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AD7845 12-Bit MDAC: Specs, Known Issues & Drop-In Replacements

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Quick-Reference Card: AD7845 at a Glance

Attribute Detail
Component Type 12-Bit Multiplying Digital-to-Analog Converter (MDAC)
Manufacturer Analog Devices Inc.
Key Spec 4-Quadrant Multiplication with On-Chip Amplifier
Supply Voltage ±12 V to ±15 V Dual Supply
Package Options DIP, PLCC, SOIC (Refer to datasheet for exact suffixes)
Lifecycle Status Obsolete / NRND (Critical procurement risk)
Best For Automatic Test Equipment (ATE) & Programmable Power Supplies


1. What Is the AD7845? (Definition + Architecture)

The AD7845 is a 12-bit multiplying digital-to-analog converter (MDAC) from Analog Devices Inc. that integrates an on-chip output amplifier to deliver complete 4-quadrant multiplication. For hardware engineers, the integration of the output amplifier is the defining feature; it eliminates the need for an external precision op-amp, removing a major source of parasitic capacitance, offset errors, and BOM bloat that plagued earlier MDAC designs.

1.1 Core Architecture & Design Philosophy

Fabricated on Analog Devices' proprietary LC2MOS process, the AD7845 combines precision bipolar analog circuitry with low-power CMOS logic. Internally, it utilizes an inverted R-2R ladder network. What makes this architecture highly reliable is the inclusion of matched application resistors directly on the silicon. Because these resistors share the same thermal gradient and manufacturing tolerances as the R-2R ladder, they guarantee monotonic performance across the entire operating temperature range.

1.2 Where It Fits in the Signal Chain / Power Path

The AD7845 sits directly between your digital control logic (MCU, DSP, or FPGA) and your high-voltage analog output stage. It is typically driven by a parallel digital bus and a precision bipolar reference voltage. Downstream, its ±10V capable output directly drives actuators, programmable attenuators, or the control loops of programmable power supplies.


2. Electrical Characteristics: The Numbers That Matter

2.1 Power Supply & Consumption Profile

The AD7845 requires a dual bipolar power supply, specifically ±12 V to ±15 V. Why it matters: Modern digital systems are heavily 3.3V or 5V single-supply. Designing in the AD7845 requires a dedicated bipolar switching regulator or charge pump if your system lacks legacy analog rails. Despite the high-voltage analog rails, the digital inputs are fully TTL and 5V CMOS compatible, saving you from needing external level shifters.

2.2 Performance Specs (Speed, Accuracy, or Efficiency)

  • Resolution: 12-Bit. Provides 4096 distinct output steps, standard for industrial control.
  • Settling Time: 2.5 μs. Why it matters: In Automatic Test Equipment (ATE), settling time dictates your test throughput. A 2.5 μs settling time is fast enough for low-to-medium frequency waveform generation but will bottleneck high-speed RF testing.
  • Total Harmonic Distortion (THD): -90 dB typ. Ensures clean signal reproduction when used as a digital audio attenuator or in precision waveform synthesis.
  • Minimum Load Resistance: 2 kΩ. Do not attempt to drive low-impedance loads (like 50Ω RF lines or speakers) directly without an external buffer.

2.3 Absolute Maximum Ratings — What Will Kill It

  • VDD to VSS Reversal: Reversing the bipolar supply rails will cause immediate catastrophic latch-up.
  • Digital Input Overvoltage: Applying logic signals higher than VDD + 0.3V or lower than GND - 0.3V will forward-bias the internal ESD diodes, potentially destroying the CMOS input stage. Always ensure the analog rails are powered up before applying digital signals.

3. Pinout & Package Guide

3.1 Pin-by-Pin Functional Groups

Pin Group Pins Function
Power VDD, VSS, GND Dual supply rails (±12V to ±15V) and system ground.
Digital Inputs DB0 to DB11 12-bit parallel data bus. DB11 is the MSB.
Control /CS, /WR Chip Select and Write logic for latching data into the DAC register.
Analog VREF, VOUT Reference voltage input and buffered analog voltage output.
Feedback RFB, ROFS Internal matched resistors for gain ranging and bipolar offset.

3.2 Package Variants & Soldering Notes

Package Pitch Thermal Pad? Soldering Method
DIP (Plastic/Ceramic) 2.54 mm No Wave solder or hand solder
PLCC 1.27 mm No Reflow or socketed
SOIC 1.27 mm No Standard IR Reflow

Note: As a legacy component, the AD7845 is often found in through-hole DIP packages, making it excellent for prototyping but bulky for modern PCB layouts.

3.3 Part Number Decoder

  • AD: Analog Devices standard prefix.
  • 7845: Base part number.
  • A/B/C/S: Indicates linearity grade and temperature range (e.g., Commercial, Industrial, Military). Refer to the datasheet for exact LSB error tolerances per grade.
  • N/R/P: Package designator (N = PDIP, R = SOIC, P = PLCC).

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 1: Ground Noise Susceptibility * Root Cause: High-frequency noise on the input common can couple directly to the output via the internal compensation capacitor. Furthermore, common impedance in the negative supply line (VSS) can cause severe interference and degrade the -90 dB THD. * Recommended Fix: Use a subtractor circuit for the analog input. Ensure the internal amplifier is powered with respect to the output signal common to maximize common-mode rejection. Implement strict star-grounding techniques separating analog and digital return paths.

Problem 2: Obsolescence and Availability * Root Cause: The LC2MOS process is heavily outdated. The AD7845 is marked as obsolete or "Not Recommended for New Designs" (NRND) across major distributors. * Recommended Fix: Do not design this into new boards. For existing product maintenance, procure remaining stock through authorized legacy distributors like Rochester Electronics. For redesigns, migrate to newer SPI/I2C precision DACs (e.g., AD57xx series).


5. Application Circuits & Integration Examples

5.1 Typical Application: 4-Quadrant Multiplying DAC

In a 4-quadrant configuration, the AD7845 can accept both positive and negative reference voltages, and output both positive and negative voltages based on the digital code. This is heavily used in programmable power supplies to digitally scale an AC reference signal. By utilizing the internal RFB and ROFS resistors, you eliminate the need for external precision resistor networks to shift the unipolar DAC output into a bipolar range.

5.2 Interface Example: Connecting to a Microcontroller

Because the AD7845 uses a legacy parallel interface, driving it requires 12 GPIO pins for data, plus 2 for control (/CS and /WR). Here is the initialization and write sequence for an STM32 or Arduino:

// Pseudocode for AD7845 Parallel Write
void write_AD7845(uint16_t dac_value) {
    // 1. Ensure /CS and /WR are HIGH (idle state)
    digitalWrite(CS_PIN, HIGH);
    digitalWrite(WR_PIN, HIGH);

    // 2. Place 12-bit value on the parallel bus (DB0-DB11)
    set_gpio_bus(dac_value & 0x0FFF); 

    // 3. Pull /CS LOW to select chip
    digitalWrite(CS_PIN, LOW);

    // 4. Pulse /WR LOW then HIGH to latch data
    digitalWrite(WR_PIN, LOW);
    delayMicroseconds(1); // Satisfy minimum write pulse width
    digitalWrite(WR_PIN, HIGH);

    // 5. Release /CS
    digitalWrite(CS_PIN, HIGH);
}

6. Alternatives, Replacements & Cross-Reference

If you are facing supply chain blocks or redesigning an older board, consider these alternatives.

6.1 Pin-Compatible Drop-In Replacements

Part Number Manufacturer Key Difference Compatible?
MX7845 Maxim Integrated Virtually identical specs and pinout ? Yes
SP7545 Sipex Similar architecture, verify settling time ?? Check Specs

6.2 Upgrade Path (Better Performance)

If redesigning the PCB, abandon the parallel interface and ±15V logic. Move to modern SPI/I2C DACs like the Analog Devices AD5722 (dual 12-bit, software-programmable bipolar output) which saves massive board space and reduces power supply complexity.

6.3 Cost-Down Alternatives

Legacy parts like the Burr-Brown (TI) DAC667 or Maxim MAX528 were historic competitors. However, finding cheap stock of any of these legacy parallel DACs is difficult today. True cost-down requires migrating to modern serial DACs.


7. Procurement & Supply Chain Intelligence

  • Lifecycle Status: Obsolete / NRND. Analog Devices has largely phased out this component.
  • Typical MOQ & Lead Time: Factory lead times are no longer applicable. Stock is limited to what is currently on distributor shelves.
  • BOM Risk Factors: Extreme risk. Single-source reliance on a discontinued part will halt production lines.
  • Recommended Safety Stock: Buy out available inventory from authorized legacy suppliers if your product cannot be redesigned.
  • Authorized Distributors: Rochester Electronics is the primary authorized source for end-of-life (EOL) Analog Devices silicon. Avoid grey-market brokers to prevent counterfeit ICs.

8. Frequently Asked Questions

Q: What is the AD7845 used for? The AD7845 is primarily used in Automatic Test Equipment (ATE), programmable power supplies, digital attenuators, and digital-to-4–20 mA converters. Its integrated amplifier makes it ideal for precision analog scaling.

Q: What are the best alternatives to the AD7845? The Maxim Integrated MX7845 is the closest pin-compatible drop-in replacement. Other historic competitors include the Burr-Brown DAC667 and Sipex SP7545, though availability for all is scarce.

Q: Is the AD7845 still in production? No, the AD7845 is generally considered obsolete and is Not Recommended for New Designs (NRND). Buyers must rely on legacy distributors for remaining stock.

Q: Can the AD7845 work with 3.3V logic? The AD7845 digital inputs are TTL and 5V CMOS compatible. While some 3.3V logic high levels may meet the minimum VIH requirements, you must verify against the datasheet to ensure reliable triggering across temperature variations.

Q: Where can I find the AD7845 datasheet and schematic symbols? Datasheets, CAD models, and legacy application notes can be found on the Analog Devices Inc. website or through authorized legacy distributors like Rochester Electronics.


9. Resources & Tools

  • Evaluation / Development Kit: No longer actively manufactured; rely on legacy stock or build a custom breakout board.
  • Reference Designs: Search the Analog Devices archive for legacy application notes on "LC2MOS MDAC Applications".
  • Community Libraries: Parallel DACs generally do not require complex libraries; simple GPIO bit-banging (as shown in Section 5) is sufficient for Arduino/STM32 integration.
  • SPICE / LTspice Model: Check the Analog Devices LTspice library for legacy macro models, though exact transient models for the internal op-amp may be limited.

AD7845JRZ-REEL Documents & Media

Download datasheets and manufacturer documentation for Analog Devices Inc. AD7845JRZ-REEL.

AD7845JRZ-REEL PCB Symbol, Footprint & 3D Model

Analog Devices Inc. AD7845JRZ-REEL

Analog Devices Inc.

DAC 1-CH R-2R 12-Bit 24-Pin SOIC T/R

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