CS5378 datasheet(62/86 Pages) CIRRUS

62 / 86 page

CS5378

DS639F1

17.6 TBS Impulse Output

If the TMODE bit in TBSCFG is set high, the TBS

generator operates in impulse mode. In this mode,

the value in TBSGAIN sets the amplitude of the

generated impulse. Impulse amplitude and period

are shown in Table 19.

To create a maximum impulse from the TBS gen-

erator, the TBSGAIN register should be set to

0x04FFFF, and the INTP bits in TBSCFG should

also be set to 0xFF. The RATE bits should always

be set to produce data at the correct rate for the in-

tended test DAC.

A rising edge on the TIMEB pin triggers the im-

pulse output. When impulse mode is enabled but

no TIMEB input is received, the TBS generator

uses a negated TBSGAIN register as a repetitive in-

put value. When a rising edge is recognized on the

TIMEB pin, a single positive TBSGAIN value is

written to the TBS generator to create the impulse.

17.7 TBS Loopback Testing

Included as part of the CS5378 test bit stream gen-

erator is a feedback path to the digital filter MDA-

TA input. This loopback mode provides a fully

digital signal path to test the TBS generator, digital

filter, and data collection interface. Digital loop-

back testing expects 512 kHz

∆Σ data into the

MDATA input.

A mismatch of the TBS generator full scale output

and the MDATA full scale input results in an am-

plitude mismatch when testing in loopback mode.

The TBS generator outputs a 75% maximum one’s

density, while the MDATA inputs expect an 86%

maximum one’s density from a

∆Σ modulator, re-

sulting in a measured full scale error of approxi-

mately -3.6 dB.

17.8 TBS Synchronization

When the TSYNC bit is set in the TBSCFG regis-

ter, the MSYNC signal resets the sine wave data

pointer and phase aligns the TBS signal output.

Once the digital filter is settled, all CS5378 devices

receiving the SYNC signal will have identical TBS

signal phase. See “Synchronization” on page 24

for more information about the SYNC and

MSYNC signals.

If TSYNC is clear, MSYNC has no effect on the

TBS data pointer and no change in the TBS output

phase will occur during synchronization.

Test Bit Stream Impulse Characteristics:

Interpolation

Selection

(INTP)

Output Rate

Selection

(RATE)

Pulse Width

from CS4373

Gain Scale

Factor

(TBSGAIN)

Pulse Height

from CS4373

0xFF

0x5

500

µs

0x04B000

~ 860 mV

0xFF

0x4

1 ms

0x04B000

~ 820 mV

0xFF

0x3

2 ms

0x04B000

~ 820 mV

0x7F

0x5

250

µs

0x04B000

~ 820 mV

0x7F

0x4

500

µs

0x04B000

~ 820 mV

0x7F

0x3

1 ms

0x04B000

~ 820 mV

Table 19. TBS Impulse Characteristics

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