TDC-GP2 acam messelectronic gmbh, TDC-GP2 Datasheet - Page 29

• Manufacturer Part Number: TDC-GP2
• Description: Precision Time Interval Measurement
• Manufacturer: acam messelectronic gmbh

5.3 Temperature Measurement
Especially for heat meter applications the TDC-GP2 has
a PicoStrain based temperature measuring unit that
offers high resolution and very low current
consumption.
The measurement is based on measuring discharge
times. Therefore a capacitor is discharged alternately
through the sense resistors and the reference
resistors.
The unit has 4 resistor ports with the following
function:
PT1
PT2
PT3
PT4
The temperature sensor should have a minimum resis-
tance of 500 Ohm. The TDC-GP2 measures the dis-
V
t rig
d o w n
V
V
lo a d
lo a d
C y c le tim e = 1 2 8 u s o r 5 1 2 u s (@ 4 M H z )
reference resistor lower temperature
sense resistor lower temperature
sense resistor lower temperature
reference resistor higher temperature
2 x P T 5 0 0 / P T 1 0 0 0
acam-messelectronic gmbh - Am Hasenbiel 27 - D-76297 Stutensee-Blankenloch - Germany - www.acam.de
2 fa k e + 4 p o rt m e a s u r e m e n ts
D is c h a rg e tim e
~ 0 .7
u p
x
P T 1
Figure 17
Figure 18
R
x
R
C , e .g . 1 5 0 µ s
R
re fd o w n
re fu p
P T 2
1 5 0 n F
P T 1
P T 2
P T 3
P T 4
P T 3
L o a d T
7 4 H C 1 G 1 4
P T 4
T D C -G P 2
S e n s e T
29
charge times of the RC-networks made of each resistor
and the capacitor.
The precision of the temperature measurement is
about 0.004°C several times better than needed for
heat meters.
The temperature measurement is fully automated. It is
triggered by the µC sending the opcode "Start_Temp".
The TDC-GP2 controls the 4 measurements by itself.
After the 4 measurements have finished the interrupt
flag is set. The four data are found in registers 0 to 3.
From Res_2/RES_1 and RES_3/RES_4 the
microcontroller can calculate the ratio Rtemp/Rref. By
means of a look-up table it can calculate the
temperature for the special type of sensor in use.
It is not possible with TDC-GP2 to use 4-wire
temperature sensors.
Configuration
Register 0, Bit 8, Tcycle sets the cycle time for the
temperature measurement.
Tcycle = 0
Tcycle = 1
Register 0, Bit9, Port# sets the number of ports that
will be used.
Port# = 0
Port# = 1
Register 0, Bit 7, Fake# sets the number of dummy
measurements at the beginning of a temperature
measurement. This is necessary to overcome
mechanical effects of the load capacitor.
Fake# = 0
Fake# = 1
A full temperature measurement last for
2 x (# of ports) + (# of fakes) cycles.
Recommended Capacitor Values
The discharge time should be about 150 µs. Therefore
the capacitor should have the following value:
PT500:
PT1000:
Please set Tcycle = 1 to avoid Timeout Error.
Recommended Capacitor Type
To get accurate results we recommend capacitor types
with very low dC/dU. We recommend:
CfCap Series from Tayo Yuden
128 µs cycle time @ 4MHz
512 µs cycle time @ 4MHz
2 ports
4 ports
2 dummy measurements
8 dummy measurements
220 nF
100 nF
5.3 Tem perature Measurem ent
1 sensor
2 sensors
TDC-GP2