OPA2634 Datasheet, PDF (11/16 Pages) Burr-Brown (TI) – Dual, Wideband, Single-Supply OPERATIONAL AMPLIFIER

frequency of 5MHz, and a –60dB stopband starting at
12MHz. This filter works well on +5V or ±5V supplies, and
with an A/D converter at 20MSPS (e.g., ADS900). V_IN
needs to be a very low impedance source, such as an op amp.
The filter transfer function was designed using Burr-Brown’s
FilterPro 42 design program (available at www.burr-
brown.com in the Applications section) with a nominal
stopband attenuation of 60dB. Table I gives the results (H₀
= DC gain, f_P = pole frequency, Q_P = pole quality, and f_Z =
zero frequency). Note that the parameters were generated at
f_–3dB = 5Hz, and then scaled to f_–3dB = 5MHz.
SECTION NO.
H₀
f_P
Q_P
1
1V/V
5.04MHz
1.77
2
1V/V
5.31MHz
0.64
3
1V/V
5.50MHz
—
TABLE I. Nominal Filter Parameters.
f_Z
12.6MHz
20.4MHz
—
The components were chosen to give this transfer function.
The 20Ω resistors isolate the amplifier outputs from capacitive
loading, but affect the response at very high frequencies only.
Figure 4 shows the nominal response simulated by SPICE; it
is very close to the ideal response.
0
–10
–20
–30
–40
–50
–60
–70
–80
1
10
100
Frequency (MHz)
FIGURE 4. Nominal Filter Response.
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 best DC offset:
where:
NG = G + ∆V_OUT/V_S
R₁ = R₄/G
R₂ = R₄/(NG – G)
R₃ = R₄/(NG –1)
NG = 1 + R₄/R₃ (Noise Gain)
V_OUT = (G)V_IN + (NG – G)V_S
R₁
V_IN
+V_S
R₂
1/2
OPA2634
V_OUT
R₃
R₄
FIGURE 5. DC Level-Shifting Circuit.
Make sure that V_IN and V_OUT stay within the specified input
and output voltage ranges.
The front page circuit is a good example of this type of
application. It was designed to take V_IN between 0V and
0.5V, and produce V_OUT between 1V and 2V, when using a
+3V supply. This means G = 2.00, and ∆V_OUT = 1.50V – G
• 0.25V = 1.00V. Plugging into the above equations gives:
NG = 2.33, R₁ = 375Ω, R₂ = 2.25kΩ, and R₃ = 563Ω. The
resistors were changed to the nearest standard values.
NON-INVERTING AMPLIFIER WITH
REDUCED PEAKING
Figure 6 shows a non-inverting amplifier that reduces peak-
ing at low gains. The resistor R_C compensates the OPA2634
to have higher Noise Gain (NG), which reduces the AC
response peaking (typically 5dB at G = +1 without R_C)
without changing the DC gain. V_IN needs to be a low
impedance source, such as an op amp. The resistor values
are low to reduce noise. Using both R_T and R_F helps
minimize the impact of parasitic impedances.
R_T
V_IN
1/2
R_{C OPA2634}
V_OUT
R_G
R_F
FIGURE 6. Compensated Non-Inverting Amplifier.
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OPA2634