OPA642N BURR-BROWN [Burr-Brown Corporation], OPA642N Datasheet - Page 8

• Manufacturer Part Number: OPA642N
• Description: Wideband, Low Distortion, Low Gain OPERATIONAL AMPLIFIER
• Manufacturer: BURR-BROWN [Burr-Brown Corporation]

APPLICATIONS INFORMATION
WIDEBAND VOLTAGE FEEDBACK OPERATION
The OPA642’s combination of speed and dynamic range is
easily achieved in a wide variety of application circuits,
providing that simple principles of good design practice 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. Proper
PC board layout and careful component selection will maxi-
mize the performance of the OPA642 in all applications, as
discussed in the remaining sections of this data sheet.
Figure 1 shows the gain of +2 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 pre-
senting 50
shunt resistor at the V
ance of the test generator, while the 50 series resistor at the
V
ment equipment load. Generally, data sheet specifications
refer to the voltage swing at the output pin (V
The 100
tances, combined with the 804
load, presents the OPA642 with an effective load of approxi-
mately 90 .
FIGURE 1. Gain of +2, High Frequency Application and
O
terminal provides a matching resistor for the measure-
50 Source
load from the series and shunt matching resis-
V
Characterization Circuit [P or U Package].
®
shunt load impedance. In Figure 1, the 50
I
50
402
OPA642
R
G
3
2
OPA642
I
7
4
+V
–V
terminal matches the source imped-
S
S
2.2µF
0.1µF
0.1µF
2.2µF
+
402
+
R
8
5
0.1µF
0.1µF
F
6
V
O
total feedback network
Gain,
50
V
V
O
I
O
= 1 +
50 Load
in Figure 1).
R
R
F
G
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 inter-
face to a very high dynamic range converter. The frequency
domain application allows the OPA642 to be operated in its
most linear region, using a signal range which swings
symmetrically around ground (0V). The 2Vp-p swing is then
level-shifted through the blocking capacitor to a DC refer-
ence level, which is created by a well-decoupled resistive
divider off the converter’s internal reference voltages. To
have a 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 OPA642 has an immeasurable effect on the
distortion of the ADS804, which achieves 80dB SFDR at
5MHz Nyquist input signal.
To achieve the lowest possible distortion in the 8-pin SO-8
or DIP package, the addition of 0.1 F decoupling capacitors
on pins 5 and 8 is 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 improve second harmonic suppression at
5MHz 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 sup-
ply connections.
Successful application of the OPA642 for ADC buffering
requires careful selection of the series resistor at the ampli-
fier output, 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 ADCs, like the ADS804,
perform better with the shunt capacitor at the input 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
OPA642. Without a series isolation resistor, the result could
be undesirable peaking or loss of stability in the amplifier.
Since the DC bias current of the CMOS A/D input is
negligible, the resistor has no effect on overall gain or offset
accuracy. Refer to the plot of “R
Typical Performance Curves to obtain a good starting value
for the series resistor. This will ensure flat frequency re-
sponse to the ADC input. Increasing the external capacitor
value will allow the series resistor to be reduced or, keeping
this resistor fixed, will band-limit the signal and reduce high
frequency noise to the input of the converter.
VIDEO LINE DRIVING
Most video distribution systems are designed with 75
series resistors to drive a matched 75
deliver a net gain of 1 to the 75 matched load, the amplifier
S
vs Capacitive Load” in the
cable. In order to