LTC2990CMSTRPBF datasheet(10/24 Pages) LINER

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LTC2990

2990f

applicaTions inForMaTion

target application, it is beneficial to configure the LTC2990

for Kelvin coded results to limit the number of math opera-

tions required in the target processor.

Unsigned

K COMP

ACT

CAL

K MEAS

(

)









(5)

Unsigned

C COMP

ACT

CAL

C MEAS

(

)









273

115 2

273 15 2

•

–

. •

(

)

(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 – 1.49) • 0.17µA

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 de-

tails must be considered. Differential voltage or current

measurementsaredirectlysampledbytheinternalADC.The

average ADC input current for each leg of the differential

input signal during a conversion is (VIN – 1.49) • 0.34µA.

The maximum source impedance to yield 14-bit results

with, 1/2LSB full-scale error is ~50Ω. In order to achieve

highaccuracy,4-point,orKelvinconnectedmeasurements

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 differential

voltage is 0.300V. The external sense resistance is then a

functionofthemaximummeasurablecurrent,orREXT_MAX

= 0.300/IMAX. For example, if you wanted to measure a

current range of ±5A, the external shunt resistance would

equal 0.300/5 = 60mΩ.

Thereexistsawaytoimprovethesenseresistor’sprecision

usingtheLTC2990.TheLTC2990measuresbothdifferential

voltage and remote temperature. It is therefore, possible

to compensate for the absolute resistance tolerance 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 be-

low 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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