ADS5547 datasheet(32/50 Pages) TI

32 / 50 page

www.ti.com

Input Common-Mode

210MSPS

(342 A

m ) xFs

(1)

Reference

VCM

REFM

REFP

INTREF

EXTREF

Internal

Reference

ADS5547

Internal Reference

External Reference

ADS5547

SLWS192 – NOVEMBER 2006

APPLICATION INFORMATION (continued)

To ensure a low-noise common-mode reference, the VCM pin is filtered with a 0.1-

µF low-inductance capacitor

connected to ground. The VCM pin is designed to directly drive the ADC inputs. The input stage of the ADC

sinks a common-mode current in the order of 342

µA (at 210 MSPS). Equation 1 describes the dependency of

the common-mode current and the sampling frequency.

This equation helps to design the output capability and impedance of the CM driving circuit accordingly.

ADS5547 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 integration of the requisite

reference capacitors on-chip 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 controlling the MODE pin 23 (see Table 7 for details) or by programming

the serial interface register bit <REF>.

Figure 41. 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.

Submit Documentation Feedback

ADS5547 Datasheet(PDF) 32 Page - Texas Instruments