ꢐ ꢕꢙ ꢚꢋ ꢛꢛ
ꢐ ꢕꢙ ꢚꢋ ꢛꢜ
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SBAS288D − JUNE 2003 − REVISED AUGUST 2004
OPEN/SHORT SENSOR DETECTION
ANALOG INPUT BUFFER
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 (ISDC) 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
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 fCLKIN is reduced by half to
3.84MHz, ZEFF for a data rate of 50SPS will double from
80MΩ to 160MΩ.
set the value of ISDC
.
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. AINP 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 AINP to AGND if AINP exceeds 3V. Note that
this clamp is activated only when the SDCS are enabled.
AIN0
AIN1
AIN2
Figure 7 shows a simplified diagram of ADS1255/6 input
structure with the external sensor modeled as resistance
RSENS between two input pins. When the SDCS are
enabled, they source ISDC to the input pin connected to
AINP and sink ISDC from the input pin connected to AINN.
The two 25Ω series resistors, RMUX, model the
ADS1255/6 internal resistances. The signal measured
with the SDCS enabled equals the total IR drop:
ISDC × (2RMUX + RSENS). Note that when the sensor is a
direct short (that is, RSENS = 0), there will still be a small
signal measured by the ADS1255/6 when the SDCS are
AIN
P
AIN3
AIN4
Input
Z
EFF
Multiplexer
AIN5
AIN
N
AIN6
AIN7
AINCOM
Figure 8. Effective Impedance with Buffer On
Table 7. Input Impedance with Buffer On
enabled: ISDC × 2RMUX
.
DATA RATE
(SPS)
Z
EFF
AVDD
(MΩ)
10
10
10
10
10
20
40
40
40
80
30,000
15,000
7,500
3,750
2,000
1,000
500
Sensor Detect
Current Source
RMUX
Ω
25
AINP
3V
Clamp
Input
Buffer
RSENS
100
RMUX
60
Ω
25
≤ 50
AINN
NOTE: f
CLKIN
= 7.68MHz.
Sensor Detect
Current Source
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.
NOTE: Arrows indicate switch positions when the SDCS are enabled.
Figure 7. Sensor Detect Circuitry
15
BB [ BURR-BROWN CORPORATION ]
