LTC1277CS datasheet(15/20 Pages) LINER

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LTC1274/LTC1277

APPLICATI

I FOR ATIO

DIGITAL INTERFACE

The ADCs are designed to interface with microproces-

sors as a memory mapped device. The CS and RD control

inputs are common to all peripheral memory interfacing.

A separate CONVST is used to initiate a conversion.

Figures 10a to 10c are the input/output characteristics of

the ADCs. The code transitions occur midway between

successive integer LSB values (i.e., 0.5LSB, 1.5LSB,

2.5LSB…FS – 1.5LSV). The output code is scaled such

that 1.0LSB = FS/4096 = 4.096V/4096 = 1.0mV.

Unipolar Offset and Full-Scale Error Adjustments

In applications where absolute accuracy is important, then

offset and full-scale errors can be adjusted to zero. Offset

error must be adjusted before full-scale error. Figure 11a

shows the extra components required for full-scale error

adjustment. If both offset and full-scale adjustments are

needed, the circuit in Figure 11b can be used. For zero

offset error, apply 0.50mV (i.e., 0.5LSB) at the input and

adjust the offset trim until the LTC1274/LTC1277 output

code flickers between 0000 0000 0000 and 0000 0000

0001. For zero full-scale error, apply an analog input of

4.0945V (i.e., FS – 1.5LSB or last code transition) at the

input and adjust R5 until the ADC output code flickers

between 1111 1111 1110 and 1111 1111 1111.

Bipolar Offset and Full-Scale Error Adjustments

Bipolar offset and full-scale errors are adjusted in a similar

fashion to the unipolar case. Again, bipolar offset must be

INPUT VOLTAGE (V)

–1

LSB

LTC1274/77 • F10c

111...111

111...110

100...001

100...000

000...000

000...001

011...110

LSB

BIPOLAR

ZERO

011...111

FS/2 – 1LSB

–FS/2

1LSB =

= 1mV

4.096V

4096

Figure 10c. LTC1277 Bipolar Transfer Characteristics

(Offset Binary)

Figure 10a. LTC1274/LTC1277 Unipolar

Transfer Characteristics

Figure 10b. LTC1274 Bipolar Transfer

Characteristics (2’s Complement)

INPUT VOLTAGE (V)

FS – 1LSB

LTC1274/77 F10a

111...111

111...110

111...101

111...100

000...000

000...001

000...010

000...011

LSB

UNIPOLAR

ZERO

1LSB = FS

4096

4.096V

4096

= 1mV

INPUT VOLTAGE (V)

–1

LSB

LTC1274/77 • F10b

011...111

011...110

000...001

000...000

100...000

100...001

111...110

LSB

BIPOLAR

ZERO

111...111

FS/2 – 1LSB

–FS/2

1LSB =

= 1mV

4.096V

4096

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