Datasheet

Temperature Measurement Operations
A temperature measurement is a time measurement of
the RC circuit connected to the temperature port device
pins T1 through T4 and TC. The TC device pin has a
driver to charge the timing capacitor. The ports that are
measured and the order in which the measurement is
performed is selected with the TP[1:0] bits in the Event
Timing 2 register.
Figure 6 depicts a 1000Ω platinum RTD with a 100nF
NPO COG 30ppm/°C capacitor. It shows two dummy
cycles with 4 temperature port evaluation measurements
and 4 real temperature port measurements. This occurs
when setting the TP[1:0] bits in the Event Timing 2 regis-
ter to 11b.
The dummy 1 and dummy 2 cycles represent preamble
measurements that are intended to eliminate the dielec-
tric absorption of the temperature measurement capaci-
tor. These dummy cycles are executed using a RTD
Emulation resistor of 1000Ω internal to the MAX35101.
This dummy path allows the dielectric absorption effects
of the capacitor to be eliminated without causing any of
the RTDs to be unduly self-heated. The number of dummy
measurements to be taken ranges from 0 to 7. This
parameter is configured by setting the PRECYC[2:0] bits
in the Event Timing 2 register.
Following the dummy cycles, an evaluation, TXevaluate,
is performed. This measurement allows the MAX35101
to maximize power efficiency by evaluating the tempera-
ture of the RTDs with a coarse measurement prior to a
real measurement. The coarse measurement provides
an approximation to the TDC converter. During the real
measurement, the TDC can then optimize its measure-
ment parameters to use power efficiently. These evaluate
cycles are automatically inserted according to the order of
ports selected with the of the Temperature Port bits. The
time from the start of one port’s temperature measure-
ment to the next port’s temperature measurement is set
using with the PORTCYC[1:0] bits in the Event Timing 2
register.
Once all the temperature measurements are completed,
the times measured for each port are reported in the cor-
responding TxInt and TxFrac Results registers. The TE bit
in the Interrupt Status register is also set and the INT pin
asserts (if enabled).
Actual temperature is determined by a ratiometric calcu-
lation. If T1 and T2 are connected to platinum RTDs and
T3 and T4 are connected to the same reference resistor
(as shown in the System Diagram), then the ratio of T1/
T3 = R
RTD1
/R
REF
and T2/T4 = R
RTD2
/R
REF
. The ratios
R
RTD1
/R
REF
and R
RTD2
/R
REF
can be determined by the
host microprocessor and the temperature can be derived
from a look-up table of Temperature vs. Resistance for
each of the RTDs utilizing interpolation of table entries if
required.
Figure 6. Temperature Command Execution Cycle Example
DUMMY1 DUMMY2 T1
EVALUATE
T3
EVALUATE
T2
EVALUATE
T4
EVALUATE
T1 T3 T2 T4
0.5
1.0
1.5
2.0
2.5
3.0
3.5
0
128 256 384 512 640 768 896 1,024 1,152 1,280 1,408
PORTCYCLE TIME
(PORTCYC1-PORTCYC0)
SET TO “00” 128µs
DRIVER TO CHANGE
TC-CONNECTED CAPACITOR
VTC
VOLTS (V)
V
TC
TIME (µs)
Figure 6
MAX35101 Time-to-Digital Converter with Analog Front-End
www.maximintegrated.com
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