Datasheet

LTC2991
12
2991fd
For more information www.linear.com/LTC2991
applicaTions inForMaTion
Compensated Kelvin temperature:
T
K _ COMP
=
η
CAL
η
ACT
• T
K _MEAS
(3)
Compensated Celsius temperature:
CCOMP
ACT
CAL
CMEAS
η
η
273.15 273.15
T
T= –+
_ _
( )
(4)
A 16-bit unsigned number is capable of representing the
ratio ηCAL/ηACT in a range of 0.00003 to 1.99997, by
multiplying the fractional ratio by 2
15
. The range of scal-
ing encompasses every conceivable target sensor value.
The ideality factor scaling granularity yields a worst-case
temperature error
of 0.01° at +125°C. Multiplying this
16-bit unsigned number and the measured Kelvin (un-
signed) temperature represented as a 16-bit number, yields
a 32-bit unsigned result. To scale this number back to a
13-bit temperature (9-bit integer part, and a 4-bit fractional
part), divide the number by 2
15
. Similarly, Celsius coded
temperature values can be scaled using 16-bit fixed-point
arithmetic, using Equation (4). In both cases, the scaled
result will have a 9-bit integer (d[12:4]) and the four LSB’s
(d[3:0]) representing the 4-bit fractional part. To convert
the corrected result to decimal, divide the final result by
24, or 16, as you would the register contents. If ideality
factor scaling is implemented in the target application, it
is beneficial to configure the LTC2991 for Kelvin coded
results to limit the number of math operations required
in the target processor.
T
T
(UNSIGNED)
KCOMP
ACT
CAL
KMEAS
_
_
=






η
η
2
2
15
15
(5)
(6)
Sampling Currents
Single-ended voltage measurements are directly sampled
by the internal ADC. The average ADC input current is a
function of the input applied voltage as follows:
I
SAMPLE
= (V
IN
– 1.49V) • 0.17[µA/V]
Inputs with source resistance less than 500Ω will yield
full-scale gain errors due to source impedance of < ½ LSB
for 14-bit conversions. The nominal conversion time is
1.5ms for single-ended conversions.
Current Measurements
The LTC2991 has the ability to perform 14-bit current
measurements with the addition of a current sense resis
-
tor (see Figure 3).
T
(UNSIGNED)
T
CCOMP
ACT
CAL
CMEAS
_
_
=






+
(
η
η
2 273.15 •2
15 4
)
2
273 15 2
15
4
– .•
Figure 3. Simplified Current Sense Schematic
In order to achieve 13-bit current sensing a few details
must be considered. Differential voltage or current mea-
surements are directly sampled by the internal ADC. The
average ADC input current for each leg of the differential
input signal during a conversion is:
I
SAMPLE
= (V
IN
– 1.49V) • 0.34[µA/V]
The maximum source impedance to yield 14-bit results
with ½ LSB full-scale error is ~50Ω.
In order to achieve 14-bit accuracy, 4-point, or Kelvin
connected measurements of the sense resistor differential
voltage are necessary.
V1 V2
LTC2991
0V – V
CC
R
SENSE
I
LOAD
2991 F03