AN-763

APPLICATION NOTE

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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

20k

Ω

+

–

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

33k

Ω

R4

33k

Ω

VOUT

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, fC, of

C1C2R1R2

π

2

1

(1)

and

R2

R1

K

Q =

(2)

where K is the dc gain.

Choosing equal capacitor values minimizes the sensitivity and

simplifies the expression for fC to

R1R2

C

π

2

1

(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

C3C4

R

fC

×

=

π

2

1

and

C4

C3

Q

2

/

1

=