AD7714* 3 V/5 V, CMOS, 500 µA Signal Conditioning ADC - dreamm
AD7714* 3 V/5 V, CMOS, 500 µA Signal Conditioning ADC - dreamm
AD7714* 3 V/5 V, CMOS, 500 µA Signal Conditioning ADC - dreamm
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AD7714<br />
Data Acquisition<br />
The AD7714 with its three differential channels (or five pseudodifferential<br />
channels) is suited to low bandwidth, high resolution<br />
data acquisition systems. In addition, the three-wire digital<br />
interface allows this data acquisition front end to be isolated<br />
with just three optoisolators. The entire system can be operated<br />
+5V<br />
from a single +3 V or +5 V supply provided that the input signals<br />
to the AD7714’s analog inputs are all of positive polarity.<br />
The low power operation of the AD7714 ensures that very little<br />
power has to be brought across the isolation barrier. Figure 15<br />
shows the AD7714 in an isolated data acquisition system.<br />
AV DD<br />
DV DD<br />
AV DD<br />
1µA<br />
AD7714<br />
IN1+<br />
AIN1<br />
IN1–<br />
AIN2<br />
AIN3<br />
AUTO-ZEROED<br />
IN2+<br />
∑∆<br />
AIN4<br />
PGA<br />
IN2–<br />
BUFFER<br />
MODULATOR<br />
+V IN<br />
AGND<br />
REGISTER BANK<br />
IN3+<br />
AIN5<br />
IN3–<br />
AIN6<br />
A = 1–128<br />
+5V<br />
1µA<br />
SERIAL INTERFACE<br />
AD780<br />
GND<br />
V OUT<br />
REF IN (+)<br />
REF IN (–)<br />
AGND<br />
SWITCHING<br />
MATRIX<br />
CHARGING BALANCING A/D<br />
CONVERTER<br />
DIGITAL<br />
FILTER<br />
CLOCK<br />
GENERATION<br />
STANDBY<br />
SYNC<br />
MCLK IN<br />
MCLK OUT<br />
RESET<br />
DRDY<br />
POL<br />
DGND BUFFER DOUT DIN CS SCLK<br />
DV DD<br />
MICROCONTROLLER<br />
OPTO-ISOLATORS<br />
Figure 15. Data Acquisition System Using the AD7714<br />
ISOLATION<br />
BARRIER<br />
MAIN TRANSMITTER ASSEMBLY<br />
ISOLATED SUPPLY<br />
3V<br />
VOLTAGE<br />
REGULATOR<br />
DV DD AV DD<br />
REF IN<br />
VOLTAGE<br />
REFERENCE<br />
V CC<br />
VOLTAGE<br />
REFERENCE<br />
SENSORS<br />
RTD<br />
mV<br />
ohm<br />
TC<br />
AMBIENT<br />
TEMP.<br />
SENSOR<br />
AD7714<br />
MCLK<br />
IN<br />
MCLK<br />
OUT<br />
MICROCONTROLLER UNIT<br />
*PID<br />
*RANGE SETTING<br />
*CALIBRATION<br />
*LINEARIZATION<br />
*OUTPUT CONTROL<br />
*SERIAL COMMUNICATION<br />
*HART PROTOCOL<br />
COM<br />
3V<br />
D/A<br />
CONVERTER<br />
HART<br />
MODEM<br />
BELL 202<br />
SIGNAL<br />
CONDITIONER<br />
WAVEFORM<br />
SHAPER<br />
INPUT/OUTPUT<br />
STAGE<br />
BANDPASS<br />
FILTER<br />
LOOP<br />
RTN<br />
4–20mA<br />
DGND<br />
AGND<br />
COM<br />
ISOLATED GROUND<br />
Smart Transmitters<br />
Another area where the low power, single supply, three-wire<br />
interface capabilities is of benefit is in smart transmitters. Here,<br />
the entire smart transmitter must operate from the 4 mA to<br />
20 mA loop. Tolerances in the loop mean that the amount of<br />
current available to power the transmitter is as low as 3.5 mA.<br />
Figure 16. Smart Transmitter Using the AD7714<br />
–34–<br />
The AD7714 consumes only <strong>500</strong> µA, leaving 3 mA available for<br />
the rest of the transmitter. Figure 16 shows a block diagram of a<br />
smart transmitter which includes the AD7714. Not shown in<br />
Figure 16 is the isolated power source required to power the<br />
front end.<br />
REV. B