REF5045 datasheet(24/62 Pages) TI2

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20001

1FFFF

00000

VREF

± 1 LSB

±VREF

+ 1 LSB

Differential Analog Input

(AINP

AINN)

±1 LSB

3FFFF

VIN

20000

ADS8881

SBAS547D – MAY 2013 – REVISED AUGUST 2015

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10.3.4 ADC Transfer Function

The ADS8881 is a unipolar, differential input device. The device output is in twos compliment format.

Figure 49 shows ideal characteristics for the device. The full-scale range for the ADC input (AINP – AINN) is

equal to twice the reference input voltage to the ADC (2 × VREF). The LSB for the ADC is given by Equation 1.

1 LSB = [2 × (VREF / 2

18)]

(1)

Figure 49. Differential Transfer Characteristics

10.4 Device Functional Modes

The ADS8881 is a low pin-count device. However, the device offers six different options for interfacing with the

digital host.

These options can be broadly classified as being either CS mode (in either a 3- or 4-wire interface) or daisy-

chain mode. The device operates in CS mode if DIN is high at the CONVST rising edge. If DIN is low at the

CONVST rising edge, or if DIN and CONVST are connected together, the device operates in daisy-chain mode.

In both modes, the device can either operate with or without a busy indicator, where the busy indicator is a bit

preceding the output data bits that can be used to interrupt the digital host and trigger the data transfer.

The 3-wire interface in CS mode is useful for applications that need galvanic isolation on-board. The 4-wire

interface in CS mode allows the user to sample the analog input independent of the serial interface timing and,

therefore, allows easier control of an individual device while having multiple, similar devices on-board. The daisy-

chain mode is provided to hook multiple devices in a chain similar to a shift register and is useful in reducing

component count and the number of signal traces on the board.

10.4.1 CS Mode

CS mode is selected if DIN is high at the CONVST rising edge. There are four different interface options

available in this mode: 3-wire CS mode without a busy indicator, 3-wire CS mode with a busy indicator, 4-wire

CS mode without a busy indicator, and 4-wire CS mode with a busy indicator. The following sections discuss

these interface options in detail.

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