LMV821 datasheet(14/24 Pages) NSC

14 / 24 page

the pass band gain. Increasing R

4 by ten percent, increased

the gain by 0.4 dB, while increasing R

5 by ten percent, de-

creased the gain by 0.4 dB.

TABLE 1.

Component (LPF)

Sensitivity (LPF)

Component (HPF)

Sensitivity (HPF)

-1.2

-0.7

-0.1

-1.0

-1.1

+0.1

+0.7

-0.1

-1.5

+0.1

-0.6

-0.1

+0.6

+0.1

Active filters are also sensitive to an op amp’s parameters

-Gain and Bandwidth, in particular. The LMV822/24 provide

a large gain and wide bandwidth. And DAAFs make excel-

lent use of these feature specifications.

Single Amplifier versions require a large open-loop to

closed-loop gain ratio - approximately 50 to 1, at the Fc of

the filter response.

Figure 12 shows an impressive photo-

graph of a network analyzer measurement (hp3577A). The

measurement was taken from a 300kHz version of

Figure

10. At 300 kHz, the open-loop to closed-loop gain ratio @ Fc

is about 5 to 1. This is 10 times lower than the 50 to 1 “rule

of thumb” for Single Amplifier Active Filters.

In addition to performance, DAAFs are relatively easy to de-

sign and implement. The design equations for the low-pass

and high-pass DAAFs are shown below. The first two equa-

tion calculate the Fc and the circuit Quality Factor (Q) for the

LPF (

Figure 10). The second two equations calculate the Fc

and Q for the HPF (

Figure 11).

To simplify the design process, certain components are set

equal to each other. Refer to

Figure 10 and Figure 11. These

equal component values help to simplify the design equa-

tions as follows:

To illustrate the design process/implementation, a 3 kHz,

Butterworth response, low-pass filter DAAF (

Figure 10)is

designed as follows:

1. Choose C

1 =C3 =C=1nF

2. Choose R

4 =R5 =1kΩ

3. Calculate R

a and R2 for the desired Fc as follows:

4. Calculate R

3 for the desired Q. The desired Q for a Butter-

worth (Maximally Flat) response is 0.707 (45 degrees into

the s-plane). R

3 calculates as follows:

Notice that R

3 could also be calculated as 0.707 of Ra or R2.

The circuit was implemented and its cutoff frequency mea-

sured. The cutoff frequency measured at 2.92 kHz.

The circuit also showed good repeatability. Ten different

LMV822 samples were placed in the circuit. The correspond-

ing change in the cutoff frequency was less than a percent.

DS100128-92

FIGURE 12. 300 kHz, Low-Pass Filter, Butterworth

Response as Measured by the HP3577A Network

Analyzer

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