M7240

CMOS IC

UNISONICTECHNOLOGIESCO.,LTD

7 of 7

www.unisonic.com.tw

QW-R502-682.a

APPLICATION INFORMATION(Cont.)

DIFFERENTIAL INPUT

The input can accept differential voltages anywhere within the common mode range of the input amplifier, or

specifically from 0.5V below the positive supply to 1V above the negative supply. In this range, the system has a

CMRR of 86dB typical. However, care must be exercised to assure the integrator output does not saturate. A worst

case condition would be a large positive common mode voltage with a near full scale negative differential input

voltage. The negative input signal drives the integrator positive when most of its swing has been used up by the

positive common mode voltage. For these critical applications the integrator output swing can be reduced to less

than the recommended 2V full scale swing with little loss of accuracy. The integrator output can swing to within 0.3V

of either supply without loss of linearity.

REFERENCE VOLTAGE CAPACITOR

The reference voltage can be generated anywhere within the power supply voltage of the converter. The main

source of common mode error is a roll-over voltage caused by the reference capacitor losing or gaining charge to

stray capacity on its nodes. If there is a large common mode voltage, the reference capacitor can gain charge

(increase voltage) when called up to de-integrate a positive signal but lose charge (decrease voltage) when called up

to de-integrate a negative input signal. This difference in reference for positive or negative input voltage will give a

roll-over error. However, by selecting the reference capacitor such that it is large enough in comparison to the stray

capacitance, this error can be held to less than 0.5 count worst case.

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 100kΩ 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 M7240, when the analog

COMMON is used as a reference, a nominal+2V full scale 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

and a 200mV scale is used, a larger value is required to prevent roll-over error. Generally 1μF will hold the roll-over

error to 0.5 count in this instance.

UTC assumes no responsibility for equipment failures that result from using products at values that

exceed, even momentarily, rated values (such as maximum ratings, operating condition ranges, or

other parameters) listed in products specifications of any and all UTC products described or contained

herein. UTC products are not designed for use in life support appliances, devices or systems where

malfunction of these products can be reasonably expected to result in personal injury. Reproduction in

whole or in part is prohibited without the prior written consent of the copyright owner. The information

presented in this document does not form part of any quotation or contract, is believed to be accurate

and reliable and may be changed without notice.