ADS1255IDB Burr-Brown Corporation, ADS1255IDB Datasheet - Page 15

• Manufacturer Part Number: ADS1255IDB
• Description: Very Low Noise/ 24-Bit Analog-to-Digital Converter
• Manufacturer: Burr-Brown Corporation

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
10 A to the sensor through the input multiplexer. The
SDCS bits in the ADCON register enable the SDCS and
set the value of I
When
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
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
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
enabled, they source I
AIN
The two 25
ADS1255/6 internal resistances. The signal measured
with the SDCS enabled equals the total IR drop:
I
direct short (that is, R
signal measured by the ADS1255/6 when the SDCS are
enabled: I
SDC
SENS
NOTE: Arrows indicate switch positions when the SDCS are enabled.
P
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and sink I
R
between two input pins. When the SDCS are
SENS
(2R
the
Figure 7. Sensor Detect Circuitry
SDC
MUX
R
R
SDCS
25
25
MUX
MUX
SDC
P
2R
+ R
SDC
series resistors, R
to AGND if AIN
SDC
MUX
from the input pin connected to AIN
SENS
.
AIN
AIN
SENS
) of approximately 0.5 A, 2 A, or
are
SDC
Clamp
AVDD
.
P
N
3V
). Note that when the sensor is a
= 0), there will still be a small
to the input pin connected to
enabled,
Sensor Detect
Current Source
Sensor Detect
Current Source
P
exceeds 3V. Note that
Buffer
Input
P
must stay below 3V
MUX,
the
ADS1255/6
model the
N
.
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 Z
input impedance scales inversely with the frequency of
CLKIN. For example, if f
3.84MHz, Z
80M
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: f CLKIN = 7.68MHz.
Figure 8. Effective Impedance with Buffer On
Table 7. Input Impedance with Buffer On
to 160M .
AINCOM
DATA RATE
EFF
AIN0
AIN1
AIN2
AIN3
AIN4
AIN5
AIN6
AIN7
30,000
15,000
(SPS)
7,500
3,750
2,000
1,000
500
100
60
50
EFF
for a data rate of 50SPS will double from
SBAS288D − JUNE 2003 − REVISED AUGUST 2004
for the different data rate settings. The
Multiplexer
Input
CLKIN
AIN P
AIN N
is reduced by half to
Z EFF
Z EFF
(M )
10
10
10
10
10
20
40
40
40
80
ADS1255
ADS1256
15