SM5844 datasheet(15/26 Pages) NPC

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SM5844AF

NIPPON PRECISION CIRCUITS—15

Attenuation (MDT, MCK, MLEN)

The digital attenuator coefﬁcients are read in as

serial data on the microcontroller interface. Data on

MDT is read into the internal shift register on the

rising edge of MCK, and then 12 bits are latched

internally on the rising edge of MLEN.

When the leading bit is 0 (B1 = LOW), the following

11 bits are read into the attenuation register and used

as an unsigned integer in MSB ﬁrst format. See

ﬁgure 3.

Although the attenuation data comprises 11 bits,

only 1025 levels are valid as given by the following.

The gain of the attenuator for values of DATT from

001H to 400H are given by the following equations.

Note that when the F12DB ﬂag is HIGH, the gain is

shifted by +12.0412 dB.

After a system reset initialization, DATT is set to

400H and the F12DB ﬂag is LOW, corresponding to

0 dB gain. (The F12DB ﬂag is described in table 2.)

Figure 3. Attenuation data format (microcontroller interface)

MCK

MDT

MLEN

MCK and MLEN can also follow the dotted lines.

LOW

B10

MSB

B11

a10

B12

LSB

DATT

–

()

∑

Gain

DATT

1024

----------------





log

[dB]

DATT

256

----------------





log

[dB]

when F12DB = LOW

when F12DB = HIGH

Table 5. Attenuator settings

Attenuation data DAT T

F12DB = LOW (default)

F12DB = HIGH

Gain (dB)

Linear expression

Gain (dB)

Linear expression

000H

−∞

0.0

−∞

0.0

001H

−60.206

1/1024

−48.165

1/256

↓↓↓↓↓

100H

−12.041

256/1024

0.0

256/256

↓↓↓↓↓

3FFH

−0.0085

1023/1024

12.032

1023/256

400H (to 7FFH)

1.0

12.041

4.0

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