Datasheet
LTC2997
8
2997fa
OPERATION
The Block Diagram shows the main components of the
LTC2997.
The LTC2997 measures temperature using either a remote
or internal diode and provides a buffered voltage propor-
tional to absolute temperature (V
PTAT
) and a buffered 1.8V
reference voltage. Remote temperature measurements
usually use a diode connected transistor as a tempera-
ture sensor, allowing the remote sensor to be a discrete
NPN (ex. MMBT3904) or an embedded PNP device in a
microprocessor or FPGA.
Temperature measurements are conducted by measur-
ing the diode voltage at multiple test currents. The diode
equation can be solved for T, where T is degrees Kelvin,
I
S
is a process dependent factor on the order of 10
–13
A,
η is the diode ideality factor, k is the Boltzmann constant
and q is the electron charge:
T =
q
η•k
•
V
DIODE
ln
I
D
I
S
⎛
⎝
⎜
⎞
⎠
⎟
This equation has a relationship between temperature and
voltage, dependent on the process-dependent variable I
S
.
Measuring the same diode (with the same value I
S
) at two
different currents yields an expression which is independent
of I
S
. The value in the natural logarithm term becomes the
ratio of the two currents, which is process independent.
T =
q
η •k
•
V
DIODE2
–V
DIODE1
ln
I
D2
I
D1
⎛
⎝
⎜
⎞
⎠
⎟
Series Resistance Cancellation
Resistance in series with the remote diode causes a positive
temperature error by increasing the measured voltage at
each test current. The composite voltage equals:
V
DIODE
+ V
ERROR
=η
kT
q
•ln
I
D
I
S
⎛
⎝
⎜
⎞
⎠
⎟
+R
S
•I
D
where R
S
is the series resistance.
The LTC2997 removes this error term from the sensor
signal by subtracting a cancellation voltage (see Figure 1).
A resistance extraction circuit uses one additional current
(I
3
) to determine the series resistance in the measurement
path. Once the correct value of the resistor is determined
V
CANCEL
equals V
ERROR
. Now the temperature to voltage
converter's input signal is free from errors due to series
resistance and the sensor temperature can be determined
using currents I
1
and I
2
.
Figure 1. Series Resistance Cancellation
V
BE
V
BE
V
ERROR
R
SERIES
V
PTAT
V
CANCEL
= V
ERROR
I
3
I
1
, I
2
D
+
2997 F01
+
–
RESISTANCE
EXTRACTION
CIRCUIT
TEMPERATURE
TO VOLT
CONVERTER
D
–