Analog Devices Inc./Maxim Integrated MAX138EQH+D

Product Summary MAX138EQH+D 3½-digit ADC with LCD Interface and Charge-Pump Voltage Converter

The MAX138EQH+D from Analog Devices Inc./Maxim Integrated is a highly integrated 3½-digit analog-to-digital converter (ADC) designed for precision measurement instruments requiring digital readout capabilities. Housed in a 44-PLCC package (16.59 x 16.59 mm), the MAX138EQH+D features an onboard LCD display driver and a charge-pump voltage inverter, supporting accurate and versatile digital conversion for panel meters, handheld digital multimeters, weighing systems, portable monitors, and digital thermometers.

Main Features and Benefits of the MAX138EQH+D

The core appeal of the MAX138EQH+D lies in its optimized combination of measurement flexibility, integration, and reliability:

Operates from a single +2.5V to +7.0V supply

Onboard charge-pump voltage inverter, enabling measurement of both positive and negative input voltages from a single supply

Integrated bandgap voltage reference for superior long-term stability (typical drift ~0.01%, max tempco 100ppm/°C)

LCD display-driver outputs with matched waveforms for direct segment control and minimal DC bias across the display

Internal oscillator requiring no external components, factory-trimmed to 40kHz nominal (conversion rate ~2.5Hz)

High common-mode rejection ratio (CMRR) of 120dB on differential inputs

Supports high input impedance sources with guaranteed maximum leakage current of 10pA at IN HI

Compact 44-PLCC format for high-density system integration

Design Structure and Conversion Process of the MAX138EQH+D

At the heart of the MAX138EQH+D is a dual-slope integration ADC core, incorporating auto-zero and zero-integrator phases for precise offset compensation and rapid overrange recovery. The analog subsystem references the IN LO pin for integration, ensuring robust normal-mode noise rejection. The conversion formula is:

Result = 1000 × (IN HI – IN LO) / (REF HI – REF LO)

with display capability up to ±1999 counts. In overrange scenarios, the lower three digits blank, and sign segments indicate polarity.

The bandgap common voltage reference, buffered by a precision op amp, establishes an accurate baseline for signal and reference voltages, surpassing the performance of legacy zener-based designs. Oscillator and charge-pump blocks are self-contained, streamlining system design and allowing supply and reference pins to be managed with minimal discrete circuitry.

System Compatibility and Electrical Specifications of the MAX138EQH+D

The device supports supply voltages of +2.5V to +7.0V, with absolute maximum ratings for supply at +7.5V. Analog and reference inputs tolerate voltages between V+ and V-, contingent on input currents being limited (<±1mA for protection clamp diodes). The common-mode voltage range for IN HI and IN LO is wide, typically ±1V around COMMON, subject to integrator and buffer input constraints (V- +1.5V to V+ – 1.5V).

Power dissipation is specified per package (1067mW for 44-PLCC), and the device supports industrial temperature ranges (–40°C to +85°C for the E-grade variant).

The LCD driver stages dynamically control output waveforms, providing direct activation of segments based on conversion phase data, and integrate seamlessly with typical LCD panels. Testing and diagnostics are supported via dedicated TEST pin, which allows for all segments to be turned on for visual inspection.

Input/Output Signal Handling and MAX138EQH+D Connectivity

Differential input measurement, high CMRR, and robust input bias management allow for direct interfacing with high-impedance sensors (e.g., pH electrodes, voltage dividers). Clamp diodes ensure input protection, provided current limiting is respected. Reference voltages can be sourced with high flexibility, spanning V+ to V-, to define full-scale counts. Display segment drivers are configured for direct LCD operation, while status indicators for overload, polarity, and battery monitoring are supported through built-in logic.

Essential Passive Component Choices for MAX138EQH+D Solutions

Proper selection and engineering of passive components are essential for optimal accuracy, noise performance, and reliability:

Integrator resistor (RINT): Determines integration current; select as 2 × VREF / 1.1μA (e.g., 1.8MΩ for 1V reference).

Integrator capacitor: Use polypropylene for minimal dielectric absorption and best linearity. Value is tuned to provide ±2V swing at INT for full-scale inputs.

Reference capacitor: Low-leakage film types (polyester or polypropylene) recommended. 0.1μF commonly sufficient; increase to 1.0μF for high-temp operation or systems with high stray capacitance.

Auto-zero capacitor: Value should be ≥4× integrator capacitor for robust noise performance (≥0.047μF for 2V range, ≥0.47μF for 200mV full scale). Polypropylene type recommended for fast stabilization.

Charge-pump capacitor: Fixed at 1μF for voltage inverter.

Minimizing stray capacitance, leakage, and proper PCB layout are essential, especially at reference terminals, which are sensitive to gain and rollover errors.

Use Cases for the MAX138EQH+D

The MAX138EQH+D is deployed across a range of precision instrumentation:

Industrial and laboratory panel meters

Portable digital multimeters requiring battery operation and LCD indication

Weigh scales and portable monitors where differential inputs and low-power operation are essential

Digital thermometers leveraging direct sensor interfacing

Systems measuring both positive and negative voltages from a single supply—thanks to the integrated charge-pump inverter

The device supports advanced metrological arrangements, such as conductance or reciprocal measurements (by swapping signal and reference terminals), RPM and frequency-to-voltage conversions, and serial-output data acquisition by monitoring the reference capacitor terminal.

Troubleshooting Approaches and Design Insights for the MAX138EQH+D

Engineering best practices for deploying the MAX138EQH+D include:

For linearity issues or saturating readings at high input voltages, verify integrator swing and component values, adjusting integration capacitor or oscillator parameters as needed.

For gain errors and rollover effects, check for excess stray capacitance or PCB leakage near reference capacitor terminals; increase reference capacitor value for high-temperature operation and thoroughly clean PCB surfaces post-soldering.

LCD segment anomalies are typically connector-related, not intrinsic to the IC; verify backplane and segment waveforms.

Noisy readings often trace to noisy input signals or insufficient integration swing—implement input filters and optimize auto-zero capacitor sizing.

Observe recommended supply filtering and bypassing to reduce ripple and logic-induced noise, especially in battery-driven designs with variable loading.

For interface with legacy ICL7136/ICL7106/ICL7137 circuits, simple changes to oscillator and reference capacitors are generally sufficient for drop-in replacement.

Alternative or Substitute Models for the MAX138EQH+D

For engineers seeking alternatives or replacements, the following device families offer comparable functionality:

ICL7136/ICL7106/ICL7137: Legacy ADC families compatible with similar dual-slope integration and 3½-digit LCD/LED drivers (note oscillator circuitry and reference differences).

MAX139/MAX140: Variants of MAX138 supporting LED displays and low segment current operation.

When migrating between these models, pay attention to subtleties in oscillator management, reference types, package configurations, and display driver requirements, as certain changes in passives or board layout may be required.

Conclusion

The MAX138EQH+D from Analog Devices Inc./Maxim Integrated delivers a highly integrated solution for precision instrumentation, combining reliable dual-slope ADC performance, onboard LCD driving, and support for single-supply, bipolar measurement scenarios. Its robust architecture, high noise immunity, and flexible input/output design make it ideal for demanding applications where accuracy, ease of integration, and operational stability are critical. By understanding the functional nuances, passive selection guidelines, and troubleshooting strategies outlined here, engineers and procurement professionals can confidently specify the MAX138EQH+D and its nearest equivalents into new and legacy system designs.