LTC1821 datasheet(12/20 Pages) LINER

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LTC2757

2757f

Figure 1. Conﬁguring the LTC2757 for

Single-Span Operation (±10V Range)

OPERATION

The most common readback task is to check the contents

of an Input register after writing to it, and before updating

the new data to the DAC register. To do this, hold UPD low

and assert READ high. The contents of the selected port’s

Input register are output to its I/O pins.

To read back the contents of a DAC register, hold UPD low

and assert READ high, then bring UPD high to select the

DAC register. The contents of the selected DAC register are

output by the selected port’s I/O pins. Note: if no update is

desired after the readback operation, UPD must be returned

low before bringing READ low, otherwise the UPD pin will

revert to its primary function and update the DAC.

Table 2. Span Codes

SPAN

Unipolar 0V to 5V

Unipolar 0V to 10V

Bipolar –5V to 5V

Bipolar –10V to 10V

Bipolar –2.5V to 2.5V

Bipolar –2.5V to 7.5V

Codes not shown are reserved and should not be used.

Readback

The contents of any one of the four interface registers can

be read back from the I/O ports by using the READ pin in

conjunction with the D/S and UPD pins.

The I/O pins and registers are grouped into two ports—Data

and Span. The Data I/O port consists of pins D0-D17, and

the Span I/O port consists of pins S0, S1 and S2.

Each I/O port has one dedicated Input register and one

dedicated DAC register. The register structure is shown

in the Block Diagram.

A Readback operation is initiated by asserting READ to

logic high after selecting the desired I/O port.

Select the I/O port (Data or Span) to be read back with

the D/S pin. The selected I/O port’s pins become logic

outputs during readback, while the unselected I/O port’s

pins remain high-impedance digital inputs.

With the I/O port selected, assert READ high and select

the desired Input or DAC register using the UPD pin. Note

that UPD is a two function pin—the Update function is only

available when READ is low. If READ is high, the Update

function is disabled and the UPD pin instead functions

as a register selector, selecting an Input or DAC register

for readback. Table 1 shows the readback functions for

the LTC2757.

Table 1. Write, Update and Read Functions

READ D/S

WR UPD

SPAN I/O

DATA I/O

Write to Input Register

Write/Update

(Transparent)

Update DAC Register

Write to Input Register

Write/Update

(Transparent)

Update DAC register

Update DAC Register

Read Input Register

Read DAC Register

Read Input Register

Read DAC Register

X = Don’t Care

LTC2757

M-SPAN

D/S

VDD

2757 F01

WR UPD READ

DATA I/O

VDD

System Offset and Gain Adjustments

Many systems require compensation for overall system

offset. This may be an order of magnitude or more greater

than the offset of the LTC2757, which is so low as to be

dominated by external output ampliﬁer errors even when

using the most precise op amps.

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