AD9545 datasheet(49/157 Pages) AD

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Data Sheet

AD9545

Rev. A | Page 49 of 157

Automatic polarity detection slightly reduces the sensitivity of

the demodulator. Therefore, it is possible for the demodulator

to detect low magnitude modulation events reliably with

automatic polarity detection disabled, but exhibit demodulation

errors with auto polarity detection enabled.

DEMODULATOR SENSITIVITY

Modulated input signals exhibiting a small change in pulse

width require more sensitivity than those that exhibit a large

change. To control the demodulator sensitivity to pulse width

changes, use the 2-bit unsigned REFx demodulator sensitivity

bit field (where x is A, AA, B, or BB) in Bits[D1:D0] of Register

0x0302 to Register 0x0303 and Register 0x0306 to Register

0x0307.

The demodulator defaults to its most sensitive setting, REFx

demodulator sensitivity = 0. Increasing the value to 1 decreases

the sensitivity with 3 constituting the least sensitive setting.

Decreased sensitivity implies the need for more robust modulation

events (that is, larger changes in pulse width) for reliable

demodulation.

DEMODULATOR PERSISTENCE

In some applications, the modulation events may not be

continuous. The modulation events may occur in a burst, for

example, as a sequence of 50 modulation events followed by no

modulation. In this case, the demod out signal (see Figure 45)

appears as a burst of 50 pulses.

The demodulator has a persistence feature; it is capable of

identifying the modulation period and then generating the

requisite pulses at the demod out output even after the input

modulation events cease.

To enable the persistence feature, use the enable REFx

demodulator persist bit (where x is A, AA, B, or BB) in Bit D6

of Register 0x0302 to Register 0x0303 and Register 0x0306 to

Logic 1 enables it.

The persistence feature requires at least five consecutive

modulation events to produce a persistent demod out signal. As

such, the recommendation is to avoid using the persistence

feature when the modulation period is greater than 100 ms (that

is, modulation frequencies below 10 Hz).

DEMODULATOR BANDWIDTH

The demodulator is capable of handling input reference carrier

frequencies from 1 MHz to 180 MHz. The demodulator splits

this into two operating ranges: 1 MHz to 30 MHz carrier

frequencies and 30 MHz to 180 MHz carrier frequencies.

Use the REFA/REFAA demodulator band select bit or the

REFB/REFBB demodulator band select bit in Bit D0 of

the lower frequency range, whereas Logic 1 selects the upper

frequency range. Note that the bandwidth setting applies to

demodulator pairs. That is, one bit affects the demodulator for

REFA and REFAA, and the other bit affects the demodulator for

REFB and REFBB.

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