OPA2631 Datasheet, PDF (11/17 Pages) Burr-Brown (TI) – Dual, Low Power, Single-Supply OPERATIONAL AMPLIFIER

APPLICATIONS INFORMATION
WIDEBAND VOLTAGE-FEEDBACK OPERATION
The OPA2631 is a unity-gain stable, very high-speed, volt-
age-feedback op amp designed for single-supply operation
(+3V to +5V). The input stage supports input voltages below
ground, and within 1.0V of the positive supply. The comple-
mentary common-emitter output stage provides an output
swing to within 30mV of ground and 130mV of the positive
supply. It is compensated to provide stable operation with a
wide range of resistive loads.
Figure 1 shows the AC-coupled, gain of +2 configuration
used for the +5V Specifications and Typical Performance
Curves. For test purposes, the input impedance is set to 50Ω
with a resistor to ground. Voltage swings reported in the
Specifications are taken directly at the input and output pins.
For the circuit of Figure 1, the total effective load on the
output at high frequencies is 150Ω || 1500Ω. The 1.50kΩ
resistors at the non-inverting input provide the common-
mode bias voltage. Their parallel combination equals the DC
resistance at the inverting input, minimizing the output DC
offset.
+V_S = 5V
6.8µF
+
1.50kΩ
0.1µF
+
0.1µF
V_IN
53.6Ω
1.50kΩ
1/2
OPA2631
0.1µF 750Ω
750Ω
V_OUT
R_L
150Ω
+V_S
2
FIGURE 1. AC-Coupled Signal—Resistive Load to Supply
Midpoint.
Figure 2 shows the DC-coupled, gain of +2 configuration
used for the +3V Specifications and Typical Performance
Curves. For test purposes, the input impedance is set to 50Ω
with a resistor to ground. Though not strictly a “rail-to-rail”
design, this part comes very close, while maintaining excel-
lent performance. It will deliver ≈ 2.9Vp-p on a single +3V
supply with 61MHz bandwidth. The 374Ω and 2.26kΩ
resistors at the input level-shift V_IN so that V_OUT is within
the allowed output voltage range when V_IN = 0. See the
Typical Performance Curves for information on driving
capacitive loads.
+V_S = 3V
6.8µF
+
2.26kΩ
0.1µF
+
374Ω
V_IN
57.6Ω
1/2
OPA2631
V_OUT
R_L
150Ω
562Ω
750Ω
+V_S
2
FIGURE 2. DC-Coupled Signal—Resistive Load to Supply
Midpoint.
SINGLE-SUPPLY ADC CONVERTER INTERFACE
The front page shows a DC-coupled, single-supply dual
ADC driver circuit. Many systems are now requiring +3V
supply capability of both the ADC and its driver. The
OPA2631 provides excellent performance in this demand-
ing application. Its large input and output voltage ranges,
and low distortion support converters such as the ADS901
shown in this figure. The input level-shifting circuitry was
designed so that V_IN can be between 0V and 0.5V, while
delivering an output voltage of 1V to 2V for the ADS901.
BANDPASS FILTER
Figure 3 shows a single OPA2631 implementing a 6th-order
bandpass filter. This filter cascades two 2nd-order Sallen-
Key sections with transmission zeros, and a double real pole
section. It has –3dB frequencies of 630kHz and 1.5MHz,
and –40dB frequencies of 230kHz and 4.2MHz. This filter
was designed to work well on +5V or ±5V supplies, while
driving an A/D converter at 6MSPS to 10MSPS (e.g., the
ADS804).
The filter transfer function is based on a 4th-order elliptic
bandpass filter, with real highpass and lowpass poles added
at the output to give a 6th-order response. The components
were chosen to give this transfer function. The 20Ω resistor
isolates the first OPA2631 output from capacitive loading,
but affects the response at very high frequencies only.
Figure 4 shows the nominal response simulated by SPICE^®.
DC LEVEL SHIFTING
Figure 5 shows a DC-coupled non-inverting amplifier that
level-shifts the input up to accommodate the desired output
voltage range. Given the desired signal gain (G), and the
amount V_OUT needs to be shifted up (∆V_OUT) when V_IN is
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OPA2631