ICL7129 - 4½-Digit Analog-to-Digital Converter with LCD Driver

Overview

The Harris ICL7129 is a high-precision 4½-digit analog-to-digital converter (ADC) with an integrated LCD driver. This CMOS integrated circuit is designed for applications requiring high-resolution measurements with minimal external components. The ICL7129 combines exceptional accuracy with low power consumption, making it particularly suitable for portable and battery-operated instruments.

With its ability to directly drive a multiplexed liquid crystal display, the ICL7129 simplifies system design while providing superior performance. The device requires only a few passive components and a reference voltage to create a complete measurement solution, offering designers an efficient path to implementing high-resolution digital measurement systems.

Key Features

• 4½-digit resolution (20,000 counts) for high-precision measurements
• Direct LCD drive capability (up to 4½ digits plus signs)
• Low power consumption (typically 1mW) ideal for battery-powered applications
• True differential input with high input impedance (>10¹²Ω)
• On-chip voltage reference (typical 3.05V)
• Auto-zero cycle for offset correction
• Built-in polarity and overrange/underrange indicators
• Single power supply operation (typically 9V)
• Low noise design with excellent temperature stability
• CMOS technology for high reliability and low power

Applications

The ICL7129's combination of high resolution, display drive capability, and low power consumption makes it suitable for a wide range of measurement applications:

Test and Measurement Equipment

• Digital multimeters
• Precision voltmeters
• Panel meters
• Data acquisition systems

Portable Instruments

• Handheld measurement devices
• Battery-powered test equipment
• Field measurement instruments

Industrial Systems

• Process control instrumentation
• Sensor signal conditioning
• Environmental monitoring equipment

Electrical Characteristics

Absolute Maximum Ratings

• Supply Voltage (V+ to V-): 15V
• Analog Input Voltage (either input): V+ to V-
• Reference Input Voltage: V+ to V-
• Operating Temperature Range: 0°C to +70°C
• Storage Temperature Range: -65°C to +150°C

Recommended Operating Conditions

• Supply Voltage: +9V typical (6V to 12V range)
• Reference Voltage: 3.05V typical (internal reference)
• Clock Frequency: 120kHz typical (conversion rate of 3 readings/sec)
• Input Voltage Range: ±2V full scale (differential)

Performance Characteristics

• Resolution: 20,000 counts (4½ digits)
• Linearity Error: ±1 count maximum
• Zero Reading Drift: ±0.2μV/°C typical
• Common Mode Rejection: 86dB minimum at 50/60Hz
• Normal Mode Rejection: 60dB minimum at 50/60Hz
• Power Consumption: 1mW typical
• Input Bias Current: 1pA typical

Functional Description

The ICL7129 employs a dual-slope integration conversion technique that provides excellent noise rejection and high accuracy. The device integrates the input signal for a fixed period, then de-integrates using a known reference voltage. The time required for de-integration is proportional to the input voltage.

Key functional blocks include:

• Analog Section: Contains the integrator, comparator, and reference circuitry
• Digital Control: Manages the conversion cycle and auto-zero functions
• LCD Driver: Provides multiplexed drive signals for up to 4½ digits plus signs
• Timing Generator: Derives all internal clocks from a single external component

Package Information

The ICL7129 is available in a 40-pin plastic dual in-line package (DIP) and 44-pin plastic leaded chip carrier (PLCC).

40-Pin DIP Package Dimensions

• Package Width: 0.6 inch (15.24mm)
• Package Length: 2.0 inches (50.8mm)
• Pin Spacing: 0.1 inch (2.54mm) between pins
• Row Spacing: 0.3 inch (7.62mm) between rows

44-Pin PLCC Package Dimensions

• Package Size: 16.66mm × 16.66mm body
• Lead Pitch: 1.27mm (50 mils)
• Lead Count: 44 (11 per side)

Typical Application Circuit

A basic application circuit for the ICL7129 requires the following external components:

• Reference capacitor (typically 0.1μF)
• Integrating capacitor (typically 0.22μF)
• Integrating resistor (typically 100kΩ)
• Oscillator components (typically 100kΩ resistor and 100pF capacitor)
• LCD display with backplane connection

The circuit can be configured for either internal or external voltage reference operation, with the internal reference providing sufficient accuracy for most applications.

Design Considerations

Layout Guidelines

To achieve optimal performance from the ICL7129:

• Keep analog signal traces short and away from digital lines
• Use a ground plane to minimize noise pickup
• Place bypass capacitors close to the power supply pins
• Use proper shielding for high-impedance input nodes

Component Selection

• Use high-quality, stable capacitors for integration and reference functions
• Select resistors with low temperature coefficients for critical paths
• Choose LCD displays with compatible drive requirements

Conclusion

The ICL7129 represents an excellent solution for designers needing high-resolution analog-to-digital conversion with integrated display capability. Its combination of precision, low power consumption, and minimal external component requirements make it particularly valuable for portable instrumentation and battery-powered applications. The device's robust design and comprehensive feature set ensure reliable performance in demanding measurement environments.