sharply to lower the
crossover frequency, and finally additional lead is introduced
at crossover to maintain good phase margin. In general, this
loop gain shaping technique allows the use of high gain
bandwidth, decompensated op amps to achieve better dynamic
performance in low signal gain applications. Refer to the
section on Low Gain Operation for further information.
The frequency domain digitizer application on the front page
allows the signal swing at the output of the OPA643 to be
operated at an optimum DC point. Centering the output
swing between the supplies is a good starting point, but
significant improvement in second-harmonic distortion can
be achieved by shifting the output DC point away from
ground. A typical signal swing of 2Vp-p, operating at either
an optimized or a ground-centered output DC voltage, is
then level shifted through the blocking capacitor to a DC
reference level at the converter input. This reference voltage
is created by a well decoupled resistive divider off the
converter’s internal reference voltages. To have negligible
effect on the rated spurious-free dynamic range (SFDR) of
the converter, the amplifier’s SFDR should be at least 10dB
greater. In the front page example, the insertion of the
OPA643 has an unmeasurable effect on the distortion of the
20MSPS ADS805, which achieves 80dB SFDR at a 10MHz
Nyquist input signal.
To deliver the lowest possible distortion using the 8-pin
SO-8 or DIP package, additional 0.1 F power supply
decoupling capacitors on pins 5 and 8 are required. These
are shown in Figure 1. Although pins 5 and 8 are internally
connected to pins 4 and 7 respectively (the standard supply
pins for 8-pin op amps), the additional capacitors help to
decouple the package lead inductances and decrease the
second-harmonic distortion for a 5MHz fundamental by
approximately 4dB. The much shorter bond wires and supply
leads of the SOT23-5 package give the best distortion
performance while requiring only two power supply
connections.
Successful application to ADC buffering requires a careful
selection of the series resistor at the output of the OPA643,
along with the additional shunt capacitor at the ADC input.
To some extent, selection of this RC network will be
determined empirically for each model of converter. Many
high performance CMOS ADC’s, like the ADS805, perform
better with an additional capacitor to ground on the input
performance. Above