AL Tech 8CH Manual de usuario descargar pdf (Pagina 29)

LTC2424/LTC2428

than 60% full scale, and only marginally increases noise

approaching full scale. Even lower power references can

be used if only the lower end of the LTC2424/LTC2428

input range is required.

2.051MHz Oscillator for 100sps Output Ratio

The oscillator circuit shown in Figure 31 can be used to

drive the F

pin, boosting the conversion rate of the

LTC2420 for applications that do not require a notch at 50

or 60Hz. This oscillator is not sensitive to hysteresis

voltage of a Schmitt trigger device as are simpler

relaxation oscillators using the 74HC14 or similar de-

vices. The circuit can be tuned over a 3-1 range with only

one resistor and can be gated. The use of transmission

gates could be used to shift the frequency in order to

provide setable conversion rates.

Pseudodifferential Multichannel Bridge Digitizer and

Digital Cold Junction Compensation

The circuit shown in Figure 32 enables pseudodifferential

measurements of several bridge transducers and abso-

lute temperature measurement.

APPLICATIONS INFORMATION

WUU

Consecutive readings are performed on each side of the

bridge by selecting the appropriate channel on the

LTC2428. Each output is digitized and the results digitally

subtracted to obtain the pseudodifferential result. Several

bridge transducers may be digitized in this manner.

In order to measure absolute temperature with a thermo-

couple, cold junction compensation must be performed.

Channel 6 measures the output of the thermocouple while

channel 7 measures the output of the cold junction sensor

(diode, thermistor, etc.). This enables digital cold junction

compensation of the thermocouple output. The tempera-

ture measurement may then be used to compensate the

temperature effects of the bridge transducers.

THERMOCOUPLE

THERMISTOR

24248 F32

CH0

CH1

CH2

CH3

CH4

CH5

CH6

CH7

8-CHANNEL

MUX

5ZS

SET

GND

1, 6, 16, 18, 22, 27, 28

CSADC

CSMUX

SCK

CLK

SDO

LTC2428

ADCINMUXOUT

7 4 3 2, 8

1µF

SET

20-BIT

∆Σ ADC

–

Figure 32. Pseudodifferential Multichannel Bridge Digitizer and Digital Cold Junction Compensation

LT1461-2.5

GND

24248 F30

10µF

16V

TANT

LTC2424/LTC2428

SET

OUT5V

0.1µF

CER

Figure 30. Low Power Reference

U1-A U1-B U1-C

U1-F

U1-E

TO LTC2424/

LTC2428

U1-D

5pF

2N3904

10k

47k

10pF

100smps, F

= 2.048MHz

30smps, F

= 614.4kHz

U1: 74HC14 OR EQUIVALENT

270pF

345 6

12 13

HALT

11 10

9 8

24248 F31

Figure 31. 2.051MHz Oscillator for 100sps Output Rate

1 2 ... 24 25 26 27 28 29 30 31 32 33 34 35 36

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