ICL7106, ICL7107, ICL7107S

10

FN3082.9

October 24, 2014

Submit Document Feedback

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

ICL7106 or the ICL7107, when the analog COMMON is used as a

reference, a nominal +2V full scale integrator swing is fine. For

the ICL7107 with +5V supplies and analog COMMON tied to

supply ground, a ±3.5V to +4V swing is nominal. For three

0.22µF and 0.10µF, respectively. Of course, if different oscillator

frequencies are 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 absorption 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 rollover error. Generally

1µF will hold the rollover error to 0.5 count in this instance.

Oscillator Components

For all ranges of frequency a 100kΩ resistor is recommended and

the capacitor is selected as shown in Equation 2:

Reference Voltage

The analog input required to generate full scale output (2000

should equal 100mV and 1V, respectively. However, in many

applications where the A/D is connected to 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 transducer is 0.662V. Instead of dividing the

input down to 200mV, the designer should use the input voltage

resistor and capacitor would be 120kΩ and 0.22µF. This makes

the system slightly quieter and also avoids a divider network on

the input. The ICL7107 with ±5V supplies can accept input

signals up to ±4V. Another advantage of this system occurs when

weighing systems with a variable fare are examples. This offset

reading can be conveniently generated by connecting the voltage

transducer between IN HI and COMMON and the variable (or

fixed) offset voltage between COMMON and IN LO.

ICL7107 Power Supplies

The ICL7107 is designed to work from ±5V supplies. However, if a

negative supply is not available, it can be generated from the

clock output with 2 diodes, 2 capacitors, and an inexpensive lC.

Figure 10 shows this application. See the ICL7660 datasheet for

an alternative.

In fact, in selected applications no negative supply is required.

The conditions to use a single +5V supply are:

1. The input signal can be referenced to the center of the

common mode range of the converter.

2. The signal is less than ±1.5V.

3. An external reference is used.

f

0.45

RC

----------- For 48kHz Clock (3 Readings/sec),

=

C

100pF.

=

(EQ. 2)

ICL7107

V+

OSC 1

V-

OSC 2

OSC 3

GND

V+

V- = 3.3V

0.047

µF

10µF

+

-

1N914

1N914

CD4009

FIGURE 10. GENERATING NEGATIVE SUPPLY FROM +5V