ADS5294 datasheet(47/67 Pages) TI1

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ADS5294

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SLAS776B – NOVEMBER 2011 – REVISED JULY 2012

External Reference Mode of Operation

The ADS5294 supports an external reference mode of operation in one of two ways:

a. By forcing the reference voltages on the REFT and REFB pins.

b. By applying the reference voltage on VCM pin.

This mode can be used to operate multiple ADS5294 chips with the same (externally applied) reference voltage.

Using the REF pins:

For normal operation, the device requires two reference voltages, REFT and REFB. By default, the device

generates these two voltages internally. To enable the external reference mode, set the register bits as shown in

Table 6 . This powers down the internal reference amplifier and the two reference voltages can be forced directly

on the REFT and REFB pins as VREFT = 1.45V ± 50mV and VREFB = 0.45 V ±50 mV.

Note that the relation between the ADC full-scale input voltage and the applied reference voltages is

Full-scale input voltage = 2 x (VREFT – VREFB)

(4)

Using the VCM pin:

In this mode, an external reference voltage VREFIN can be applied to the VCM pin such that

Full-scale input voltage = 2 x VREFIN

(5)

To enable this mode, set the register bits as shown in Table 6. This changes the function of the VCM pin to an

external reference input pin. The voltage applied on VCM must be 1.5 V ±50 mV.

Table 6. External reference function

Function

EN_HIGH_ADDRS

EN_EXT_REF

EXT_REF_VCM

External reference using REFT/REFB pins

External reference using VCM pin

DATA OUTPUT FORMAT MODES

ADDR.

D15

D14

D13

D12

D11

D10

NAME

(HEX)

BTC_MODE

MSB_FIRST

The ADC output, by default, is in Straight offset binary mode. Programming the BTC_MODE bit to '1' inverts the

MSB, and the output becomes Binary 2’s complement mode. Also, by default, the first bit of the frame (following

the rising edge of CLKP) is the LSB of the ADC output. Programming the MSB_FIRST mode inverts the bit order

in the word, and the MSB is output as the first bit following CLKP rising edge.

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