ADC32RF44IRMPR datasheet(30/135 Pages) TI2

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CLKIN

SYSREF

(1)

st SYSREF pulse.

The input clock divider

is reset.

Time > tDLL + 2 x tSYSREF

MASK NCO SYSREF Register Bit

(2)

1 (The NCO and LMFC counter of the JESD

interface ignore the SYSREF pulses.)

Mask SYSREF to the NCO after

resetting the NCO phase.

The NCO phase is reset here for

the last time.

Then, the NCO mask is set high to

ignore further SYSREF pulses.

ADC32RF44

SBAS809A – FEBRUARY 2017 – REVISED MARCH 2017

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(1)

fS = sampling (device) clock frequency.

(2)

LCM = least-common multiple.

(3)

K = number of frames per multi-frame.

(4)

S = samples per frame.

(5)

D = decimation ratio.

2. SYSREF is applied as a periodic pulse.

Figure 70 shows how SYSREF can be applied as a continuous periodic waveform.

tSYSREF is a period of the SYSREF waveform.

Alternatively, the SYSREF buffer can be powered down using the PDN SYSREF bit.

Figure 70. SYSREF Used as a Periodic Waveform

After applying the SYSREF signal, DLL must be allowed to lock, and the NCO phase and LMFC counter

must be allowed to reset by waiting for at least the tDLL (40 µs) + 2 × tSYSREF time. Then, the SYSREF going

to the NCO and JESD can be masked by setting the MASK NCO SYSREF register bit.

9.3.3.2 Frequency of the SYSREF Signal

When SYSREF is a periodic signal, its frequency is required to be a sub-harmonic of the internal local multi-

frame clock (LMFC) frequency, as described in Equation 1. The LMFC frequency is determined by the selected

decimation, frames per multi-frame setting (K), samples per frame (S), and device input clock frequency.

SYSREF = LMFC / N

where

•

N is an integer value (1, 2, 3, and so forth)

(1)

In order for the interleaving correction engine to synchronize properly, the SYSREF frequency must also be a

multiple of fS / 64. Table 2 provides a summary of the valid LMFC clock settings.

Table 2. . SYSREF and LMFC Clock Frequency

OPERATING MODE

LMFS SETTING

LMFC CLOCK FREQUENCY

SYSREF FRQUENCY

Bypass mode

82820

(1) / (20 × K)

fS / (N × LCM

(2) (64, 20 × K(3)))

Bypass mode

8224

fS/(4 × K)

fS / (N × LCM (64, 4 × K))

Decimation

Various

fS / (D × S

(4) × K)

fS / (N × LCM (64, D

(5) × S × K))

The SYSREF signal is recommended to be a low-frequency signal less than 5 MHz in order to reduce coupling to

the signal path both on the printed circuit board (PCB) as well as internal to the device.

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