OPA2690 Datasheet, PDF (14/30 Pages) Burr-Brown (TI) – Dual, Wideband, Voltage-Feedback OPERATIONAL AMPLIFIER with Disable

OPA2690
SBOS238D − JUNE 2002 − REVISED DECEMBER 2004
Again, an additional resistor (50Ω in this case) is included
directly in series with the noninverting input. This minimum
recommended value provides part of the DC source
resistance matching for the noninverting input bias
current. It is also used to form a simple parasitic pole to roll
off the frequency response at very high frequencies
( > 500MHz) using the input parasitic capacitance. The
gain resistor (R_G) is AC-coupled, giving the circuit a DC
gain of +1, which puts the input DC bias voltage (2.5V) on
the output as well. The output voltage can swing to within
1V of either supply pin while delivering > 100mA output
current. A demanding 100Ω load to a midpoint bias is used
in this characterization circuit. The new output stage circuit
used in the OPA2690 can deliver large bipolar output
currents into this midpoint load with minimal crossover
distortion, as shown in the +5V supply harmonic distortion
plots.
SINGLE-SUPPLY ADC INTERFACE
Most modern, high-performance ADCs (such as the TI
ADS8xx and ADS9xx series) operate on a single +5V (or
lower) power supply. It has been a considerable challenge
for single-supply op amps to deliver a low distortion input
signal at the ADC input for signal frequencies exceeding
5MHz. The high slew rate, exceptional output swing, and
high linearity of the OPA2690 make it an ideal
single-supply ADC driver. The circuit on the front page
shows one possible interface paricularly suited to
differential I/O, AC-coupled requirements. Figure 4 shows
the test circuit of Figure 2 modified for a capacitive (ADC)
load and with an optional output pull-down resistor (R_B).
This circuit would be suitable to dual-channel ADC driving
with a single-ended I/O.
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The OPA2690 in the circuit of Figure 4 provides > 200MHz
bandwidth for a 2V_PP output swing. Minimal 3rd-harmonic
distortion or 2-tone, 3rd-order intermodulation distortion
will be observed due to the very low crossover distortion
in the OPA2690 output stage. The limit of output
Spurious-Free Dynamic Range (SFDR) will be set by the
2nd-harmonic distortion. Without R_B, the circuit of Figure 4
measured at 10MHz shows an SFDR of 57dBc. This can
be improved by pulling additional DC bias current (I_B) out
of the output stage through the optional R_B resistor to
ground (the output midpoint is at 2.5V for Figure 4).
Adjusting I_B gives the improvement in SFDR shown in
Figure 3. SFDR improvement is achieved for I_B values up
to 5mA, with worse performance for higher values. Using
the dual OPA2690 in an I/Q receiver channel will give
matched AC performance through high frequencies.
70
68 ^{VO = 2VPP, 10MHz}
66
64
62
60
58
56
54
52
50
0 123 456 78
Output Pull−Down Current (mA)
Figure 3. SFDR versus I_B
9 10
+5V
V_I
1V_PP
0.1µF
698Ω
50Ω
59Ω
698Ω
Power−supply decoupling not shown.
1/2
O PA2690
R_S
30Ω
50pF
2.5V DC
±1V AC
ADC Input
402Ω
402Ω
0.1µF
R_B
I_B
Figure 4. SFDR versus I_B
14