LTC2986 datasheet(11/26 Pages) AD

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LTC2990

Rev. F

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APPLICATIONS INFORMATION

temperature error of 0.01° at 125°C. Multiplying this

16-bit unsigned number and the measured Kelvin

(unsigned) 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 per Equation (5).

Similarly, Celsius coded temperature values can be scaled

using 16-bit fixed-point arithmetic, using Equation (6).

In both cases, the scaled result will have a 9-bit integer

(d[12:4]) and the 4LSBs (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 reg-

ister contents. If ideality factor scaling is implemented

in the target application, it is beneficial to configure the

LTC2990 for Kelvin coded results to limit the number of

math operations required in the target processor.

TK _COMP =

Unsigned

(

)

ηCAL

ηACT

215

⎛

⎝

⎜

⎞

⎠

⎟TK _MEAS

215

(5)

TC_COMP =

Unsigned

(

)

ηCAL

ηACT

215

⎛

⎝

⎜

⎞

⎠

⎟ TC _MEAS + 273.15 • 2

(

)

215

– 273.15 • 24

(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:

IIN(AVG) = (VIN – 1.49V) • 0.17[µA/V]

Inputs with source resistance less than 200Ω will yield

full-scale gain errors due to source impedance of <1/2LSB

for 14-bit conversions. The nominal conversion time is

1.5ms for single-ended conversions.

Current Measurements

The LTC2990 has the ability to perform 14-bit current

measurements with the addition of a current sense resis-

tor (see Figure 3).

In order to achieve accurate current sensing a few details

must be considered. Differential voltage or current

measurements are directly sampled by the internal ADC.

The average ADC input current for each leg of the differ-

ential input signal during a conversion is (VIN – 1.49V)

• 0.34[µA/V]. The maximum source impedance to yield

14-bit results with, 1/2LSB full-scale error is ~50Ω. In

order to achieve high accuracy 4-point, or Kelvin con-

nected 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 LTC2990. The full-scale dif-

ferential 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 current range of ±5A, the external shunt

resistance would equal 0.300V/5A = 60mΩ.

There exists a way to improve the sense resistor’s preci-

sion using the LTC2990. The LTC2990 measures both dif-

ferential voltage and remote temperature. It is therefore,

possible to compensate for the absolute resistance toler-

ance of the sense resistor and the temperature coefficient

of the sense resistor in software. The resistance would be

measured by running a calibrated test current through the

discrete resistor. The LTC2990 would measure both the

differential voltage across this resistor and the resistor

temperature. From this measurement, RO and TO in the

equation below would be known. Using the two equations,

the host microprocessor could compensate for both the

absolute tolerance and the TCR.

RT = RO • [1 + α(T – TO)]

where:

α = +3930 ppm/°C for copper trace

α = ±2 to ~+200ppm/°C for discrete R

(7)

I = (V1 – V2)/RT

(8)

Figure 3. Simplified Current Sense Schematic

LTC2990

0V – VCC

RSENSE

ILOAD

2990 F03

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