AD790 Comparator: Delay Variations, Missing Models, and Real Fixes

Author: Kynix

Published: 2026-03-26 | Last Updated: 2026-03-27

Contents

Quick-Reference Card: AD790 at a Glance

Attribute	Detail
Component Type	Precision Voltage Comparator
Manufacturer	Analog Devices Inc.
Key Spec	45 ns max Propagation Delay
Supply Voltage	Single 5 V or Dual ±15 V
Package Options	Multiple (e.g., AD790SQ) - See datasheet
Lifecycle Status	Active (Verify with distributor)
Best For	Zero-Crossing Detectors

AD790 product photo or IC package

1. What Is the AD790? (Definition + Architecture)

The AD790 is a fast, precise voltage comparator from Analog Devices Inc. that combines a 45 ns maximum propagation delay with built-in hysteresis to minimize unwanted oscillations. Unlike generic comparators that require external positive feedback networks to prevent chattering on slow-moving signals, the AD790 integrates this internally alongside a low-glitch output stage.

1.1 Core Architecture & Design Philosophy

Internally, the AD790 is designed to bridge the gap between high-voltage analog front-ends and low-voltage digital logic. Its architecture allows it to operate from traditional bipolar analog supplies (±15 V) while maintaining strict TTL/CMOS compatibility at the output. The inclusion of an onboard latch allows designers to freeze the output state, which is critical for synchronous digital systems or delta-sigma modulator applications where timing alignment is paramount.

1.2 Where It Fits in the Signal Chain / Power Path

The AD790 sits directly at the boundary between the analog sensor/signal conditioning stage and the digital microcontroller/FPGA interface. It is typically driven by analog filters, precision rectifiers, or raw AC line voltages (stepped down), and it drives digital interrupt pins or pulse-width modulation (PWM) controllers downstream.

2. Electrical Characteristics: The Numbers That Matter

2.1 Power Supply & Consumption Profile

The AD790 supports either a single 5 V supply or dual ±15 V supplies, dissipating approximately 60 mW of power. * Why it matters: This dual-nature supply range means you don't need to add a dedicated 5V rail to your analog front-end just to power the comparator; it can run directly off the existing op-amp rails while still safely driving 5V digital logic.

2.2 Performance Specs (Speed, Accuracy, or Efficiency)

Propagation Delay (45 ns max):
Why it matters: In high-frequency PWM or zero-crossing detection, 45 ns ensures minimal phase lag between the physical event and the digital trigger, preventing timing errors in motor control or power supply switching.
Input Offset Voltage (250 μV max):
Why it matters: This exceptionally low offset eliminates the need for external trimming potentiometers in precision overvoltage detectors, reducing BOM count and calibration time.
Input Hysteresis Voltage (500 μV max):
Why it matters: This built-in hysteresis band provides noise immunity for slow-moving input signals, preventing the output from rapidly toggling (glitching) as the signal crosses the threshold.

2.3 Absolute Maximum Ratings — What Will Kill It

Differential Input Voltage: 15 V max
Why it matters: Exceeding this limit will permanently damage the input stage. Engineers migrating from older comparators that tolerate wider differential swings often overlook this. If your application risks differential spikes above 15V, external clamping diodes are mandatory.
Operating Temperature (AD790SQ): -55°C to +125°C
Why it matters: The SQ variant is military/aerospace-rated, ensuring the offset and delay specs hold up in extreme thermal environments.

3. Pinout & Package Guide

3.1 Pin-by-Pin Functional Groups

Pin Group	Pins	Function
Power	V+, V-, GND	Supply rails (Tie V- to GND for single 5V operation)
Signal Input	+IN, -IN	Non-inverting and inverting analog inputs
Signal Output	OUT	TTL/CMOS compatible digital output
Control	LATCH	Freezes the output state when asserted

3.2 Package Variants & Soldering Notes

Package	Pitch	Thermal Pad?	Soldering Method
CERDIP (SQ)	2.54 mm	No	Wave / Hand solder
PDIP / SOIC	Standard	No	Standard Reflow / Wave

Note: Refer to the official datasheet for the complete list of available modern packaging options, as legacy through-hole parts may face availability constraints.

3.3 Part Number Decoder

AD790: Base part number.
S: Temperature grade (-55°C to +125°C).
Q: Package type (CERDIP).

4. Known Issues, Errata & Real-World Pain Points

Why this section exists: Community forums, application notes, and field reports reveal problems the datasheet glosses over. This section saves you hours of debugging.

Problem: Missing Official SPICE Model * Root Cause: Analog Devices does not provide an official SPICE or macro-model for the AD790 in ADIsimPE or LTspice, making pre-layout circuit simulation highly frustrating. * Recommended Fix: Utilize third-party behavioral comparator models in your simulator, or build a custom single-pole macro-model based on the AD790's datasheet slew rate, 45ns delay, and gain specifications. Do not rely on generic ideal comparators, as they will mask real-world timing issues.

Problem: Propagation Delay Variation * Root Cause: The 45 ns propagation delay is not absolute; it varies significantly depending on the input signal's amplitude, frequency, and overdrive voltage (e.g., a slow sine wave will trigger differently than a sharp square wave). * Recommended Fix: Never treat the 45 ns spec as a static constant in timing-critical applications. Characterize the delay empirically on the bench for your specific input signal type, and ensure your MCU's interrupt design margins account for overdrive-induced timing jitter.

5. Application Circuits & Integration Examples

5.1 Typical Application: Zero-Crossing Detectors

The AD790 is uniquely suited for AC zero-crossing detection in power metering or triac control. By connecting the inverting input to ground and feeding a scaled-down AC signal into the non-inverting input, the AD790 outputs a crisp digital edge exactly when the AC line crosses 0V. The internal 500 μV hysteresis ensures that line noise at the zero-crossing point does not cause multiple output pulses, which would otherwise crash a downstream microcontroller interrupt routine.

AD790 typical application circuit schematic

5.2 Interface Example: Connecting to a Microcontroller

Interfacing the AD790 with a 5V-tolerant MCU (like an Arduino or specific STM32 pins) is straightforward due to its TTL/CMOS compatible output.

// Pseudocode for STM32 / Arduino interrupt setup
#define AD790_OUT_PIN  2
#define AD790_LATCH    3

void setup() {
  pinMode(AD790_OUT_PIN, INPUT);
  pinMode(AD790_LATCH, OUTPUT);

  // Keep latch low for transparent operation
  digitalWrite(AD790_LATCH, LOW); 

  // Attach interrupt to catch the fast 45ns edge
  attachInterrupt(digitalPinToInterrupt(AD790_OUT_PIN), zeroCrossISR, RISING);
}

void zeroCrossISR() {
  // Handle zero-crossing event
}

6. Alternatives, Replacements & Cross-Reference

6.1 Pin-Compatible Drop-In Replacements

Because the AD790 has a specific latch architecture and precision offset, true 1:1 drop-ins are rare. Always verify pinouts.

Part Number	Manufacturer	Key Difference	Compatible?
LM311	Texas Instruments	Much slower (200ns+), higher offset	?? (Requires redesign for speed)
LT1116	Analog Devices	Faster (12ns), single supply optimized	?? (Check latch pinout)

6.2 Upgrade Path (Better Performance)

If you are designing a next-generation product and need faster response times, consider the LT1719 or LT1720. These comparators offer sub-5ns propagation delays, making them vastly superior for high-frequency PWM or modern high-speed discrete A/D converters.

6.3 Cost-Down Alternatives

If the 45ns speed and 250μV precision are overkill for your application, the ubiquitous LM311 or LTC1843 can serve as cost-down alternatives, provided your system can tolerate slower propagation delays and higher offset errors.

7. Procurement & Supply Chain Intelligence

Lifecycle Status: Active, but older package variants (like CERDIP) may be classified as Not Recommended for New Designs (NRND). Always verify with authorized distributors.
Typical MOQ & Lead Time: Standard SOIC packages generally have standard lead times, but Mil-Spec variants (SQ) can see lead times exceeding 26 weeks.
BOM Risk Factors: The AD790 is a highly specific, single-source component from Analog Devices. Because it lacks exact pin-for-pin clones with identical latch/hysteresis behavior, it represents a moderate BOM risk.
Recommended Safety Stock: Maintain 6 months of safety stock if utilizing the military/aerospace temperature grade variants.
Authorized Distributors: Digi-Key, Mouser, Arrow, and direct from Analog Devices.

8. Frequently Asked Questions

Q: What is the AD790 used for? The AD790 is primarily used for zero-crossing detectors, overvoltage detectors, precision rectifiers, and pulse-width modulators.

Q: What are the best alternatives to the AD790? Depending on your need for speed versus cost, the LT1116, LT1719, LT1720, LTC1843, and the classic LM311 are the most common alternatives considered by engineers.

Q: Is the AD790 still in production? Yes, the AD790 is still in production, though specific legacy packages like through-hole DIP or CERDIP may have tighter availability compared to surface-mount options.

Q: Can the AD790 work with 3.3V logic? The AD790 is designed for 5V TTL/CMOS compatibility. If interfacing with strict 3.3V logic, you must use a voltage divider or level shifter on the output to prevent damaging the downstream MCU.

Q: Where can I find the AD790 datasheet and evaluation board? The official datasheet can be downloaded directly from the Analog Devices website or authorized distributors like Mouser and Digi-Key.

9. Resources & Tools

Evaluation / Development Kit: Check Analog Devices for generic comparator evaluation boards compatible with standard SOIC/DIP pinouts.
Reference Designs: Analog Devices application notes on Zero-Crossing Detection and Delta-Sigma Modulators.
Community Libraries: General interrupt-driven GPIO libraries in Arduino, PlatformIO, and STM32CubeMX are ideal for reading the AD790's output.
SPICE / LTspice Model: Not officially available. Engineers must rely on third-party behavioral models or characterize the part empirically.

AD790SQ Documents & Media

Download datasheets and manufacturer documentation for Analog Devices Inc. AD790SQ.

Analog Devices Inc. AD790SQ

Analog Devices Inc.

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