LTC2991 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTC2991' PDF : 32 Pages View PDF
LTC2991
APPLICATIONS INFORMATION
Compensated Kelvin temperature:
Sampling Currents
TK _ COMP
=
ηCAL
ηACT
• TK _ MEAS
(3)
Compensated Celsius temperature:
( ) TC_COMP =
ηCAL
ηACT
TC_MEAS + 273.15 – 273.15 (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 215. 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 215. 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.
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:
ISAMPLE = (VIN – 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).
0V – VCC
RSENSE
V1
V2
LTC2991
ILOAD
2991 F03
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:
TK _COMP
=
(UNSIGNED)
ηCAL
ηACT
215
215
TK _MEAS
(5)
( ) TC
_
COMP
=
(UNSIGNED)
ηCAL
ηACT
215
215
TC_MEAS +273.15 • 24
– 273.15 • 24
(6)
ISAMPLE = (VIN – 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.
In the case of current measurements, the external sense
resistor is typically small, and determined by the full-scale
input voltage of the LTC2991. The full-scale differential
voltage is 0.300V. The external sense resistance, is then a
function of the maximum measurable current, or REXT_MAX
= 0.300V/IMAX. For example, if you wanted to measure a
2991ff
12
For more information www.linear.com/LTC2991
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