Analog Devices Inc. AD637JD/+

Overview

AD637JD/+ from Analog Devices is a high-accuracy, wideband true RMS-to-DC converter IC designed for precision measurement of complex AC/DC waveforms in instrumentation and test systems. It delivers 0.02% maximum nonlinearity (0–2 V rms), 8 MHz bandwidth at 2 V rms input, and computes true RMS, mean square, absolute value, and dB output (60 dB range) with chip select power-down.

For engineers reviewing the AD637JD/+ datasheet, AD637JD/+ pinout, AD637JD/+ application, or AD637JD/+ equivalent, key selection criteria include crest factor compensation up to CF = 10 (<1% additional error), ±15 V dual-supply operation, 14-lead SBDIP hermetic package, and on-chip buffer amplifier usable as input buffer or active filter.

Technical Context

The AD637JD/+ implements an implicit analog solution to the RMS equation V_OUT = √(V_IN²)_AVG using a four-section architecture: absolute value circuit, squarer/divider, low-pass filter with external C_AV, and buffer amplifier. Its denominator current is derived internally or externally via DEN INPUT (Pin 6) to support RMS, mean square, or absolute value modes.

Chip Select (Pin 5) enables analog three-state operation and reduces quiescent current from 2.2 mA to 350 µA; during CS low, RMS OUT enters high-impedance state, enabling parallel connection of multiple AD637JD/+ units for wideband RMS multiplexing without signal contention.

Key Specifications

Pinout & Package

The AD637JD/+ is supplied in a hermetically sealed 14-lead SBDIP (Small Outline Ceramic Dual In-line Package) with 0.300" body width, rated for 0°C to +70°C operation and optimized for high-reliability instrumentation environments.

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar RMS-to-DC conversion applications.

Compared with AD637JRZ-REEL7, the AD637JD/+ offers superior low-temperature stability and hermetic reliability but requires through-hole assembly; versus AD736JRZ-REEL, it delivers 20× higher bandwidth and 15× better nonlinearity at the cost of dual-supply complexity and higher BOM count.

Availability

AD637JD/+ is available at Aetrix Electronics and suitable for power quality monitoring, audio test equipment, industrial motor drives, and calibration standards requiring stable component supply with long-term obsolescence management.

Supply support for AD637JD/+ 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

Analog Devices, Inc. is a global leader in high-performance analog, mixed-signal, and digital signal processing semiconductors, headquartered in Norwood, MA, with design centers worldwide and ISO 9001-certified manufacturing.

The AD637 belongs to Analog Devices' precision signal conditioning product line, engineered specifically for metrology-grade RMS measurement in test & measurement, energy, and aerospace systems where analog integrity and long-term calibration stability are critical.

FAQ

What is the maximum input frequency the AD637JD/+ can accurately measure at 200 mV rms?

The AD637JD/+ maintains ≤1% additional error up to 66 kHz at 200 mV rms input, and its ±3 dB bandwidth extends to 1 MHz at that level. This makes it suitable for measuring switched-mode power supply ripple, PWM motor drive fundamentals, and intermediate-frequency signals in communications test gear. Performance is verified per Table 1 in the Rev. L datasheet, with no derating required within the specified operating temperature range.

Does the AD637JD/+ require external passive components for basic RMS operation?

Yes - the AD637JD/+ requires only one external component for standard RMS operation: a capacitor connected to CAV (Pin 8) to set the averaging time constant. A 1 µF capacitor yields τ = 25 ms, providing 0.1% low-frequency error at 100 Hz. No resistors, op amps, or trimming pots are needed for basic functionality, as laser wafer trimming ensures initial accuracy without user adjustment.

Can the AD637JD/+ compute RMS of bipolar signals without AC coupling?

Yes - the AD637JD/+ accepts true ac+dc inputs directly at VIN (Pin 13). Its absolute value circuit and implicit RMS algorithm inherently handle bipolar waveforms, delivering correct RMS output regardless of DC offset. For example, a 1 V rms sine wave superimposed on 2 V dc produces the same RMS output (1 V dc) as the pure 1 V rms sine wave, confirming dc-insensitive RMS computation.

How does the chip select (CS) function affect the output impedance of AD637JD/+?

When CS (Pin 5) is pulled below 0.2 V, the AD637JD/+ disables its internal output driver stage, placing RMS OUT (Pin 9) in a high-impedance state with resistance >100 kΩ. This allows multiple AD637JD/+ units to share a common output bus without contention - essential for building wideband RMS multiplexers in automated test equipment. The output returns to normal 0.5 Ω drive strength when CS is open or >2.4 V.

What is the purpose of the DEN INPUT pin on the AD637JD/+ and how is it used?

The DEN INPUT (Pin 6) provides external control of the denominator current in the AD637JD/+'s squarer/divider core. Applying a stable reference voltage here configures the device for mean square computation (V_OUT = V_IN²/V_DEN) or ratio measurement - e.g., computing power (V·I) by feeding scaled current into DEN while applying voltage to VIN. Input resistance is 25 kΩ typical, and offset is ±0.2 mV, supporting precision ratio applications.