REF5045 datasheet(22/62 Pages) TI2

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AINP

AINN

4 pF

55 pF

REF

GND

Device in Hold Mode

REF

AINP

AINM

Binary

Algorithm

(SAR)

Common Mode Voltage

Detection and Rejection

Block

ADS8881

SBAS547D – MAY 2013 – REVISED AUGUST 2015

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Product Folder Links: ADS8881

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Feature Description (continued)

Most differential input SAR ADCs prohibit the input common-mode voltage, VCM (that is, the average voltage

between the inputs), at AINP or AINM from varying more than approximately 10% beyond the mid-scale input

value. As shown in Figure 46, the device has a unique common-mode voltage detection and rejection block that

does not have this restriction and thus allows VCM to be set to any value between 0 V and VREF without

degrading device performance.

Figure 46. Conceptual Diagram: True Differential Input Structure

Table 1 shows the full-scale input range of the device as a function of input common-mode voltage. The device

offers a maximum dynamic range for VCM = VREF / 2. The differential input with wide common-mode range allows

connecting differential signals from sensors without any signal conditioning.

Table 1. Full-Scale Input Range

VCM

ABSOLUTE INPUT RANGE

FULL SCALE INPUT RANGE (VFS)

VAINP

VAINN

VCM < VREF / 2

0 to 2 × VCM

(–2 × VCM) to (2 × VCM)

VCM = VREF / 2

0 to VREF

(–VREF) to (VREF)

VCM > VREF / 2

(2 × VCM – VREF) to VREF

[–2 × (VCM – VREF)] to [2 × (VCM – VREF)]

Figure 47 shows an equivalent circuit of the input sampling stage. The sampling switch is represented by a 96-Ω

resistance in series with the ideal switch; see the ADC Input Driver section for more details on the recommended

driving circuits.

Figure 47. Input Sampling Stage Equivalent Circuit

Figure 45 and Figure 47 illustrate electrostatic discharge (ESD) protection diodes to REF and GND from both

analog inputs. Make sure that these diodes do not turn on by keeping the analog inputs within the specified

range.

REF5045 Datasheet(PDF) 22 Page - Texas Instruments

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