LTC6907 datasheet(13/30 Pages) AD

13 / 30 page

LTC6990

Rev. D

For more information www.analog.com

OPERATION

Changing DIVCODE After Start-Up

Following start-up, the A/D converter will continue moni-

toring VDIV for changes. Changes to DIVCODE will be

recognized slowly, as the LTC6990 places a priority on

eliminating any “wandering” in the DIVCODE. The typi-

cal delay depends on the difference between the old and

new DIVCODE settings and is proportional to the master

oscillator period.

tDIVCODE = 16 •(∆DIVCODE + 6)• tMASTER

A change in DIVCODE will not be recognized until it is

stable, and will not pass through intermediate codes. A

digital filter is used to guarantee the DIVCODE has settled

to a new value before making changes to the output. Then

the output will make a clean (glitchless) transition to the

new divider setting.

Start-Up Time

When power is first applied to the LTC6990 the power-on

reset (POR) circuit will initiate the start-up time, tSTART.

The OUT pin is floated (high-impedance) during this time.

The typical value for tSTART ranges from 0.5ms to 8ms

depending on the master oscillator frequency (indepen-

dent of NDIV):

tSTART(TYP) = 500 • tMASTER

The start-up time may be longer if the supply or DIV

pin voltages are not stable. For this reason, it is recom-

mended to minimize the capacitance on the DIV pin so it

will properly track V+.

Figure 5. DIVCODE Change from 5 to 2

Figure 6. Typical Start-Up

100µs/DIV

6990 F05

DIV

1V/DIV

OUT

1V/DIV

V+ = 3.3V

RSET = 200k

576µs

100µs/DIV

6990 F06

V+

1V/DIV

OUT

1V/DIV

V+ = 2.5V

DIVCODE = 4

RSET = 50k

OUTPUT CONNECTED

TO 1.25V THROUGH 25k

TO SHOW Hi-Z

470µs

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