M7240

CMOS IC

UNISONICTECHNOLOGIESCO.,LTD

6 of 7

www.unisonic.com.tw

QW-R502-682.a

APPLICATION INFORMATION

The M7240 is the production of the basic application of LCD digital DC voltage meter and digital multimeter, by

selecting different parameters of the external components, you can create different range gauge outfit. The 200mV

range gauge outfit which the most widely used because it is three and a half the basic components of digital

multimeter. As M7240 of 3V low-voltage power supply, making use M7240 production of the digital DC voltage meter

input signal can be a total of COM and common GND two access methods, Figure 1 is the 200mv typical application

circuit.

In Fig.1 circuit, when moving the jumper J is connected to INLO and COM, the composition of the input signal

connected in COM, when moving the jumper J is connected to INLO and GND, the composition of the input signal

connected in GND, for the production of digital multimeter, two input signal connection method in general to meet the

application requirements. Use the COM method to make the gauge outfit digital multimeter, the circuit is relatively

simple form, but because of COM client can tolerate a few mA maximum current, the need for greater measurement

poured into the current project, not applicable. Use the GND method to make the gauge outfit produced digital

multimeter, there is no current problem of affordability, but measurement converter circuit is slightly changed.

In addition, if you live in the city to use electricity for 60HZ, in order to reduce the frequency of the circuit city

electrical effects, the connection between the RX and power resistors, please use 680KΩ.

ANALOG COMMON

The COM pin is used to set the common-mod voltage for the system in which the input signals are floating with

respect to the power supply of the M7240. In most of the applications, INLO, RFLO and COM pins are tied to the

same point, so that the common mode voltage can be removed from the reference system and the converter.

The COM pin is also used as a voltage reference. It sets a voltage of around 2.5 volts more negative than the

positive supply.

The analog COM is tied internally to an NMOS capable of sinking 30mA. This NMOS will hold the COM voltage

at 2.5 volts when an external load attempts to pull the COM voltage toward the positive supply.

The source current of COM is only 10μA, so it is easy to pull COM voltage to a more negative voltage with

respect to the positive supply.

REFERENCE VOLTAGE

For a 1000 counts reading, the input signal must be equal to the reference voltage. As a result, it requires the

input signal be twice the reference voltage for a 2000 counts full-scale reading. Thus, for the 200.0mV and 2.000V

full-scale, the reference voltage should equal 100.0mV and 1.000V, In some applications the full-scale input voltage

my be other than 200mV or 2V, but 600mV. For example, the reference voltage should be set to 300mV and the

input signal can be used directly without being divided.

The differential reference can be used during the measurement of resistor by the ratio metric method and when

a digital reading of zero is desired for Vin≠0. A compensating offset voltage can be applied between COM and INLO

and the voltage of being measured is connected between COM and INHI.

AUTO-ZERO PHASE

During auto-zero three things happen. First, input high and low are disconnected from the pins and internally

shorted to analog COM. Second, the reference capacitor is charged to the reference voltage. Third, a feedback loop

amplifier, integrator, and comparator. Since the comparator is included in the loop, the AZ accuracy is limited only by

the noise of the system. In any case, the offset referred to the input is less than 10μV.

SIGNAL INTEGRATE PHASE

During signal integrate phase, the auto-zero loop is opened, the internal short is removed, and the internal input

high and low are connected to the external pins. The converter then integrates the differential voltage between INHI

and INLO for a fixed time. This differential voltage can be within a wide common mode range: up to 1V from either

supply. if, on the other hand, the input signal has no return with respect to the converter power supply, INLO can be

tied to analog COM to establish the correct common mode voltage. At the end of this phase, the polarity of the

integrated signal is determined.