ADS1258IPHPREP datasheet(25/57 Pages) TI

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SDC

Ω

AVDD

ADCINP

Ω

AVSS

ADCINN

MUXOUTP

MUXOUTN

SDC

ADS1258-EP

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SBAS445C – MARCH 2009 – REVISED DECEMBER 2009

SENSOR BIAS

The current source is connected to the output of the

multiplexer. For unselected channels, the current

An integrated current source provides a means to

source is not connected. This configuration means

bias an external sensor (for example, a diode

that when a new channel is selected, the current

junction); or, it verifies the integrity of a sensor or

source charges stray sensor capacitance, which may

sensor connection. When the sensor fails to an open

slow the rise of the sensor voltage. The automatic

condition, the current sources drive the inputs of the

switch time delay feature can be used to apply an

converter to positive full-scale. The biasing is in the

appropriate time delay before a conversion is started

form of differential currents (programmable 1.5

μA or

to provide fully settled data (see the Switch Time

μA), connected to the output of the multiplexer.

Delay section).

Figure 48 shows a simplified diagram of ADS1258

The time to charge the external capacitance is given

input structure with the external sensor modeled as a

in Equation 6:

resistance RS between two input pins. The two 80Ω

series resistors, RMUX, model the ADS1258 internal

resistances.

represents

the

effective

input

(6)

resistance of the ADC input or external buffer. When

It is also important to note that the low impedance

the sensor bias is enabled, they source ISDC to one

(65k

Ω) of the direct ADC inputs or the impedance of

selected input pin (connected to the MUXOUTP

the external signal conditioning loads the current

channel) and sink ISDC from the other selected input

sources. This low impedance limits the ability of the

pin (connected to the MUXOUTN channel). The

current source to pull the inputs to positive full-scale

signal measured with the biasing enabled equals the

for open-channel detection.

total IR drop: ISDC[(2RMUX + RS)

׀׀

RL]. Note that when

the sensor is a direct short (that is, RS = 0), there is

OPEN-SENSOR DETECTION

still a small signal measured by the ADS1258 when

the biasing is enabled: ISDC[2RMUX

׀׀

RL].

For open-sensor detection, set the biasing to either

1.5

μA or 24μA. Then select the channel and read the

output code. When a sensor opens, the positive input

is pulled to AVDD and the negative input is pulled to

AVSS. Because of this configuration, the output code

trends toward positive full-scale. Note that the

interaction of the multiplexer resistance with the

current source may lead to degradation in converter

linearity. It is recommended to enable the current

source only periodically to check for open inputs and

discard the associated data.

EXTERNAL DIODE BIASING

The current source can be used to bias external

diodes for temperature sensing. Scan the appropriate

channels with the current source set to 24µA.

Re-scan the same channels with the current source

set to 1.5µA. The difference in diode voltage readings

resulting from the two bias currents is directly

proportional to temperature.

Note that errors in current ratio, diode and cable

Figure 48. Sensor Bias Structure

resistance, or the non-ideality factor of the diode can

lead to errors in temperature readings. These effects

can be compensated by characterization or by

calibrating the diode at known temperatures.

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