LTC1598 datasheet(6/12 Pages) LINER

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LTC1598

APPLICATIONS INFORMATION

INPUT DATA WORD

The LTC1598 uses its Chip Select and DIN pins to select

one of its eight channels as shown in the operating

sequence figures and Table 1. For this discussion we will

assume that CS MUX and CS ADC are tied together and will

refer to them as simply, CS.

When CS is high, the input data on the DIN pin is latched

into the 4-bit shift register on the rising edge of the clock.

The input data word consists of an “EN” bit and a string of

three bits for channel selection. If the “EN” bit is logic high

as illustrated in Operating Sequence 1, it enables the

selected channel. To ensure correct operation, the CS

must be pulled low before the next rising edge of the clock.

More than four input bits can be sent to the ADC without

problems. The channel will be determined by the last four

bits clocked in before CS falls.

Once the CS is pulled low, all channels are simulta-

neously switched off to ensure a break-before-make

interval. After a delay of tON, the selected channel is

switched on, allowing signal transmission. The selected

channel remains on, until the next falling edge of CS. After

a delay of tOFF, it terminates the analog signal transmis-

sion and switches to the next selected channel. If the

“EN” bit is logic low, as illustrated in Operating Sequence

2, it disables all channels. Table 1 shows the various bit

combinations for channel selection.

Table 1. Logic Table for Channel Selection

CHANNEL STATUS

All Off

CH0

CH1

CH2

CH3

CH4

CH5

CH6

CH7

ANALOG CONSIDERATIONS

Grounding

The LTC1598 should be used with an analog ground plane

and single-point grounding techniques. To achieve the

optimum performance use a printed circuit board. The

Ground pins (Pins 4 and 9) should be tied directly to the

ground plane with minimum lead length.

Bypassing

For good performance, the LTC1598 VCC and VREF pins

must be free of noise and ripple. Any changes in the VCC

and VREF voltages with respect to ground during the

conversion cycle can induce errors or noise in the output

code. Bypass the VCC and VREF pins directly to the analog

ground plane with minimum of 0.1

µF capacitors and lead

lengths as short as possible.

Analog Inputs

Because of the capacitive redistribution A/D conversion

techniques used, the analog inputs of the LTC1598 have

capacitive switching input current spikes. These current

spikes settle quickly and do not cause a problem. But if

large source resistances are used, or if slow settling op

amps drive the inputs, take care to ensure the transients

caused by the current spikes settle completely before the

conversion begins.

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