Datasheet

External Reference
(
V
)
+
Code
786432
Total Analog Supply Voltage
(
V
)
+
Code
786432
Temperature( C)=°
TempReading( V) 168,000 V- mm
TempSensorCoefficient
+25 C°
Device Gain
ǒ
VńV
Ǔ
+
Code
7864320
ADS1258-EP
www.ti.com SBAS445D –MARCH 2009– REVISED MARCH 2011
INTERNAL SYSTEM READINGS The scale factor of Equation 9 converts the code
value to external reference voltage:
Analog Power-Supply Reading (VCC)
(9)
The analog power-supply voltage of the ADS1258
can be monitored by reading the VCC register. The
This readback function can be used to check for
supply voltage is routed internal to the ADS1258 and
missing or an out-of-range reference. If the reference
is measured and scaled using an internal reference.
input pins are floating (not connected), internal
The supply readback channel outputs the difference
biasing pulls them to the AVSS supply. This causes
between AVDD and AVSS (AVDD – AVSS), for both
the output code to tend toward '0'. Bypass capacitors
single and dual configurations. Note that it is required
connected to the external reference pins may slow
to disable chopping (CHOP = 0) prior to taking this
the response of the pins when open. When reading
reading.
this register immediately after power-on, verify that
the reference has settled to ensure an accurate
The scale factor of Equation 7 converts the code
reading. Note that it is required to disable chopping
value to volts:
(CHOP = 0) prior to taking this reading.
(7)
Temperature Reading (TEMP)
When the power supply falls below the minimum
The ADS1258 contains an on-chip temperature
specified operating voltage, the full operation of the
sensor. This sensor uses two internal diodes with one
ADS1258 cannot be ensured. Note that when the
diode having a current density of 16x of the other.
total analog supply voltage falls to below
The difference in current densities of the diodes
approximately 4.3 V the returned data is set to zero.
yields a difference voltage that is proportional to
The SUPPLY bit in the status byte is then set. The bit
absolute temperature.
is cleared when the total supply voltage rises
approximately 50 mV higher than the lower trip point. As a result of the low thermal resistance of the
package to the printed circuit board (PCB), the
The digital supply (DVDD) may be monitored by
internal device temperature tracks the PCB
looping-back the supply voltage to an input channel.
temperature closely. Note also that self-heating of the
A resistor divider may be required for bipolar supply
ADS1258 causes a higher reading than the
operation to reduce the DVDD level to within the
temperature of the surrounding PCB. Note that it is
range of the analog supply.
required to disable chopping (CHOP = 0) prior to
taking this reading.
Gain Reading (GAIN)
The scale factor of Equation 10 converts the
In this configuration, the external reference is
temperature reading to °C. Before using the equation,
connected both to the analog input and to the
the temperature reading code must first be scaled to
reference input of the ADC. The data from this
μV.
register indicates the gain of the device.
The following scale factor (Equation 8) converts the
code value to device gain:
(10)
(8)
Where Temp Sensor Coefficient = 563µV/°C (if the
ADS1258 and test PCB temperatures are forced
To correct the device gain error, the user software
together), or 394µV/°C (if only the ADS1258
can divide each converter data value by the device
temperature is forced and the test PCB is in free-air).
gain. Note that this corrects only for gain errors
originating within the ADC; system gain errors
Offset Reading (OFFSET)
because of an external gain stage error or because of
reference errors are not compensated. Note that it is
The differential output of the multiplexer is shorted
required to disable chopping (CHOP = 0) also prior to
together and set to a common-mode voltage of
taking this reading.
(AVDD – AVSS)/2. Ideally, the code from this register
function is 0h, but varies because of the noise of the
Reference Reading (REF)
ADC and offsets stemming from the ADC and
external signal conditioning. This register can be used
In this configuration, the external reference is
to calibrate or track the offset of the ADS1258 and
connected to the analog input and an internal
external signal conditioning. The chop feature of the
reference is connected to the reference of the ADC.
ADC can automatically remove offset and offset drift
The data from this register indicates the magnitude of
from the external signal conditioning; see the External
the external reference voltage.
Chopping section.
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