10

FN3084.5

Component Value Selection

Integrating Resistor

Both the buffer amplifier and the integrator have a class A

output stage with 6

µA of quiescent current. They can

supply ~1

µA 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, 1.8M

Ω is near

optimum and similarly a 180k

Ω for a 200mV scale.

Integrating Capacitor

The integrating capacitor should be selected to give the

maximum voltage swing that ensures tolerance build-up will

not saturate the integrator swing (approximately. 0.3V from

either supply). When the analog COMMON is used as a

reference, a nominal

±2V full-scale integrator swing is fine. For

are 0.047

µF, for 1/s (16kHz) 0.15µF. Of course, if different

oscillator frequencies are used, these values should be

changed in inverse proportion to maintain the same

output swing.

The integrating capacitor should have a low dielectric

absorption to prevent roll-over errors. While other types may

be adequate for this application, polypropylene capacitors

give undetectable errors at reasonable cost.

At three readings/sec, a 750

Ω resistor should be placed in

series with the integrating capacitor, to compensate for

comparator delay.

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 0.32

µF capacitor is recommended. On the

2V scale, a 0.33

µF capacitor increases the speed of

recovery from overload and is adequate for noise on this

scale.

Reference Capacitor

A 0.1

µF 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-over

error. Generally 1

µF will hold the roll-over error to 0.5 count

in this instance.

Oscillator Components

For all ranges of frequency a 50pF capacitor is recommended

and the resistor is selected from the approximation equation

Reference Voltage

The analog input required to generate full-scale output (2000

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 0.682V. Instead of

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

for integrating resistor 330k

Ω. This makes the system slightly

quieter and also avoids a divider network on the input. Another

advantage of this system occurs when a digital reading of zero

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.

Typical Applications

The ICL7126 may be used in a wide variety of

configurations. The circuits which follow show some of the

possibilities, and serve to illustrate the exceptional versatility

of these A/D converters.

The following application notes contain very useful

information on understanding and applying this part and are

available from Intersil Corporation.

Application Notes

f

0.45

RC

-----------

For 48kHz clock (3 readings/sec), R = 180k

Ω

•

∼

NOTE #

DESCRIPTION

AN016

“Selecting A/D Converters”

AN017

“The Integrating A/D Converter”

AN018

“Do’s and Don’ts of Applying A/D Converters”

AN023

“Low Cost Digital Panel Meter Designs”

AN032

“Understanding the Auto-Zero and Common Mode

Performance of the ICL7136/7/9 Family”

AN046

“Building a Battery-Operated Auto Ranging DVM with

the ICL7106”

AN052

ICL7126