OPA2830 Datasheet, PDF (26/43 Pages) National Semiconductor (TI) – Dual, Low-Power, Single-Supply, Wideband OPERATIONAL AMPLIFIER

OPA2830
SBOS309D – AUGUST 2004 – REVISED AUGUST 2008.................................................................................................................................................. www.ti.com
DIFFERENTIAL LOW-PASS ACTIVE FILTERS
The dual OPA2830 offers an easy means to
implement low-power differential active filters. On a
single supply, one way to implement a 2nd-order,
low-pass filter is shown in Figure 79. This circuit
provides a net differential gain of 1 with a precise
5MHz Butterworth response. The signal is
AC-coupled (giving a high-pass pole at low
frequencies) with the DC operating point for the
circuit set by the unity-gain buffer—the BUF602. This
buffer gives a very low output impedance to high
frequencies to maintain accurate filter characteristics.
If the source is a DC-coupled signal already biased
into the operating range of the OPA2830 input CMR,
these capacitors and the midpoint bias may be
removed. To get the desired 5MHz cutoff, the input
resistors to the filter is actually 119Ω. This is
implemented in Figure 79 as the parallel combination
of the two 238Ω resistors on each half of the
differential input as part of the DC biasing network. If
the BUF602 is removed, these resistors should be
collapsed back to a single 119Ω input resistor.
150pF
+5V
0.1µF
238Ω
506Ω
1/2
OPA 2 830
+5V
238Ω
100pF
750Ω
2.5V
V_I
5kΩ
BUF602
0.1µF 5kΩ
1500Ω
V_O
750Ω
238Ω
100pF
0.1µF
238Ω
506Ω
1/2
OPA 2 830
150pF
Figure 79. Single-Supply, 2nd-Order, Low-Pass
Sallen-Key Filter
Implementing the DC bias in this way also attenuates
the differential signal by half. This is recovered by
setting the amplifier gain at 2V/V to get a net
unity-gain filter characteristic from input to output. The
filter design shown here has also adjusted the
resistor values slightly from an ideal analysis to
account for the 100MHz bandwidth in the amplifier
stages. The filter capacitors at the noninverting inputs
are shown as two separate capacitors to ground.
While it is certainly correct to collapse these two
capacitors into a single capacitor across the two
inputs (which would be 50pF for this circuit) to get the
same differential filtering characteristic, tests have
shown two separate capacitors to a low impedance
point act to attenuate the common-mode feedback
present in this circuit giving more stable operation in
actual implementation. Figure 80 shows the
frequency response for the filter of Figure 79.
0
−1
−2
−3
−4
−5
−6
−7
−8
−9
−10
−11
−12
10²
10³
10⁴
10⁵
10⁶
Frequency (Hz)
Figure 80. 5MHz, 2nd-Order, Butterworth
Low-Pass Filter
HIGH-PASS FILTERS
Another approach to mid-supply biasing is shown in
Figure 81. This method uses a bypassed divider
network in place of the buffer used in Figure 79. The
impedance is set by the parallel combination of the
resistors forming the divider network, but as
frequency increases it looks more and more like a
short due to the capacitor. Generally, the capacitor
value needs to be two to three orders of magnitude
greater than the filter capacitors shown for the circuit
to work properly.
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