ADC32RF83 datasheet(35/138 Pages) TI

35 / 138 page

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.

ADC32RF80, ADC32RF83

www.ti.com

SBAS774A – MAY 2016 – REVISED DECEMBER 2016

Product Folder Links: ADC32RF80 ADC32RF83

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(1)

fS = sampling (device) clock frequency.

(2)

S = samples per frame.

(3)

K = number of frames per multi-frame.

(4)

LCM = least-common multiple.

(5)

D = decimation ratio.

2. SYSREF is applied as a periodic pulse.

Figure 92 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 92. 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.

8.3.3.2 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

Decimation

Various

(1) / (D × S(2) × K(3))

fS / (N × LCM

(4) (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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