Datasheet

ADS1246
ADS1247
ADS1248
www.ti.com SBAS426G –AUGUST 2008–REVISED OCTOBER 2011
INTERNAL VOLTAGE REFERENCE The two matched current sources can be connected
to dedicated current output pins IOUT1 and IOUT2
The ADS1247/8 includes an onboard voltage
(ADS1248 only), or to any AIN pin (ADS1247/8); refer
reference with a low temperature coefficient. The
to the ADS1247/48 Detailed Register Definitions
output of the voltage reference is 2.048V with the
section for more information. It is possible to connect
capability of both sourcing and sinking up to 10mA of
both current sources to the same pin. Note that the
current.
internal reference must be turned on and properly
compensated when using the excitation current
The voltage reference must have a capacitor
source DACs.
connected between VREFOUT and VREFCOM. The
value of the capacitance should be in the range of
1μF to 47μF. Large values provide more filtering of SENSOR DETECTION
the reference; however, the turn-on time increases
The ADS1246/7/8 provide a selectable current
with capacitance, as shown in Table 10. For stability
(0.5μA, 2μA, or 10μA) to help detect a possible
reasons, VREFCOM must have a path with an
sensor malfunction.
impedance less than 10Ω to ac ground nodes, such
as GND (for a 0V to 5V analog power supply), or
When enabled, two burnout current sources flow
AVSS (for a ±2.5V analog power supply). In case this
through the selected pair of analog inputs to the
impedance is higher than 10Ω, a capacitor of at least
sensor. One sources the current to the positive input
0.1μF should be connected between VREFCOM and
channel, and the other sinks the same current from
an ac ground node (for example, GND). Note that
the negative input channel.
because it takes time for the voltage reference to
When the burnout current sources are enabled, a
settle to the final voltage, care must be taken when
full-scale reading may indicate an open circuit in the
the device is turned off between conversions. Allow
front-end sensor, or that the sensor is overloaded. It
adequate time for the internal reference to fully settle.
may also indicate that the reference voltage is
absent. A near-zero reading may indicate a
Table 10. Internal Reference Settling Time
short-circuit in the sensor.
VREFOUT SETTLING TIME TO REACH THE
CAPACITOR ERROR SETTLING ERROR
BIAS VOLTAGE GENERATION
±0.5% 70μs
1μF
A selectable bias voltage is provided for use with
±0.1% 110μs
ungrounded thermocouples. The bias voltage is
±0.5% 290μs
(AVDD + AVSS)/2 and can applied to any analog
4.7μF
±0.1% 375μs
input channel through internal input multiplexer. The
±0.5% 2.2ms
bias voltage turn-on times for different sensor
47μF
capacitances are listed in Table 11.
±0.1% 2.4ms
The internal bias generator, when selected on
The onboard reference is controlled by the registers;
multiple channels, causes them to be internally
by default, it is off after startup (see the ADS1247/48
shorted. Because of this, it is important that care be
Detailed Register Definitions section for more details).
taken to limit the amount of current that may flow
Therefore, the internal reference must first be turned
through the device. It is recommended that under no
on and then connected via the internal reference
circumstances more than 5mA be allowed to flow
multiplexer. Because the onboard reference is used
through this path. This applies when the device is in
to generate the current reference for the excitation
operation and when it is in shutdown mode.
current sources, it must be turned on before the
excitation currents become available.
Table 11. Bias Voltage Settling Time
SENSOR CAPACITANCE SETTLING TIME
EXCITATION CURRENT SOURCE DACS
0.1μF 220μs
The ADS1247/8 provide two matched excitation
1μF 2.2ms
current sources for RTD applications. For three- or
10μF 22ms
four-wire RTD applications, the matched current
sources can be used to cancel the errors caused by
200μF 450ms
sensor lead resistance. The output current of the
current source DACs can be programmed to 50μA,
100μA, 250μA, 500μA, 750μA, 1000μA, or 1500μA.
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