OPA2631 Datasheet, PDF (12/18 Pages) Burr-Brown (TI) – Dual, Low Power, Single-Supply OPERATIONAL AMPLIFIER

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
at the center of its range, the following equations give the
resistor values that produce the desired performance. Start
by setting R₄ between 200Ω and 1.5kΩ.
NG = G + ∆V_OUT/V_S
R₁ = R₄/G
R₂ = R₄/(NG – G)
R₃ = R₄/(NG –1)
where:
NG = 1 + R₄/R₃ (Noise Gain)
V_OUT = (G)V_IN + (NG – G)V_S
2.2nF
1% Resistors
5% Capacitors
60.4Ω
7.32kΩ
V_IN
1.2nF
1.2nF
681Ω
1/2
OPA2631
200Ω
59Ω
3.9nF
V_OUT
86.6Ω
1.8nF
1.2nF
130Ω
1/2
OPA2631
200Ω
133Ω
73.2Ω
330pF
20Ω 27pF 3.9nF
46.4Ω
FIGURE 3. Bandpass Filter.
10
0
–10
–20
–30
–40
–50
–60
10K
100K
1M
10M
Frequency (Hz)
FIGURE 4. Nominal Filter Response.
12
100M
R₁
V_IN
+V_S
R₂
1/2
OPA2631
V_OUT
R₃
R₄
FIGURE 5. DC Level Shifting Circuit.
OPA2631
SBOS067A