ADS5295IPFP datasheet(61/93 Pages) TI2

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Gain Error (%) = Error in Full-Scale Voltage ´

100

Ideal Full-Scale Voltage

= 2x [Error in (V

)]

REFT

REFB

100

2.0

Internal

Reference

ADC

REFT

REFB

INTREF/EXTREF

Device

INTREF/EXTREF

REF Amp

Full-Scale Input Voltage = 2

)

REFT

REFB

ADS5295

www.ti.com

SBAS595 – DECEMBER 2012

EXTERNAL REFERENCE MODE OF OPERATION

For normal operation, the device requires two reference voltages (REFT and REFB) that are generated internally

by default, as shown in Figure 55. The value of the reference voltage determines the actual ADC full-scale input

voltage, as shown in Equation 1:

(1)

Figure 55. Reference Equivalent Circuit

Any error in the reference results in a deviation of the full-scale input range from its ideal value of 2.0 VPP, as

shown in Equation 2:

Error in Full-Scale Voltage = 2x [Error in (VREFT – VREFB)]

(2)

The reference inaccuracy results in a gain error, which is defined as Equation 3:

(3)

To minimize the gain error, the internal reference voltages are trimmed to an accuracy of ±1.5% (or ±30 mV).

To obtain even lower gain error, the ADS5295 supports an external reference mode of operation. In this mode,

the internal reference amplifiers are powered down and an external amplifier must force the reference voltages

on the REFT and REFB pins. For example, this mode can be used to ensure that multiple ADS5295 chips in the

system have nearly the same full-scale voltage.

To enable the external reference mode, set the register bits as shown in Table 8. These settings power down the

internal reference amplifier and the two reference voltages can be forced directly on the REFT and REFB pins as

VREFT = 1.45 V and VREFB = 0.45 V.

Table 8. External Reference Function

FUNCTION

EN_HIGH_ADDRS

EN_EXT_REF

External reference using the REFT, REFB pins

Because the internal reference amplifiers are powered down, the accuracy of the full-scale voltage is determined

by the accuracy of (VREFT – VREFB), where VREFT is the voltage forced on REFT and VREFB is the voltage forced

on REFB.

Note that although the nominal value of (VREFT – VREFB) = 1.0 V, ensure that:

[(VREFT + VREFB) / 2 = 0.950 V ± 50 mV].

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