Application
Report
June 1999
Mixed-Signal Products
SLOA016
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Copyright  1999, Texas Instruments Incorporated
Contents
1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
2 Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
3 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
List of Figures
1
2
3
4
5
6
Layout of Area 100 of the EVA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Equivalent Circuit for the Bandpass Filter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Simulated Frequency Response . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Power Supply Bypass . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Bandpass Filter Layout on the Universal Op Amp Board . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Frequency Response of the Bandpass Filter Measured on a Network Analyzer . . . . . . . . . . . . . . . . . . . . . . . . . . .
Universal Operational Amplifier Evaluation Board for Designing a Two-Stage Bandpass Filter
2
3
3
4
5
6
iii
iv
SLOA016
Universal Operational Amplifier Evaluation Board for Designing a
Two-Stage Bandpass Filter
Shawn T. Workman
ABSTRACT
This application report describes the Universal Operational Amplifier board and how to
use it to demonstrate and evaluate the performance of TI’s surface-mounted two-stage
operational bandpass filters.
1 Introduction
This report uses the design goal of a band-pass filter with a bandwidth from
100 Hz to 10 kHz and unity gain as an example for other two-stage bandpass
filters. A Butterworth configuration with a Q of 0.707 is chosen to obtain a
maximally flat pass-band response. Beyond cutoff frequencies of 100 Hz and
10 kHz, the attenuation of the filter is 40 dB/dec. The amplifier is implemented in
a gain block by cascading a two-pole, high-pass Sallen-Key stage with a two-pole
low-pass Sallen-Key stage. The operational amplifier (op amp) chosen for this
design is the TLV2442. The TLV2442 is a dual low-voltage operational amplifier
from Texas Instruments. For the operational amplifier, the most essential design
parameters are the bandwidth and the slew rate. The unity gain bandwidth of the
TLV2442 is 1.8 MHz, which far exceeds the cut-off frequencies of the band-pass
filter and will not be a limiting factor in the design. Since the maximum input
amplitude is not specified, the input signal is assumed to swing to the rails. The
output swing of the TLV2442 at 5 V and 10 kHz requires a maximum slew rate
of 0.15 V/µS. The TLV2442 has a typical slew rate of 1.4 V/µS at a 5 V. Therefore
the bandwidth and slew rate requirements are well within the capabilities of the
TLV2442. For this particular application, the high input impedance and low noise
of the device make the TLV2442 excellent for cascading active filter stages.
2 Description
The EVM board consists of four separate circuit development areas. The four
areas are provided to evaluate specific op amp package options. Voltage
references, capacitors, and resistors are not area dependent, and can be either
surface-mount or through-hole devices. For this particular design a TLV2442 in
an SOIC package is used, therefore all design references for the remainder of this
report refer to area 100-SOIC of the EVM board. Figure 1 shows the layout of area
100 for the EVM. Area 100 uses the 1xx-reference designators and is compatible
with dual op amps packaged as an 8-pin SOICs.
1
Description
Area 100 – SOIC
UNIVERSAL OP AMP EVM
SOIC SLOP120-1 1998
R101
R110
C101
R111
B104+
R102
C107
B103+
R103
R112
B102–
R104
R113
B101–
C102
C108
B1OUT
C103
R114
V1+
C104
U102
GND1
VREF1
U101
V1–
A1OUT
R115
R116
C105
C109
A101–
R105
C110
A102–
R106
C111
A103+
R107
R117
A104+
C106
R118
R108
C112
R109
R119
Figure 1. Layout of Area 100 of the EVA
The first stage will be the high-pass stage to take advantage of the capacitive
input of the high-pass stage, which alleviates dc biasing of the source.
Determine the cut-off frequency and Q for the circuit by using: f c 1
and Q 2
1
2C R 1R 2
R1
R2
Where C= C1 = C2. Use the relation for Q to determine the R1/R2 ratio. A Q of
0.707 leaves R1 twice as large as R2. One can now choose any convenient set
of standard values for these resistors. R2 = 10 kΩ and R2 = 20 kΩ will satisfy the
Q of 0.707. In order to determine the value for C, solve the cut-off frequency
equation and plug in fc = 100 Hz and the resistor values obtained above into:
C
1
2f c R 1R 2
Choosing the closest standard value, C1 = C2 = 0.1 µF. The second stage of the
band-pass filter is a low pass filter with a cut-off frequency of 10 kHz and a Q of
0.707. The relationship for the low-pass filter Q and cut-off frequency are
1
respectively: Q 2
C2
1
f C 1 and c
2R C 2C 1
Where R = R1 = R2. Solving for C1 and C2 with a Q = 0.707 reveals that C2 is twice
as large as C1. One can now choose any convenient combination that satisfies
this condition.
2
SLOA016
Description
Choosing C1 = 330 pF, and C2 = 680 pF as standard capacitor values, R can be
determined by:
R+
1
2f c ǸC 1
C2
A standard value of R = R1 = R2 = 33 kΩ can be used. The circuit schematic and
the results of the simulation for the band-pass filter are shown in Figures 2 and
3 respectively.
High Pass Filter
R2
10 kΩ
C1
C2
Low Pass Filter
TLV2442
_
+
0.1 µF 0.1 µF
+
C2
680 pF
R1
R2
33 kΩ
33 kΩ
TLV2442
_
+
C1
330 pF
R1
20 kΩ
V_in
2.5 V
Figure 2. Equivalent Circuit for the Bandpass Filter
ELECTRONICS WORKBENCH SIMULATED FREQUENCY RESPONSE
10
Gain – dB
–4
–18
–32
–46
–60
1M
10 M
1
10 M
1
100
10 k
1M
100 M 1000 M
100
10 k
1M
100 M 1000 M
Phase – Deg
720
432
144
–144
–432
–720
1M
f – Frequency – Hz
Figure 3. Simulated Frequency Response
Universal Operational Amplifier Evaluation Board for Designing a Two-Stage Bandpass Filter
3
Description
After analysis and simulation the circuit is constructed on area 100 of the EVM
board using the Sallen-Key op amp example from the Universal Op Amp User’s
Guide. The power supply bypass circuit is configured for single supply operation,
using a jumper from the V1-to ground across reference designator C109. A
0.1 µF capacitor for high frequency bypassing and a 1 µF capacitor for low
frequency bypassing are connected from ground to V1+ using reference
designators C104 and C103 respectively. The bypass circuit and the
corresponding reference designators for the EVM board are shown in Figure 4.
The layout of the band-pass filter as seen on the EVM board with the
corresponding reference designators is shown in Figure 5. Biasing of the circuit
is accomplished using a TL431ACLP adjustable precision shunt regulator to
generate VREF1. The TL431, configured as shown, provides a low impedance
reference for the circuit of 2.5 V (1/2 V1+ in a 5 V system). VREF1 is jumped to
B104+. This biases the positive input of the high-pass filter op amp and thus the
entire circuit, see Figure 5.
V1
V1
C104
0.1 µF
C103
1 µF
GND1
Jumper
C109†
1
V1–
V1–
† The Universal Operational Amplifier board was designed for various configurations. These on-board
reference designators have different components inserted/deleted for the two-stage bandpass filter.
Figure 4. Power Supply Bypass
The 0.1 µF capacitors for the high-pass filter stage are soldered in place at
reference designators R101 and R111. The input resistor (20 kΩ) is soldered in
place at R102 and the feedback resistor (10 kΩ) at C101. A jumper that is not
shown in Figure 5, placed across R104 completes the connection from the output
to the negative input. The output is jumped from B1 out to A103+, the input to the
low-pass filter stage.
The 33-kΩ resistors for the low-pass stage are soldered at R119 and R108. The
330-pF capacitor is soldered at C111 and the 680-pF capacitor is soldered at
C106. Also, a jumper that is not shown in Figure 5, is placed across R105 to
complete the connection from the output to the negative input. The output of the
band-pass filter is taken from A1out. The TLV2442 is U101.
4
SLOA016
Description
High Pass Filter
V1
VREF1 = 2.5 V
R114
2.2 kΩ
R102†
B104
5
Jumper
+
1
+
R101† R111†
0.1 µF 0.1 µF
Jumper R115†
C
A
7
1/2 Dual
Op Amp
B1OUT
20 kΩ
B103
R
u101b
6 _
VI
U102
TL431ACLP
C101†
10 kΩ
–
2
Voltage
Reference
Band-Pass
Filter
u101a
1
+
A1OUT
Low Pass Filter
Stage
_ 2
3
1/2 Dual
Op Amp
R119
R108
33 kΩ
33 kΩ
A103
C111
330 pF
C106
1
680 pF
† The Universal Operational Amplifier board was designed for various configurations. These on-board
reference designators have different components inserted/deleted for the two-stage bandpass filter.
Figure 5. Bandpass Filter Layout on the Universal Op Amp Board
The circuit was tested and the frequency response of the circuit was measured
on an AP Instruments Model 102B analog network analyzer. The output of the
network analyzer is shown in Figure 6. The lower cut-off frequency is at 100 Hz
and the upper cut-off frequency is at 10 kHz. This response shows a flat
pass-band region with 40-dB roll-off beyond both cut-off frequencies. Comparing
the actual circuit frequency response in Figure 6 to the simulated frequency
response shown in Figure 3 shows good correlation between the two graphs.
Universal Operational Amplifier Evaluation Board for Designing a Two-Stage Bandpass Filter
5
5
160
0
120
–5
80
–10
40
–15
0
–20
–40
–25
–80
–30
–120
–35
–160
10
100
1k
10 k
Phase – Deg
Meg – dB
References
100 k
Figure 6. Frequency Response of the Bandpass Filter Measured on a Network Analyzer
3 References
1. David L. Terrell, Op Amps Design, Application, and Troubleshooting,
Butterworth-Heinemann, 1996.
2. Sedra/Smith, Microelectronic Circuits, Third Edition, Oxford University Press,
1991
3. S. Franco, Design with Operational Amplifiers and Analog Integrated Circuits,
McGraw-Hill 1988
4. Universal Operational Amplifier EVM User’s Manual, literature #SLVU006
6
SLOA016