ADS5221RGZ datasheet(13/19 Pages) TI

13 / 19 page

ADS5221

SBAS262A

www.ti.com

The ADS5221 requires solid bypassing for all reference pins

to keep the effects of clock feedthrough to a minimum and to

achieve the specified levels of performance. Figure 7 to

Figure 10 show the recommended decoupling scheme. All

0.1

µF capacitors must be located as close to the pins as

possible. In addition, pins REFT, VREF, and REFB must be

decoupled with tantalum surface-mount capacitors (2.2

µF,

4.7

µF, 10µF or higher).

INTERNAL REFERENCE

There are two internal fixed reference modes and one

internal programmable reference mode as shown in Table I

and Figure 7 through Figure 9. Setting RSEL to ground (or

< 0.2V) provides an internal reference voltage of +1.0V at

VREF pin, +2V at REFT, and +1V at REFB pin. In this case,

the input FSR is +2V peak-to-peak. Connecting RSEL to the

VREF pin provides an internal reference voltage of +0.5V at

VREF, +1.75V at REFT, and +1.25V at REFB. In this case, the

input FSR is +1V peak-to-peak. Setting the resister divider as

in Figure 9 provides an internal voltage between +0.5V and

+1V at VREF, which is as follows:

VREF = 0.5 • (1+R2/R1)

In this case, the voltage at REFT and REFB and input FSR

is calculated based on Table I.

ADS5221

RSEL

VREF

REFT

REFB

2.2

µF

0.1

µF

Ω

µF

2.2

µF

Ω

2.2

µF

402

Ω

402

Ω

3.3V

Output

ADS5221

RSEL

VREF

REFT

REFB

µF

0.1

µF

Ω

µF

2.2

µF

Ω

2.2

µF

402

Ω

402

Ω

3.3V

0.5V

Output

ADS5221

RSEL

VREF

REFT

REFB

µF

0.1

µF

Ω

µF

2.2

µF

Ω

2.2

µF

402

Ω

402

Ω

3.3V

EXTERNAL REFERENCE

For even more design flexibility, the ADS5221 can be oper-

ated with external references. Utilization of an external refer-

ence voltage may be considered for applications requiring

higher accuracy, improved temperature stability, or flexible

full-scale range. Particularly in multi-channel applications,

the use of a common external reference offers the benefit of

improving gain matching between converters. Setting RSEL

to AVDD (+3.3V) provides an external reference mode for the

ADS5221. In this case, the internal VREF amplifier is powered

down, and the VREF pin requires an external reference

voltage between +0.5V to +1V to provide an input FSR

voltage of +1V to +2V. The REFT and REFB will appear with

the voltage as shown in Table I and input FSR is always

twice the voltage at the VREF pin. A voltage reference

(REF1004 or TPS79225) and a single-supply amplifier

(OPA2234 or OPA4227) can be used to generate a precision

external reference.

ADS5221

RSEL

VREF

REFT

REFB

µF

0.1

µF

Ω

µF

2.2

µF

Ω

2.2

µF

402

Ω

402

Ω

3.3V

Input

0.5V to 1V

FIGURE 7. Internal Reference Mode for VREF = 1V.

FIGURE 8. Internal Reference Mode for VREF = 0.5V.

FIGURE 9. Internal Reference Mode for VREF = 0.5 • (1 + R2/R1).

FIGURE 10. External Reference Configuration.

ADS5221RGZ Datasheet(PDF) 13 Page - Texas Instruments

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