Datasheet

ADS1146
ADS1147
ADS1148
www.ti.com SBAS453F –JULY 2009–REVISED APRIL 2012
INTERNAL VOLTAGE REFERENCE The two matched current sources can be connected
to dedicated current output pins IOUT1 and IOUT2
The ADS1147 and ADS1148 include an onboard
(ADS1148 only), or to any AIN pin (ADS1147 and
voltage reference with a low temperature coefficient.
ADS1148); refer to the ADS1147/48 Detailed
The output of the voltage reference is 2.048V with the
Register Definitions section for more information. It is
capability of both sourcing and sinking up to 10mA of
possible to connect both current sources to the same
current.
pin. Note that the internal reference must be turned
on and properly compensated when using the
The voltage reference must have a capacitor
excitation current 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 ADS1146/7/8 provide a selectable current
with capacitance, as shown in Table 6. 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 full-
settle to the final voltage, care must be taken when
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 short-
Table 6. Internal Reference Settling Time
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 7.
±0.1% 2.4ms
The internal bias generator when selected on multiple
The onboard reference is controlled by the registers;
channels causes them to be internally shorted.
by default, it is off after startup (see the ADS1147/48
Because of this, it is important that care be taken to
Detailed Register Definitions section for more details).
limit the amount of current that may flow through the
Therefore, the internal reference must first be turned
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 7. Bias Voltage Settling Time
SENSOR CAPACITANCE SETTLING TIME
EXCITATION CURRENT SOURCE DACS
0.1μF 220μs
The ADS1147 and ADS1148 provide two matched
1μF 2.2ms
excitation current sources for RTD applications. For
10μF 22ms
three- or four-wire RTD applications, the matched
current sources can be used to cancel the errors
200μF 450ms
caused by 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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