LMP90100MHE National Semiconductor, LMP90100MHE Datasheet - Page 29

• Manufacturer Part Number: LMP90100MHE
• Description: AFE, SENSOR, 28TSSOP
• Manufacturer: National Semiconductor

Specifications of LMP90100MHE

• Brief Features: 24bit Low Power Sigma Delta ADC, Automatic Channel Sequencer
• Supply Voltage Range: 2.7V To 5.5V
• Operating Temperature Range: -40°C To +105°C
• Digital Ic Case Style: TSSOP
• No. Of Pins: 28
• Base: RoHS Compliant
• Ic Function: Multi-Channel, Low Power 24-bit Sensor AFE
• Rohs Compliant: Yes

16.4 SENSOR INTERFACE
LMP90100 contains two types of current sources: excitation
currents (IB1 & IB2) and burnout currents. They are described
in the next sections.
16.4.1 IB1 & IB2 - Excitation Currents
IB1 and IB2 can be used for providing currents to external
sensors, such as RTDs or bridge sensors. 100µA to 1000µA,
in steps of 100µA, can be sourced by programming the
ADC_AUXCN: RTD_CUR_SEL bits.
Refer to
can be used to source a 3-wire RTD.
16.4.2 Burnout Currents
As shown in
10 µA burnout current sources, one sourcing current from VA
to VINP, and the other sinking current from VINN to ground.
These currents are used for sensor diagnostics and can be
enabled for each channel using the CHx_INPUTCN:
BURNOUT_EN bit.
Section 17.6.1 3–Wire RTD
Figure
FIGURE 14. Burnout Currents
14, the LMP90100 contains two internal
to see how IB1 and IB2
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Burnout Current Injection:
Burnout currents are injected differently depending on the
channel scan mode selected.
When BURNOUT_EN = 1 and the device is operating in
ScanMode0, 1, or 2, the burnout currents are injected into all
the channels for which the BURNOUT_EN bit is selected.
This will cause problems and hence in this mode, more than
one channel should not have its BURNOUT_EN bit selected.
Also, the burnout current will interfere with the signal and in-
troduce a fixed error depending on the particular external
sensor.
When BURNOUT_EN = 1 and the device is operating in
ScanMode3, burnout currents are injected into the last sam-
pled channel on a cyclical basis
burnout currents injection is truly done in the background
without affecting the accuracy of the on-going conversion.
Operating in this mode is recommended.
16.4.3 Sensor Diagnostic Flags
Burnout currents can be used to verify that an external sensor
is still operational before attempting to make measurements
on that channel. A non-operational sensor means that there
is a possibility the connection between the sensor and the
LMP90100 is open circuited, short circuited, shorted to VA or
GND, overloaded, or the reference may be absent. The sen-
sor diagnostic flags diagram can be seen in
FIGURE 15. Burnout Currents Injection for ScanMode3
(Figure
15). In this mode,
Figure
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