ADS1258IPHPREP datasheet(17/57 Pages) TI

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ESD

Diodes

ESD

Diodes

3pF

eff = 40kΩ

CLK = 16MHz)

AVDD

AVSS

VREFP

VREFN

AVSS * 100mV t VREFP or VREFN t AVDD ) 100mV

ADS1258-EP

www.ti.com

SBAS445C – MARCH 2009 – REVISED DECEMBER 2009

MULTIPLEXER INPUTS

A simplified diagram of the input multiplexer is

illustrated in Figure 36. The multiplexer connects one

of 16 single-ended external inputs, one of eight

differential external inputs, or one of the on-chip

internal variables to the ADC inputs. The output of the

channel multiplexer can be routed to external pins

and then to the input of the ADC. This flexibility

allows for use of external signal conditioning. See the

External Multiplexer Loop section.

ESD diodes protect the analog inputs. To keep these

diodes from turning on, make sure the voltages on

the input pins do not go below AVSS by more than

100 mV, and likewise do not exceed AVDD by more

than 100 mV:

AVSS – 100mV < (Analog Inputs) < AVDD + 100 mV.

Overdriving the multiplexer inputs may affect the

conversions

other

channels.

See

the

Input

Overload Protection description in the Hardware

Figure 35. Simplified Reference Input Circuit

Considerations segment of the Applications section.

ESD diodes protect the reference inputs. To keep

The converter supports two modes of channel access

these diodes from turning on, make sure the voltages

through the multiplexer: the Auto-Scan mode and the

on the reference pins do not go below AVSS by more

Fixed-Channel mode. These modes are selected by

than 100mV, and likewise do not exceed AVDD by

the

MUXMOD

bit

CONFIG0.

The

100mV, as described in Equation 1:

Auto-Scan

mode

scans

through

the

selected

channels

automatically,

with

break-before-make

(1)

switching. The Fixed-Channel mode requires the user

A high-quality reference voltage is essential for

set

the

channel

address

for

each

channel

achieving the best performance from the ADS1258.

measured.

Noise and drift on the reference degrade overall

system performance. It is especially critical that

VOLTAGE REFERENCE INPUTS

special care be given to the circuitry that generates

(VREFP, VREFN)

the reference voltages and the layout when operating

The voltage reference for the ADS1258 ADC is the

in the low-noise settings (that is, with low data rates)

differential voltage between VREFP and VREFN:

prevent

the

voltage

reference

from

limiting

VREF = VREFP – VREFN. The reference inputs use a

performance. See the Reference Inputs description in

structure similar to that of the analog inputs with the

the

Hardware

Considerations

segment

the

circuitry on the reference inputs shown in Figure 35.

Applications section.

The load presented by the switched capacitor can be

modeled with an effective resistance (Reff) of 40 kΩ

for fCLK = 16 MHz. Note that the effective impedance

of the reference inputs loads an external reference

with a non-zero source impedance.

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