AN-763
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Dual Universal Precision Op Amp Evaluation Board
Rev. C | Page 1 of 8
INTRODUCTION
The EVAL-PRAOPAMP-2RZ, EVAL-PRAOPAMP-2RMZ, and
EVAL-PRAOPAMP-2CPZ are universal precision evaluation
boards that accommodate dual op amps in 8-pin SOIC,
MSOP, and LFCSP packages, respectively. For the exposed pad
connection for the LFCSP package, see the appropriate product
data sheet.
These PRAOPAMP evaluation boards provide multiple choices
and extensive flexibility for different application circuits and
configurations.
These boards are not intended to be used with high frequency
components or high speed amplifiers. However, they provide
the user with many combinations for various circuit types,
including active filters, instrumentation amplifiers, composite
amplifiers, and external frequency compensation circuits.
Several examples of application circuits are provided in this
application note.
TWO STAGE BAND-PASS FILTER
4
V–
V1
R1
20kΩ
+
–
V+
8
7
1/2 ADA4077-2
6
5
C2
10nF
C1
10nF
R2
10k
Ω
4
V–
C4
330pF
V+
8
1
1/2 ADA4077-2
2
3
C3
680pF
R3
33kΩ
R4
33kΩ
V
OUT
05284-001
Figure 1. KRC Filter
The low offset voltage and high CMRR makes the ADA4077-2
a great choice for precision filters, such as the KRC filter shown
in Figure 1.
This particular filter implementation offers the flexibility to
tune the gain and the cut-off frequency independently.
Since the common-mode voltage into the amplifier varies
with the input signal in the KRC filter circuit, a high CMRR
amplifier, such as the ADA4077-2, is required to minimize
distortion. Furthermore, the low offset voltage of the ADA4077-2
allows a wider dynamic range when the circuit gain is chosen to
be high.
The circuit shown in Figure 1 consists of two stages. The first
stage is a simple high-pass filter with a corner frequency, f
C
, of
(1)
and
(2)
where K is the dc gain.
Choosing equal capacitor values minimizes the sensitivity and
simplifies the expression for f
C
to
(3)
The value of Q determines the peaking of the gain vs. frequency
(generally ringing in the time domain). Commonly chosen
values for Q are near unity.
Setting Q = 1/√2 yields minimum gain peaking and minimum
ringing. Use Equation 3 to determine the values for R1 and R2.
For example, set Q = 1/√2 and R1/R2 = 2 in the circuit example,
and pick R1 = 5 kΩ and R2 = 10 kΩ for simplicity. The second
stage is a low-pass filter whose corner frequency can be deter-
mined in a similar fashion.
R3 = R4 = R
and
