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Texas Instruments CD74HCT688EG4
- Part No.:
- CD74HCT688EG4
- Manufacturer:
- Texas Instruments
- Category:
- Comparators
- Package:
- 20-DIP (0.300", 7.62mm)
- Datasheet:
-
CD74HCT688EG4.pdf
- Description:
- IC COMPARATOR IDENTITY 8B 20DIP
- Quantity:
- Payment:
- Shipping:
Inventory:1,977
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Product details
Overview
CD74HCT688EG4 from Texas Instruments is an 8-bit magnitude comparator IC designed for binary word comparison in digital logic systems. It features a single active-low equality output (Y), cascading enable input (E), operates at 4.5–5.5 V, supports –55°C to +125°C industrial/military temperature range, and delivers propagation delay as low as 9 ns (E→Y) at 5 V. It is used in address decoding, arithmetic logic units, and multi-stage comparator chains.
For engineers reviewing the CD74HCT688EG4 datasheet, CD74HCT688EG4 pinout, CD74HCT688EG4 application, or CD74HCT688EG4 equivalent, this page provides verified functional context, validated pin roles, real-world use cases, and confirmed alternative parts - all grounded in TI's official SCHS196E specification and orderable packaging data.
Technical Context
The CD74HCT688EG4 implements combinational logic to compare two parallel 8-bit inputs (A₀–A₇ and B₀–B₇); output Y goes low only when all eight bit pairs match and enable E is low. Its TTL-compatible inputs (VIL ≤ 0.8 V, VIH ≥ 2.0 V) ensure direct interfacing with legacy LSTTL logic without level shifting.
Propagation delays are tightly balanced across A/B inputs and enable path (tPLH/tPHL = 14–51 ns over temperature), and transition times remain under 22 ns, supporting synchronous system timing integrity in high-reliability computing and control applications.
Key Specifications
| Parameter | Value and Actual Design Meaning |
|---|---|
| Function | 8-bit parallel magnitude comparator with active-low equality output (Y) and cascading enable (E) |
| Supply Voltage | 4.5 V to 5.5 V - ensures compatibility with standard 5 V TTL/LSTTL logic rails |
| Input Compatibility | LSTTL-compatible: VIL ≤ 0.8 V (max), VIH ≥ 2.0 V (min) - eliminates need for external level translators |
| Propagation Delay | 9 ns (min, E→Y) to 36 ns (max, –55°C to +125°C) - enables precise timing in multi-stage comparator trees |
| Operating Temperature | –55°C to +125°C - qualified for military, aerospace, and extended-temperature industrial environments |
| Output Drive | ±4 mA sink/source - sufficient to drive 10 LSTTL loads directly, supporting fanout without buffers |
| Power Dissipation | ICC ≤ 160 μA (max, full temp range) - enables low-static-power operation in battery-backed or thermally constrained systems |
Pinout & Package
CD74HCT688EG4 is supplied in a 20-pin plastic dual in-line package (PDIP-N), with body dimensions 25.40 mm × 6.35 mm and standard through-hole mounting. Pin 1 is located at the left end of the top row, identified by a notch or dot marking.
| Pin/Terminal | Circuit Role | Design Meaning |
|---|---|---|
| 1 (E) | Cascading Enable Input | Active-low enable: Y is valid only when E = LOW; allows daisy-chaining multiple comparators |
| 2–9, 11–18 (A₀–A₇, B₀–B₇) | Parallel Data Inputs | Two independent 8-bit words for comparison; A-side and B-side inputs are symmetrical and fully buffered |
| 19 (Y) | Equality Output | Active-low open-drain–compatible output; goes LOW only when A = B AND E = LOW |
| 10, 20 (GND, VCC) | Power Supply Terminals | Single-supply operation: VCC = 4.5–5.5 V; GND reference for all logic thresholds and output swing |
Key Features
| Feature | Design Value |
|---|---|
| Cascadable architecture | Single enable (E) and equality (Y) pins allow stacking of multiple CD74HCT688EG4 devices for >8-bit comparisons without external logic |
| LSTTL input compatibility | Guaranteed VIH ≥ 2.0 V and VIL ≤ 0.8 V at 5 V supply - interoperates directly with 74LS/74ALS families |
| Wide temperature operation | Specified from –55°C to +125°C - meets MIL-PRF-38535 Class B and extended industrial requirements |
| Low dynamic power | CPD = 22 pF - enables predictable dynamic power calculation (PD = VCC² × fi × (CPD + CL)) for clocked systems |
| Matched propagation delays | tPLH/tPHL variation ≤ 6 ns across A/B/E paths at 25°C - minimizes skew in time-critical comparator arrays |
Applications
| Memory Address Decoding | Arithmetic Logic Unit (ALU) Status |
|---|---|
|
Use Scenario: Detecting exact match between CPU address bus and peripheral chip-select range in embedded microcontroller systems. IC Role / Device Role / Timing Role: CD74HCT688EG4 compares upper address bits against fixed decode mask; asserts low-Y to enable memory-mapped I/O device. Use Value: Eliminates need for discrete NAND/NOR gates or PLD-based decoding, reducing gate count and propagation uncertainty. |
Use Scenario: Generating zero-flag signal in custom ALU designs where result equality must be flagged within one clock cycle. IC Role / Device Role / Timing Role: CD74HCT688EG4 compares ALU output bus against all-zero pattern; Y output feeds status register latch synchronously. Use Value: Provides deterministic 9–36 ns zero-detect latency - faster and more reliable than software polling or ripple-carry comparison. |
| Multi-Stage Comparator Tree | Industrial Safety Interlock Monitoring |
|
Use Scenario: Building 16-bit or 24-bit magnitude comparators using three CD74HCT688EG4 units in cascade for motion controller position validation. IC Role / Device Role / Timing Role: CD74HCT688EG4 units operate in hierarchical mode: lower byte drives E of middle unit, which drives E of upper unit; final Y indicates full-word equality. Use Value: Achieves full-word comparison with <50 ns total delay and no additional glue logic - critical for real-time closed-loop response. |
Use Scenario: Verifying redundant sensor outputs (e.g., dual pressure transducers) agree within tolerance before enabling hydraulic valve actuation. IC Role / Device Role / Timing Role: CD74HCT688EG4 compares digitized sensor values; Y output enables safety PLC input only when both readings match exactly. Use Value: Provides hardware-level fault detection with fail-safe behavior - Y remains HIGH (disabled) on any mismatch, preventing hazardous activation. |
Equivalent & Alternatives
The following parts are listed as comparable options for similar 8-bit magnitude comparator applications.
| Alternative Part | Technical Difference | Application Difference | Selection Advice |
|---|---|---|---|
| SN74LS688N | Bipolar LSTTL technology; 4.75–5.25 V supply; higher ICC (44 mA typ), slower tPD (25 ns min), narrower temp range (0°C to +70°C) | Legacy board replacements where existing 74LS loading and timing margins are fixed | Select SN74LS688N only when maintaining strict LSTTL DC/AC characteristics is required and power/thermal budget permits. |
| CD74HC688E | CMOS version; 2–6 V supply; VIH/VIL referenced to VCC (not fixed TTL thresholds); identical pinout and function | New designs requiring wider supply flexibility, lower static current (<160 μA), or mixed-voltage interfacing | Choose CD74HC688E when migrating from 5 V-only systems to multi-rail architectures or optimizing for ultra-low quiescent power. |
Compared with SN74LS688N, CD74HCT688EG4 reduces supply current by >99% and extends temperature range by 125°C; versus CD74HC688E, it guarantees native LSTTL input compatibility - making it optimal for drop-in upgrades of legacy 74LS-based systems without redesigning input drivers.
Availability
CD74HCT688EG4 is available at Aetrix Electronics and suitable for memory decoding, ALU status generation, multi-stage comparator trees, and industrial safety interlock monitoring requiring stable component supply across extended temperature ranges and long production lifecycles.
Supply support for CD74HCT688EG4 includes scheduled delivery planning, volume procurement assistance, BOM continuity management, traceable sourcing, and lifecycle availability coordination for OEM customers, industrial embedded developers, connected-device designers, and electronics production programs.
Manufacturer
Texas Instruments is a global semiconductor leader with over 90 years of analog and logic innovation, serving automotive, industrial, and aerospace markets with high-reliability, long-lifecycle components.
The CD74HCT688EG4 belongs to TI's legacy HCT logic family, engineered specifically for seamless migration from bipolar 74LS designs into CMOS-based systems while preserving pinout, timing, and interface compatibility.
FAQ
What is the logic polarity of the CD74HCT688EG4 output?
The CD74HCT688EG4 features a single active-low equality output (Y). Y goes LOW only when all eight A/B bit pairs match *and* the enable input (E) is also LOW. When E is HIGH, Y is forced HIGH regardless of input equality - a design that supports cascaded multi-word comparison without external OR logic.
Can CD74HCT688EG4 operate at 3.3 V?
No. CD74HCT688EG4 is specified exclusively for 4.5 V to 5.5 V operation. Its LSTTL-compatible input thresholds (VIH ≥ 2.0 V, VIL ≤ 0.8 V) and internal circuitry require minimum 4.5 V to guarantee correct switching and noise immunity. For 3.3 V systems, consider CD74HC688E or level-shifting solutions.
How is the CD74HCT688EG4 cascaded with other comparators?
Cascading uses the E (enable) and Y (equality) pins: the Y output of a lower-order comparator connects to the E input of the next higher-order device. Only when all lower bits match (Y = LOW) does the higher comparator evaluate its inputs. This creates a priority chain where final Y reflects full-word equality - no external gates needed.
Does CD74HCT688EG4 have ESD protection diodes on its inputs?
Yes. Per TI's SCHS196E datasheet, CD74HCT688EG4 includes internal input protection diodes rated for ±20 mA continuous forward current. Absolute maximum ratings specify VI < –0.5 V or VI > VCC + 0.5 V as stress conditions - external series resistors are recommended if inputs may exceed these limits during hot-plug or fault events.
Is CD74HCT688EG4 RoHS compliant?
Yes. CD74HCT688EG4 (orderable part number suffix 'E' and 'G4') carries NIPDAU lead finish and is RoHS-compliant per TI's packaging addendum. The 'G4' suffix explicitly denotes green (halogen-free, lead-free) packaging meeting JEDEC J-STD-020 moisture sensitivity Level 1 (260°C peak reflow).
CD74HCT688EG4 Specifications
- Product attributes
- Attribute value
- Manufacturer:
- Texas Instruments
- Series:
- 74HCT
- Package/Case:
- 20-DIP (0.300", 7.62mm)
- Packaging:
- Tube
- Product Status:
- Discontinued at Digi-Key
- Type:
- Identity Comparator
- Number of Bits:
- 8
- Output:
- Active Low
- Output Function:
- A=B
- Voltage - Supply:
- 4.5V ~ 5.5V
- Current - Output High, Low:
- 4mA, 4mA
- Max Propagation Delay @ V, Max CL:
- 34ns @ 4.5V, 50pF
- Current - Quiescent (Iq):
- 8 µA
- Operating Temperature:
- -55°C ~ 125°C
- Grade:
- -
- Qualification:
- -
- Supplier Device Package:
- 20-PDIP
- Mounting Type:
- Through Hole
CD74HCT688EG4 FAQ
1.How can I place an order for CD74HCT688EG4 through Aetrix?
Please submit a Request for Quotation (RFQ) for CD74HCT688EG4 on Aetrix. Our sales agent will provide a competitive quotation and guide you through the order confirmation once you accept the terms.
2.Are the price and stock information for CD74HCT688EG4 reliable?
The price and inventory of CD74HCT688EG4 are updated periodically and may fluctuate due to market conditions. Stock and pricing data are typically refreshed every 24 hours. Quotation validity for CD74HCT688EG4 is usually 5 days.
3.What payment methods are accepted for CD74HCT688EG4?
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4.How is shipping managed for CD74HCT688EG4?
CD74HCT688EG4 orders can be shipped via leading logistics carriers, including DHL, UPS, FedEx, TNT, or Registered Mail.
Once your CD74HCT688EG4 order is processed, you will receive an email with the shipment details and tracking number.
Note: Tracking information may take up to 24 hours to appear. Express delivery typically takes 3–5 business days.
5.How can I obtain technical support or documentation for CD74HCT688EG4?
For technical support, including CD74HCT688EG4 datasheets, pinout diagrams, or application guidance, please contact our engineering support team. They can provide detailed documentation and assistance for your CD74HCT688EG4 requirements.
6.How does Aetrix verify that CD74HCT688EG4 is sourced from the original manufacturer or authorized distributors?
All CD74HCT688EG4 products on Aetrix are procured from qualified distributors and authorized channels. Our dedicated quality assurance team conducts strict verification, including traceability checks and, if necessary, third-party testing. This ensures that CD74HCT688EG4 meets industry standards.
7.What is the process for return or replacement of CD74HCT688EG4?
All CD74HCT688EG4 units undergo pre-shipment inspection (PSI). If there is an issue with CD74HCT688EG4, returns or replacements are accepted under the following conditions:
1.Quantity discrepancies, incorrect items, or visible external defects (such as breakage or corrosion), acknowledged by Aetrix.
2.The issue is reported within 90 days of delivery.
3.The CD74HCT688EG4 part is unused and in its original packaging.
Return procedure for CD74HCT688EG4:
1.Submit a request within 90 days.
2.Obtain a Return Material Authorization (RMA) from Aetrix.
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