Datasheet

dV
dt
+
I
SDC
C
80
Ω
AVDD
R
L
R
S
ADCINP
80
Ω
AVSS
ADCINN
MUXOUTP
MUXOUTN
I
SDC
I
SDC
ADS1258-EP
www.ti.com SBAS445D –MARCH 2009– REVISED MARCH 2011
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 1.5μA or
to provide fully settled data (see the Switch Time
24μA), 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 R
S
between two input pins. The two 80Ω
series resistors, R
MUX
, model the ADS1258 internal
resistances. R
L
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 I
SDC
to one
(65kΩ) 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 I
SDC
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: I
SDC
[(2R
MUX
+ R
S
) ׀׀ R
L
]. Note that when
the sensor is a direct short (that is, R
S
= 0), there is
OPEN-SENSOR DETECTION
still a small signal measured by the ADS1258 when
the biasing is enabled: I
SDC
[2R
MUX
׀׀ R
L
].
For open-sensor detection, set the biasing to either
1.5μA 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.
Copyright © 2009–2011, Texas Instruments Incorporated Submit Documentation Feedback 23
Product Folder Link(s): ADS1258-EP