LTC1417AIGN datasheet(18/32 Pages) LINER

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LTC1417

Conversion Clock Selection

In Figure 15, the conversion clock controls the internal

ADC operation. The conversion clock can be either inter-

nal or external. By connecting EXTCLKIN high, the inter-

nal clock is selected. This clock generates 16 clock cycles

which feed into the SAR for each conversion.

To select an external conversion clock, apply an external

conversion clock to EXTCLKIN (Pin 6). (When an external

shift clock (SCLK) is used during a conversion, the SCLK

should be used as the external conversion clock to avoid

the noise generated by the asynchronous clocks. To

maintain accuracy, the external conversion clock fre-

quency must be between 50kHz and 9MHz.) The SAR

sends an end of conversion signal, EOC, that gates the

external conversion clock so that only 16 clock cycles can

go into the SAR, even if the external clock, EXTCLKIN,

contains more than 16 cycles.

When RD is low, these 16 cycles of conversion clock

(whether internally or externally generated) will appear

on CLKOUT during each conversion and then CLKOUT

will remain low until the next conversion. If desired,

CLKOUT can be used as a master clock to drive the serial

port. Because CLKOUT is running during the conversion,

it is important to avoid excessive loading that can cause

large supply transients and create noise. For the best

performance, limit CLKOUT loading to 20pF.

Serial Port

The serial port in Figure 15 is made up of a 16-bit shift

trolled by two inputs: SCLK and RD. The serial port has

one output, DOUT, that provides the serial output data.

The SCLK is used to clock the shift register. Data may be

clocked out with the internal conversion clock operating

as a master by connecting CLKOUT (Pin 8) to SCLK

(Pin 7) or with an external data clock applied to SCLK.

The minimum number of SCLK cycles required to trans-

fer a data word is 14. Normally, SCLK contains 16 clock

cycles for a word length of 16 bits; 14 bits with MSB first,

followed by two trailing zeros.

A logic high on RD disables SCLK and three-states DOUT.

In case of using a continuous SCLK, RD can be controlled

to limit the number of shift clocks to the desired number

(i.e., 16 cycles) and to three-state DOUT after the data

transfer.

In power shutdown mode (SHDN = low), a high RD

selects Sleep mode while a low RD selects Nap mode.

DOUT outputs the serial data; 14 bits, MSB first, on the

falling edge of each SCLK (see Figures 16 and 17). If 16

SCLKs are provided, the 14 data bits will be followed by

two zeros. The MSB (D13) will be valid on the first rising

and the first falling edge of the SCLK. D12 will be valid on

the second rising and the second falling edge as will all

the remaining bits. The data may be captured using either

edge. The largest hold time margin is achieved if data is

captured on the rising edge of SCLK.

BUSY gives the end-of-conversion indication. When the

LTC1417 is configured as a serial bus master, BUSY can

be used as a framing pulse. To three-state the serial port

after transferring the serial output data, BUSY and RD

should be connected together at the ADC (see Figure 17).

Figures 17 to 20 show several serial modes of operation,

demonstrating the flexibility of the LTC1417 serial interface.

APPLICATIONS INFORMATION

Figure 16. SCLK to DOUT Delay

t12

t11

SCLK

VIL

VOH

VOL

DOUT

1417 F16

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