ADS5271 datasheet(36/42 Pages) TI2

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VREF =1.5V

1.5V

VREF =1.5V+

1.5V

REF

SET

0.1 F

2.2 F

56.2kW

2.2 F

0.1 F

ADS5287

SBAS428D – JANUARY 2008 – REVISED JUNE 2012

www.ti.com

resistor at ISET reduces the reference bias current and

The device also supports the use of external

thereby scales down the device operating power.

reference voltages. There are two methods to force

However, it is recommended that the external resistor

the references externally. The first method involves

be within 10% of the specified value of 56.2k

Ω so

pulling INT/EXT low and forcing externally REFT and

that the internal bias margins for the various blocks

REFB to 2.5V and 0.5V nominally, respectively. In this

are proper.

mode, the internal reference buffer goes to a 3-state

output. The external reference driving circuit should

Buffering the internal bandgap voltage also generates

be designed to provide the required switching current

the common-mode voltage VCM, which is set to the

for the eight ADCs inside the chip. It should be noted

midlevel of REFT and REFB, and is accessible on pin

that in this mode, VCM and ISET continue to be

53. It is meant as a reference voltage to derive the

generated from the internal bandgap voltage, as in

input common-mode if the input is directly coupled. It

the internal reference mode. It is therefore important

can also be used to derive the reference common-

to ensure that the common-mode voltage of the

mode voltage in the external reference mode. The

externally-forced

reference

voltages

matches

suggested decoupling for the reference pins is shown

within 50mV of VCM.

in Figure 40.

The second method of forcing the reference voltages

externally can be accessed by pulling INT/EXT low,

and programming the serial interface to drive the

external reference mode through the VCM pin (register

bit called EXT_REF_VCM). In this mode, VCM

becomes configured as an input pin that can be

driven from external circuitry. The internal reference

buffers driving REFT and REFB are active in this

mode. Forcing 1.5V on the VCM pin in the mode

results in REFT and REFB coming to 2.5V and 0.5V,

respectively. In general, the voltages on REFT and

REFB in this mode are given by Equation 4 and

Equation 5, respectively:

(4)

Figure 40. Suggested Decoupling on the

Reference Pins

(5)

Table 7 describes the state of the reference voltage

internal buffers during various combinations of the

PD, INT/EXT, and EXT_REF_VCM register bits.

Table 7. State of Reference Voltages for Various Combinations of PD, INT/EXT, and EXT_REF_VCM

INTERNAL BUFFER STATE

INT/EXT

EXT_REF_VCM

REFT buffer

3-state

2.5V

3-state

2.5V(1)

1.5V + VCM/1.5V

Do not use

2.5V(1)

Do not use

REFB buffer

3-state

0.5V

3-state

0.5V(1)

1.5V – VCM/1.5V

Do not use

0.5V(1)

Do not use

VCM pin

1.5V

Force

Do not use

Force

Do not use

(1)

Weakly forced with reduced strength.

sections, while LVDD and LVSS are used to denote

the digital supplies. Care is taken to ensure that there

NOISE COUPLING ISSUES

is minimal interaction between the supply sets within

High-speed mixed signals are sensitive to various

the

device.

The

extent

noise

coupled

and

types of noise coupling. One primary source of noise

transmitted from the digital to the analog sections

is the switching noise from the serializer and the

depends on:

output buffers. Maximum care is taken to isolate

1. The effective inductances of each of the supply

these noise sources from the sensitive analog blocks.

and ground sets.

As a starting point, the analog and digital domains of

2. The isolation between the digital and analog

the device are clearly demarcated. AVDD and AVSS

supply and ground sets.

are used to denote the supplies for the analog

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