LTC2221 datasheet(24/32 Pages) LINER

24 / 32 page

LTC2220/LTC2221

22201fa

As with all high speed/high resolution converters, the

digital output loading can affect the performance. The

digital outputs of the LTC2220/LTC2221 should drive a

minimal capacitive load to avoid possible interaction be-

tween the digital outputs and sensitive input circuitry. The

output should be buffered with a device such as an

ALVCH16373 CMOS latch. For full speed operation the

capacitive load should be kept under 10pF.

Lower OVDD voltages will also help reduce interference

from the digital outputs.

Digital Output Buffers (LVDS Mode)

Figure 13b shows an equivalent circuit for a differential

output pair in the LVDS output mode. A 3.5mA current is

steered from OUT+ to OUT– or vice versa which creates a

±350mV differential voltage across the 100Ω termination

resistor at the LVDS receiver. A feedback loop regulates

the common mode output voltage to 1.25V. For proper

operation each LVDS output pair needs an external 100

Ω

termination resistor, even if the signal is not used (such as

OF+/OF– or CLKOUT+/CLKOUT–). To minimize noise the

PC board traces for each LVDS output pair should be

routed close together. To minimize clock skew all LVDS PC

board traces should have about the same length.

APPLICATIO S I FOR ATIO

Table 3. MODE Pin Function

Clock Duty

MODE Pin

Output Format

Cycle Stablizer

Offset Binary

Off

1/3VDD

Offset Binary

2/3VDD

2’s Complement

VDD

2’s Complement

Off

Data Format

The LTC2220/LTC2221 parallel digital output can be se-

lected for offset binary or 2’s complement format. The

format is selected with the MODE pin. Connecting MODE

to GND or 1/3VDD selects offset binary output format.

Connecting MODE to 2/3VDD or VDD selects 2’s comple-

ment output format. An external resistor divider can be

used to set the 1/3VDD or 2/3VDD logic values. Table 3

shows the logic states for the MODE pin.

LTC2220/LTC2221

22201 F13b

OVDD

LVDS

RECEIVER

OGND

1.25V

OUT+

OUT–

100

Ω

–

3.5mA

10k

Figure 13b. Digital Output in LVDS Mode

Overflow Bit

An overflow output bit indicates when the converter is

overranged or underranged. In CMOS mode, a logic high

on the OFA pin indicates an overflow or underflow on the

A data bus, while a logic high on the OFB pin indicates an

overflow or underflow on the B data bus. In LVDS mode,

a differential logic high on the OF+/OF– pins indicates an

overflow or underflow.

Output Clock

The ADC has a delayed version of the ENC+ input available

as a digital output, CLKOUT. The CLKOUT pin can be used

to synchronize the converter data to the digital system. This

is necessary when using a sinusoidal encode. In all CMOS

modes, A bus data will be updated just after CLKOUTA rises

and can be latched on the falling edge of CLKOUTA. In demux

CMOS mode with interleaved update, B bus data will be

updated just after CLKOUTB rises and can be latched on the

falling edge of CLKOUTB. In demux CMOS mode with si-

multaneous update, B bus data will be updated just after

CLKOUTB falls and can be latched on the rising edge of

CLKOUTB. In LVDS mode, data will be updated just after

CLKOUT+/CLKOUT– rises and can be latched on the falling

edge of CLKOUT+/CLKOUT–.

LTC2221 датащи(PDF) 24 Page - Linear Technology

LTC2221 датащи(HTML) 24 Page - Linear Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32

датащи скачать

ссылки URL