UTC7106

CMOS IC

10

QW-R502-018,B

40

39

38

TEST

CLOCK

INTERNAL TO PART

FIGURE 6A

40

39

38

RC OSCILLATOR

CLOCK

INTERNAL TO PART

FIGURE 6B

R

C

FIGURE 6. CLOCK CIRCUITS

COMPONENT VALUE SELECTION

Integrating Resistor

Both the buffer amplifier and the integrator have a class A output stage with 100μA of quiescent current. They

can supply 4μA of drive current with negligible nonlinearity. The integrating resistor should be large enough to

remain in this very linear region over the input voltage range, but small enough that undue leakage requirements are

not placed on the PC board. For 2V full scale, 470kΩ is near optimum and similarly a 47kΩ for a 200mV scale.

Integrating Capacitor

The integrating capacitor should be selected to give the maximum voltage swing that ensures tolerance buildup

will not saturate the integrator swing(approximately. 0.3V from either supply).In the UTC 7106, when the analog

COMMON is used as a reference, a nominal+2V fullscale integrator swing is fine. For three readings/second (48kHz

used, these values should be changed in inverse proportion to maintain the same output swing.

An additional requirement of the integrating capacitor is that it must have a low dielectric absorptiont to prevent

roll-over errors. While other types of capacitors are adequate for this application, polypropylene capacitors give

undetectable errors at reasonable cost.

Auto-Zero Capacitor

The size of the auto-zero capacitor has some influence on the noise of the system. For 200mV full scale where

noise is very important, a 0.47μF capacitor is recommended. On the 2V scale, a 0.047μF capacitor increases the

speed of recovery from overload and is adequate for noise on this scale.

Reference Capacitor

A 0.1μF capacitor gives good results in most applications. However, where a large common mode voltage exists

(i.e.,the REF LO pin is not at analog COMMON)and a 200mV scale is used, a larger value is required to prevent

roll-ovre error. Generally 1μF will hold the roll-over error to 0.5 count in this instance.

Oscillator Components

For all ranges of frequency a 91kΩ resistor is recommended and the capacitor is selected from the equation:

f= 0.45/RC For 48kHz Clock (3 Readings/sec), C=100pF.

Reference Voltage

.Thus, for the 200mV and 2V

a transducer, there will exist a scale factor other than unity between the input voltage and the digital reading. For

instance, in a weighing system, the designer might like to have a full scale reading when the voltage from the