LTC2412

33

2412f

APPLICATIO S I FOR ATIO

Figure 38. Input Normal Mode Rejection vs Input Frequency with Input Perturbation of 100% Full Scale (50Hz Notch)

of full scale. In many industrial applications, it is not un-

common to have to measure microvolt level signals super-

imposed over volt level perturbations and LTC2412 is

eminently suited for such tasks. When the perturbation is

differential, the specification of interest is the normal mode

rejection for large input signal levels. With a reference

tial input range of 5V peak-to-peak. Figures 39 and 40 show

measurement results for the LTC2412 normal mode rejec-

tion ratio with a 7.5V peak-to-peak (150% of full scale)

input signal superimposed over the more traditional nor-

mal mode rejection ratio results obtained with a 5V peak-

to-peak (full scale) input signal. In Figure 39, the LTC2412

= LOW) and in Figure 40 it uses the internal oscillator with

rejection performance is maintained with no compromises

in this extreme situation. When operating with large input

signal levels, the user must observe that such signals do

not violate the device absolute maximum ratings.

Measuring Barometric Pressure and Temperature

with a Single Sensor

Figure 41 shows the LTC2412 measuring both tempera-

ture and pressure from an Intersema model MS5401-BM

absolute pressure sensor. The bridge has a nominal

impedance of 3.4k, a temperature coefficient of resistance

of 2900ppm/

°C and a temperature coefficient of span of

–1900ppm/

°C. R1 provides first order temperature com-

pensation of the output span by causing the bridge voltage

to increase by 1900ppm/

°C, offsetting the –1900ppm/°C

TC of span. R1 should have a much smaller TC than that

of the bridge resistance; 50ppm/

°C or less is satisfactory.

In addition to compensating the bridge output span, this

circuit also provides a convenient way to measure ambient

temperature. Channel 1 of the LTC2412 measures the

bridge excitation voltage, which has a slope of approxi-

mately 3.2mV/

°C. Channel 0 measures the bridge output,

which has a slope of 50mV/bar. The temperature reading

can also be used for second order compensation of the

pressure reading.

INPUT FREQUENCY (Hz)

0

12.5

25

37.5

50

62.5

75

87.5 100 112.5 125 137.5 150 162.5 175 187.5 200

2412 F38

0

–20

–40

–60

–80

–100

–120

MEASURED DATA

CALCULATED DATA