A 2 MHz SINGLE-SUPPLY, BIQUAD BAND-PASS
Figure 47 shows a circuit for a single-supply, biquad band-pass
filter with a center frequency of 2 MHz. A 2.5 V bias level is
easily created by connecting the noninverting inputs of all three
op amps to a resistor divider consisting of two 1 kΩ resistors
connected between 5 V and ground. This bias point is also
decoupled to ground with a 0.1 μF capacitor. The frequency
response of the filter is shown in Figure 48.
To maintain an accurate center frequency, it is essential that the
op amp have sufficient loop gain at 2 MHz. This requires the
choice of an op amp with a significantly higher unity gain,
crossover frequency. The unity gain, crossover frequency of the
AD8031/AD8032 is 40 MHz. Multiplying the open-loop gain by
the feedback factors of the individual op amp circuits yields the
loop gain for each gain stage. From the feedback networks of
the individual op amp circuits, it can be seen that each op amp
has a loop gain of at least 21 dB. This level is high enough to
ensure that the center frequency of the filter is not affected by
the op amp’s bandwidth. If, for example, an op amp with a gain
bandwidth product of 10 MHz was chosen in this application,
the resulting center frequency would shift by 20% to 1.6 MHz.
Figure 47. A 2 MHz, Biquad Band-Pass Filter Using AD8031/AD8032
Figure 48. Frequency Response of 2 MHz Band-Pass Filter
HIGH PERFORMANCE, SINGLE-SUPPLY LINE DRIVER
Even though the AD8031/AD8032 swing close to both rails, the
AD8031 has optimum distortion performance when the signal
has a common-mode level half way between the supplies and
when there is about 500 mV of headroom to each rail. If low
distortion is required in single-supply applications for signals
that swing close to ground, an emitter-follower circuit can be
used at the op amp output.
Figure 49. Low Distortion Line Driver for Single-Supply, Ground Referenced Signals
Figure 49 shows the AD8031 configured as a single-supply, gain-
of-2 line driver. With the output driving a back-terminated
50 Ω line, the overall gain from VIN to VOUT is unity. In addition
to minimizing reflections, the 50 Ω back termination resistor
protects the transistor from damage if the cable is short circuited.
The emitter follower, which is inside the feedback loop, ensures
that the output voltage from the AD8031 stays about 700 mV
above ground. Using this circuit, low distortion is attainable
even when the output signal swings to within 50 mV of ground.
The circuit was tested at 500 kHz and 2 MHz.
Rev. C | Page 16 of 20