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ICL7109

12-Bit, Microprocessor Compatible A/D Converter

Intersil 

ICL7109

Reference Voltage

The analog input required to generate a full scale output of

4096 counts is VIN = 2VREF . For normalized scale, a refer-

ence of 2.048V should be used for a 4.096V full scale, and

204.8mV should be used for a 0.4096V full scale. However,

in many applications where the A/D is sensing the output of

a transducer, there will exist a scale factor other than unity

between the absolute output voltage to be measured and a

desired digital output. 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 driving the

input down to 409.6mV, the input voltage should be mea-

sured directly and a reference voltage of 0.341V should be

used. Suitable values for integrating resistor and capacitor

are 33kΩ and 0.15µF. This avoids a divider on the input.

Another advantage of this system occurs when a zero read-

ing is desired for non-zero input. Temperature and weight

measurements with an offset or tare are examples. The off-

set may be introduced by connecting the voltage output of

the transducer between common and analog high, and the

offset voltage between common and analog low, observing

polarities carefully. However, in processor-based systems

using the ICL7109, it may be more efficient to perform this

type of scaling or tare subtraction digitally using software.

Reference Sources

The stability of the reference voltage is a major factor in the

overall absolute accuracy of the converter. The resolution of

the ICL7109 at 12 bits is one part in 4096, or 244ppm. Thus

if the reference has a temperature coefficient of 80ppm/oC

(onboard reference) a temperature difference of 3oC will

introduce a one-bit absolute error.

For this reason, it is recommended that an external high-

quality reference be used where the ambient temperature is

not controlled or where high-accuracy absolute measure-

ments are being made.

The ICL7109 provides a REFerence OUTput (Pin 29) which

may be used with a resistive divider to generate a suitable

reference voltage. This output will sink up to about 20mA

without significant variation in output voltage, and is provided

with a pullup bias device which sources about 10µA. The

output voltage is nominally 2.8V below V+, and has a tem-

perature coefficient of ±80ppm/oC (Typ). When using the

onboard reference, REF OUT (Pin 29) should be connected

to REF- (Pin 39), and REF+ should be connected to the

wiper of a precision potentiometer between REF OUT and

V+. The circuit for a 204.8mV reference is shown in the test

circuit. For a 2.048mV reference, the fixed resistor should be

removed, and a 25kΩ precision potentiometer between REF

OUT and V+ should be used.

Note that if Pins 29 and 39 are tied together and Pins 39 and

40 accidentally shorted (e.g., during testing), the reference

supply will sink enough current to destroy the device. This can

be avoided by placing a 1kΩ resistor in series with Pin 39.

Detailed Description

Digital Section

The digital section includes the clock oscillator and scaling

circuit, a 12-bit binary counter with output latches and TTL-

compatible three-state output drivers, polarity, over-range

and control logic, and UART handshake logic, as shown in

Figure 4.

Throughout this description, logic levels will be referred to as

“low” or “high”. The actual logic levels are defined in the Elec-

trical Specifications Table. For minimum power consumption,

all inputs should swing from GND (low) to V+ (high). Inputs

driven from TTL gates should have 3-5kΩ pullup resistors

INTEGRATOR

OUTPUT

INTERNAL CLOCK

AZ PHASE I

INTERNAL LATCH

STATUS OUTPUT

2048 COUNTS

MINIMUM

POLARITY

DETECTED

INT PHASE II

FIXED 2048

COUNTS

NUMBER OF COUNTS TO ZERO CROSSING

PROPORTIONAL TO VIN

ZERO CROSSING

OCCURS

ZERO CROSSING

DETECTED

DEINT PHASE III

AZ

4096 COUNTS

MAX

AFTER ZERO CROSSING

ANALOG SECTION WILL

BE IN AUTOZERO

CONFIGURATION

MODE Input

FIGURE 3. CONVERSION TIMING (RUN/HOLD PIN HIGH)

The MODE input is used to control the output mode of the

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