XTR108EA-2K5 BURR-BROWN [Burr-Brown Corporation], XTR108EA-2K5 Datasheet - Page 11

• Manufacturer Part Number: XTR108EA-2K5
• Manufacturer: BURR-BROWN [Burr-Brown Corporation]

The circuit is designed for compliance with NAMUR NE43
recommendation for sensor interfaces. The limit levels are
listed in Tables VII and VIII. Because of the large step sizes,
units that use this feature should be checked if the value is
critical. The under-scale limit circuit will override the Zero
DAC level if it is set lower and there is not enough sensor
offset at the PGA input.
It may be necessary to disable limiting if the XTR108 is used
in applications other than a 4-20mA transmitter, where the
PGA output is between 0.5V and 4.5V.
SENSOR FAULT DETECTION CIRCUIT
To detect sensor burnout and/or short, a set of four compara-
tors is connected to the inputs of the PGA. If any of the
inputs are taken outside of the PGA’s common-mode range,
the corresponding comparator sets a sensor fault flag that
causes the PGA output to go either to the upper or lower
error limit. The state of the fault condition can be read in the
digital form from register 3. The direction of the analog
output is set according to the “Alarm Configuration Regis-
ter” (see Table X). The level of the output is produced as
follows: if the over-scale/under-scale limiting is enabled, the
error levels are: over-scale limit +2LSBs of the over-scale
DAC, about 1mA referred to I
of under-scale limit –2LSBs of the under-scale DAC, about
0.4mA referred to I
scale/under-scale limiting is disabled, the PGA output volt-
age will go to within 150mV of either positive or negative
supply (V
bit corresponding to the error condition.
OUTPUT CURRENT AMPLIFIER + R
To produce the 4-20mA output, the XTR108 uses a current
amplifier with a fixed gain of 50A/A. The voltage from the
PGA is converted to current by the external resistor, R
I
ground), is connected to the output and inverting input of the
amplifier. This allows collecting all external and internal
supply currents, sensor return current, and leakage currents
from the different parts of the system and accounting for
them in the output current. The current from R
the pin I
therefore, is at ground potential as well. The ratio of two
XTR108
TABLE II. Equations for Calculating Zero Output.
RET
SBOS187C
OVERALL
PROGRAM
COARSE DAC
FINE DAC
NOTE: N
, the common potential of the circuit (substrate and local
13
IN
S
and N
that is connected to the noninverting input and
or I
12
RET
are assigned decimal values of registers 13 and 12, respectively.
), depending on the alarm configuration
OUT
or 0.05V referred to V
V
ZERO
OUT
VOLTAGE REFERRED TO V
= V
or 0.125V referred to V
WITH RESPECT TO I
V
V
Z PROGRAM
V
Z COARSE
Z PROGRAM
Z FINE
VI
V
+ V
RESISTOR
80
V
REF
O
80
REF
VI
3 5
. If the over-
Z COARSE
.
V
flows into
8
N
64
REF
N
12
RET
4
13
VI
O
+ V
. Pin
PIN
www.ti.com
Z FINE
O
,
matched internal resistors determines a current gain of this
block. Note that the I
substrate potential.
EXCITATION CURRENT DACS AND R
Two matched adjustable reference current sources are avail-
able for sensor excitation. The defining equations are given
in Table III. Both current sources are controlled simulta-
neously by the coarse and fine DACs with a pedestal.
The external resistor R
into the reference current for the sensor excitation DACs.
The total current output of the DACs is split, producing two
references: I
match very closely over the full adjustment range without
mismatched differential steps. Both current reference out-
puts must be within the compliance range, i.e.: one reference
cannot be floated since it will change the value of the other
current source.
The recommended value of R
100 RTD sensors. This generates I
when both coarse and fine DACs are set to zero. The value
of the R
currents are required, i.e.: for 1000
sensor.
Similar to the Zero DACs, the outputs of the fine and coarse
DAC are summed together with the pedestal I
Each of the excitation DACs has 8-bit resolution (256 steps)
with 4-bit overlap between the coarse and the fine. This
TABLE III. Equations for Calculating the Values of Each
OVERALL
PROGRAM
COARSE DAC
FINE DAC
NOTE: N
respectively.
11
SET
and N
REF1
Reference Current.
resistor can be increased if lower reference
10
are the decimal values of registers 11 and 10,
I
and I
ZERO
CURRENT REFERRED TO I
I
REF1, 2
= I
SET
OUT
REF2
I
Z COARSE
I
I
Z PROGRAM
Z PROGRAM
Z FINE
= I
is used to convert the REF voltage
. Both of the current references
pin is always biased below the
REF PROGRAM
REFERENCE CURRENT
I
REF COARSE
I
REF FINE
I
REF PROGRAM
5
SET
8
V
5
+ I
R
8
V
REF
R
VI
175
REF
Z COARSE
VI
is 12.1k
REF1, 2
8
R
V
R
V
+ I
N
VI
REF
REF
SET
64
R
V
N
12
REF COARSE
REF
SET
4
13
RTD or a bridge
OUT
5
+ I
R
= 492 A currents
SET
V
SET
1024
REF
Z FINE
N
PIN
10
N
64
RESISTOR
REF PROGRAM
11
for use with
+ I
REF FINE
11
.