ADS1258IPHPREP datasheet(44/57 Pages) TI

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Input

AINx

AVDD

BAT54SWTI

AVSS

10k

Ω

typ.

ADS1258-EP

SBAS445C – MARCH 2009 – REVISED DECEMBER 2009

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HARDWARE CONSIDERATIONS

The following summarizes the design and layout

considerations when using the ADS1258:

a. Power Supplies: The converter accepts a single

+5V supply (AVDD = 5 V and AVSS = AGND) or

dual, bipolar supplies (typically AVDD = 2.5 V,

AVSS

–2.5

V).

Dual

supply

operation

accommodates true bipolar input signals, within a

±2.5-V range. Note that the maximum negative

input voltage to the multiplexer is limited to AVSS

– 100 mV, and the maximum positive input

voltage is limited to AVDD + 100 mV. The range

Figure 63. Input Overload Protection

for the digital power supply (DVDD) is 2.7 V to

d. ADC Inputs: The external multiplexer loop of the

5.25 V. For all supplies, use a 10-

μF tantalum

ADS1258 allows for the inclusion of signal

capacitor,

bypassed

with

0.1-

μF

ceramic

conditioning between the output of the multiplexer

capacitor, placed close to the device pins.

and the input of the ADC. Typically, an amplifier

Alternatively, a single 10-

μF ceramic capacitor

is used to provide gain, buffering, and/or filtering

can be used. The supplies should be relatively

to the input signal. For best performance, the

free from noise and should not be shared with

ADC inputs should be driven differentially. A

devices that produce voltage spikes (such as

differential

in/differential

out

relays, LED display drivers, etc.). If a switching

single-ended-to-differential

driver

recom-

power supply is used, the voltage ripple should

mended. If the driver uses higher supply voltages

be low (< 2 mV). The analog and digital power

than the device itself (for example, ±15V),

supplies may be sequenced in any order.

attention

should

paid

power-supply

b. Analog (Multiplexer) Inputs: The 16-channel

sequencing

and

potential

over-voltage

fault

analog input multiplexer can accommodate 16

conditions. Protection resistors and/or external

single-ended inputs, eight differential input pairs,

clamp diodes may be used to protect the ADC

or combinations of either. These options permit

inputs. A 1-nF or higher capacitor should be used

freedom in choosing the input channels. The

directly across the ADC inputs.

channels do not have to be used consecutively.

e. Reference Inputs: It is recommended to use a

Unassigned channels are skipped by the device.

10-

μF tantalum with a 0.1-μF ceramic capacitor

In the Fixed-Channel mode, any of the analog

directly across the reference pins, VREFP and

inputs (AIN0 to AIN15) can be addressed for the

VREFN. The reference inputs should be driven

positive input and for the negative input. The

low-impedance

source.

For

rated

full-scale range of the device is 2.13 VREF, but the

performance, the reference should have less than

absolute analog input voltage is limited to 100 mV

μVRMS broadband noise. For references with

beyond the analog supply rails. Input signals

higher noise, external filtering may be necessary.

exceeding the analog supply rails (for example,

Note that when exiting the sleep mode, the

±10 V) must be divided prior to the multiplexer

device begins to draw a small current through the

inputs.

reference pins. Under this condition, the transient

c. Input Overload Protection: Overdriving the

response of the reference driver should be fast

multiplexer inputs may affect the conversions of

enough to settle completely before the first

other channels. In the case of input overload,

reading is taken, or simply discard the first

external

Schottky

diode

clamps

and

series

several readings.

resistor are recommended, as shown in Figure

61.

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