Datasheet
ADS1255
ADS1256
SBAS288K − JUNE 2003 − REVISED SEPTEMBER 2013
www.ti.com
15
OPEN/SHORT SENSOR DETECTION
The sensor detect current sources (SDCS) provide a
means to verify the integrity of the external sensor
connected to the ADS1255/6. When enabled, the SDCS
supply a current (I
SDC
) of approximately 0.5μA, 2μA, or
10μA to the sensor through the input multiplexer. The
SDCS bits in the ADCON register enable the SDCS and
set the value of I
SDC
.
When the SDCS are enabled, the ADS1255/6
automatically turns on the analog input buffer regardless
of the BUFEN bit setting. This is done to prevent the input
circuitry from loading the SDCS. AIN
P
must stay below 3V
to be within the absolute input range of the buffer. To
ensure this condition is met, a 3V clamp will start sinking
current from AIN
P
to AGND if AIN
P
exceeds 3V. Note that
this clamp is activated only when the SDCS are enabled.
Figure 7 shows a simplified diagram of ADS1255/6 input
structure with the external sensor modeled as resistance
R
SENS
between two input pins. When the SDCS are
enabled, they source I
SDC
to the input pin connected to
AIN
P
and sink I
SDC
from the input pin connected to AIN
N
.
The two 25Ω series resistors, R
MUX,
model the
ADS1255/6 internal resistances. The signal measured
with the SDCS enabled equals the total IR drop:
I
SDC
× (2R
MUX
+ R
SENS
). Note that when the sensor is a
direct short (that is, R
SENS
= 0), there will still be a small
signal measured by the ADS1255/6 when the SDCS are
enabled: I
SDC
× 2R
MUX
.
NOTE: Arrows indicate switch positions when the SDCS are enabled.
AVDD
Input
Buffer
3V
Clamp
Sensor Detect
Current Source
Sensor Detect
Current Source
R
MUX
25
Ω
AIN
N
AIN
P
R
MUX
25
Ω
R
SENS
Figure 7. Sensor Detect Circuitry
ANALOG INPUT BUFFER
To dramatically increase the input impedance presented
by the ADS1255/6, the low-drift chopper-stabilized buffer
can be enabled via the BUFEN bit in the STATUS register.
The input impedance with the buffer enabled can be
modeled by a resistor, as shown in Figure 8. Table 7 lists
the values of Zeff for the different data rate settings. The
input impedance scales inversely with the frequency of
CLKIN. For example, if f
CLKIN
is reduced by half to
3.84MHz, Zeff for a data rate of 50SPS will double from
80MΩ to 160MΩ.
AIN
P
AIN
N
Input
Multiplexer
AIN0
AIN1
AIN2
AIN3
AIN4
AIN5
AIN6
AIN7
AINCOM
ADS1256 Only
Zeff
Figure 8. Effective Impedance with Buffer On
Table 7. Input Impedance with Buffer On
DATA RATE
(SPS)
Zeff
(MΩ)
30,000 10
15,000 10
7,500 10
3,750 10
2,000 10
1,000 20
500 40
100 40
60 40
≤ 50 80
NOTE: f
CLKIN
= 7.68MHz.
With the buffer enabled, the voltage on the analog inputs
with respect to ground (listed in the Electrical
Characteristics as Absolute Input Voltage) must remain
between AGND and AVDD − 2.0V. Exceeding this range
reduces performance, in particular the linearity of the
ADS1255/6. This same voltage range, AGND to
AVDD − 2.0V, applies to the reference inputs when
performing a self gain calibration with the buffer enabled.