ICL7126 View Datasheet(PDF) - Renesas Electronics

Part Name

Description

Manufacturer

ICL7126

3 1/2 Digit, Low Power, Single Chip A/D Converter

Renesas Electronics 

ICL7216

Component Value Selection

Integrating Resistor

Both the buffer amplifier and the integrator have a class A

output stage with 6A of quiescent current. They can supply

~1A 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 is near optimum and

similarly a 180k 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 three

readings/second (48kHz clock) nominal values for ClNT are

0.047F, for 1/s (16kHz) 0.15F. 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 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F capacitor is recommended. On the 2V

scale, a 0.33F capacitor increases the speed of recovery from

overload and is adequate for noise on this scale.

Reference Capacitor

A 0.1F 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F 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

f  0--R--.--4-C--5--  For 48kHz clock (3 readings/sec), R = 180k

Reference Voltage

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

counts) is: VlN = 2VREF. Thus, for the 200mV and 2V scale,

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

many applications where the A/D is connected to a transducer,

FN3084 Rev.5.00

Oct 25, 2004

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 input voltage

directly and select VREF = 0.341V. Suitable values for integrating

resistor 330k. 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 is desired for VIN 

0. Temperature and 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.

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

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

AN052

“Building a Battery-Operated Auto Ranging DVM with

the ICL7106”

“Tips for Using Single-Chip 31/2 Digit A/D Converters”

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