6

LTC1598

INPUT DATA WORD

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.

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

switched on, allowing signal transmission. The selected

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

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

EN

D2

D1

DO

All Off

0

X

X

X

CH0

1

0

0

0

CH1

1

0

0

1

CH2

1

0

1

0

CH3

1

0

1

1

CH4

1

1

0

0

CH5

1

1

0

1

CH6

1

1

1

0

CH7

1

1

1

1

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

conversion cycle can induce errors or noise in the output

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.