LTC1289BCSW datasheet(12/28 Pages) LINER

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LTC1289

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APPLICATI

I FOR ATIO

The following discussion will demonstrate how the two

reference pins are to be used in conjunction with the

analog input multiplexer. In unipolar mode the input span

of the A/D is set by the difference in voltage on the REF+ pin

and the REF– pin. In the bipolar mode the input span is

twice the difference in voltage on the REF+ pin and the

REF– pin. In the unipolar mode the lower value of the input

span is set by the voltage on the COM pin for single-ended

inputs and by the voltage on the minus input pin for

differential inputs. For the bipolar mode of operation the

voltage on the COM pin or the minus input pin sets the

center of the input span.

The upper and lower value of the input span can now be

summarized in the following table:

Example 2 (Diff.): IN–

≤ IN+ ≤ IN– + 2V

Example 3 (Diff.): IN– – 2V

≤ IN+ ≤ IN– + 2V.

MSB-First/LSB-First Format (MSBF)

The output data of the LTC1289 is programmed for MSB-

first or LSB-first sequence using the MSBF bit. For MSB-

first output data, the input word clocked to the LTC1289

should always contain a logical one in the sixth bit location

(MSBF bit). Likewise for LSB-first output data the input

word clocked to the LTC1289 should always contain a zero

in the MSBF bit location. The MSBF bit affects only the

order of the output data word. The order of the input word

is unaffected by this bit.

The reference voltages REF+ and REF

– can fall between

VCC and V

–, but the difference (REF+– REF–) must be less

than or equal to VCC. The input voltages must be less than

or equal to VCC and greater than or equal to V

–.

The following examples are for a single-ended input con-

figuration.

Example 1: Let VCC = 3.3V, V

– = 0V, REF+ = 3V, REF– = 1V

and COM = 0V. Unipolar mode of operation. The resulting

input span is 0V

≤ IN+ ≤ 2V.

Example 2: The same conditions as Example 1 except

COM = 1V. The resulting input span is 1V

≤ IN+ ≤ 3V. Note

if IN+

≥ 3VtheresultingDOUTwordisall1’s.IfIN+≤1Vthen

the resulting DOUT word is all 0’s.

Example 3: Let VCC = 3.3V, V– = –3.3V, REF+ = 3V, REF–

= 1V and COM = 1V. Bipolar mode of operation. The

resulting input span is –1V

≤ IN+ ≤ 3V.

For differential input configurations with the same condi-

tions as in the above three examples the resulting input

spans are as follows:

Example 1 (Diff.): IN–

≤ IN+ ≤ IN– + 2V

Word Length (WL1, WL0) and Power Shutdown

The last two bits of the input word (WL1 and WL0)

program the output data word length and the power

shutdown feature of the LTC1289. Word lengths of 8, 12

or 16 bits can be selected according to the following table.

The WL1 and WL0 bits in a given DIN word control the

length of the present, not the next, DOUT word. WL1 and

WL0 are never “don’t cares” and must be set for the

correct DOUT word length even when a “dummy” DIN word

is sent. On any transfer cycle, the word length should be

made equal to the number of SCLK cycles sent by the

MPU. Power down will occur when WL1 = 0 and WL0 = 1

is selected. The previous result will be clocked out as a 10

bit word so a “dummy”conversion is required before

powering down the LTC1289. Conversions are resumed

once CS goes low or an SCLK is applied, if CS is already

low.

INPUT

CONFIGURATION

UNIPOLAR MODE

BIPOLAR MODE

Single-Ended

Lower Value COM

–(REF

+ – REF–) + COM

Upper Value (REF

+ – REF–) + COM (REF+ – REF–) + COM

Differential

Lower Value IN

–

–(REF

+ – REF–) + IN–

Upper Value (REF

+ – REF–) + IN–

(REF

+ – REF–) + IN–

MSBF

OUTPUT FORMAT

LSB-First

MSB-First

LTC1289 AI06

WL1

OUTPUT WORD LENGTH

8 Bits

Power Shutdown

12 Bits

16 Bits

LTC1289 AI07

WL0

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