TH7301 datasheet(5/14 Pages) MELEXIS

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TH7301 Dual-Channel Programmable Low-Pass Filter

Definition of

Terms

cset

cutoff frequency setting

cutoff frequency (-3dB bandwidth)

step

-step size

voltage gain

Frequency

Setting

To maintain frequency resolution at low frequen-

cies a companding law is applied to the fre-

quency code. The frequency range is selected as

one of the five octaves in the total filter range

controlled by bits FC(2...0). Each octave is divided

linearly into 128 equally sized steps decoded by

bits FS(6...0). This frequency range is for an

external resistor value of 4.5 k

Ω between pin

RDAC and VEE. The range is designed to

guarantee 0.8 MHz to 22.4 MHz cutoff range,

taking process variations in top account without

the need to change this external resistor. If there

is a need to fine tune this frequency range for any

reason, it may be re-centered by selecting a

suitably valued external resistor.

The tables below show the selectable frequency

settings.

The 128 steps within each octave are decoded by

FS(6...0). The according control mechanism

applies identically to all octaves refering to the

appropriate step sizes of each octave, as shown

in the tables and the example below.

Example:

FC(2...0) = 000

(frequency range 0.8 ... 1.5937 MHz,

Step size = 6.25 kHz)

FS(6...0) = 0000011

cset = 0.81875 MHz

(2..0)

Frequency range

fmin ... fmax (MHz)

Step size

fstep (kHz)

000

0.8 ... 1.5937

6.25

001

1.6 ... 3.1875

12.5

010

3.2 ... 6.375

011

6.4 ... 12.75

1xx

12.8 ... 25.5

100

Gain Setting

The gain control bits AC(5...0) are decoded to

select a voltage gain between -3 dB and 20.5 dB

in 0.5 dB steps. The gain control is divided into 8

steps controlled by AC(2...0) with additive step

sizes of 0.5 dB decoded by AC(5...3).

For example, to set the gain to 16 dB

AC(5...0) = 011001.

The table below shows the gain setting.

AC(2...0)

Gain (dB)

AC(5...3)

Gain (dB)

000

2.5

001

2.0

010

1.5

011

1.0

100

0.5

101

0.0

110

0.0

111

-3

111

0.0

(6...0)

Step N

Cutoff Frequency

fcset = fmin + N*fstep

(MHz)

0000000

fcset = fmin

0000001

fcset = fmin + fstep

0000010

fcset = fmin + 2*fstep

0000011

fcset = fmin + 3*fstep

1111111

127

fcset = fmin + 127*fstep

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