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REFERENCE

S0165-04

VCM

REFM

REFP

INTREF

EXTREF

4kW

1kW

Internal

Reference

ADS6xxx

Internal Reference

External Reference

Full−scale differential input pp + (Voltage forced on VCM)

1.33

(2)

ADS6445, ADS6444

ADS6443, ADS6442

SLAS531 – MAY 2007

APPLICATION INFORMATION (continued)

The ADS644X has built-in internal references REFP and REFM, requiring no external components. Design

schemes are used to linearize the converter load seen by the references; this and the on-chip integration of the

requisite reference capacitors eliminates the need for external decoupling. The full-scale input range of the

converter can be controlled in the external reference mode as explained below. The internal or external

reference modes can be selected by programming the register bit <REF> (refer to Table 13).

Figure 88. Reference Section

When the device is in internal reference mode, the REFP and REFM voltages are generated internally.

Common-mode voltage (1.5 V nominal) is output on VCM pin, which can be used to externally bias the analog

input pins.

When the device is in external reference mode, the VCM acts as a reference input pin. The voltage forced on

the VCM pin is buffered and gained by 1.33 internally, generating the REFP and REFM voltages. The differential

input voltage corresponding to full-scale is given by Equation 2.

In this mode, the range of voltage applied on VCM should be 1.45 V to 1.55 V. The 1.5-V common-mode voltage

to bias the input pins has to be generated externally.

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