ADS1258IPHPREP datasheet(29/57 Pages) TI

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External Reference (V) +

Code

786432

Total Analog Supply Voltage (V) +

Code

786432

Temperature(

°C) +

Temp Reading(mV)

* 168, 000mV

394mV

°C

) 25°C

Device Gain V V

Code

7864320

ADS1258-EP

www.ti.com

SBAS445C – MARCH 2009 – REVISED DECEMBER 2009

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

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

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

this

configuration,

the

external

reference

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.

The following scale factor (Equation 8) converts the

code value to device gain:

(10)

(8)

Offset Reading (OFFSET)

To correct the device gain error, the user software

The differential output of the multiplexer is shorted

can divide each converter data value by the device

together and set to a common-mode voltage of

gain. Note that this corrects only for gain errors

(AVDD – AVSS)/2. Ideally, the code from this register

originating within the ADC; system gain errors

function is 0h, but varies because of the noise of the

because of an external gain stage error or because of

ADC and offsets stemming from the ADC and

reference errors are not compensated. Note that it is

external signal conditioning. This register can be used

required to disable chopping (CHOP = 0) also prior to

to calibrate or track the offset of the ADS1258 and

taking this reading.

external signal conditioning. The chop feature of the

ADC can automatically remove offset and offset drift

Reference Reading (REF)

from the external signal conditioning; see the External

Chopping section.

this

configuration,

the

external

reference

connected to the analog input and an internal

reference is connected to the reference of the ADC.

The data from this register indicates the magnitude of

the external reference voltage.

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