Analog Devices Inc. AD637SD

Overview

AD637SD from Analog Devices is a high-precision, wideband true RMS-to-DC converter IC designed for accurate power and signal integrity measurement in analog signal chains. It delivers 0.02% max nonlinearity (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) - used in precision test equipment, power analyzers, and RF envelope detection.

For engineers reviewing the AD637SD datasheet, AD637SD pinout, AD637SD application, or AD637SD equivalent, key selection criteria include crest factor compensation up to CF = 10 (<1% additional error), chip select–enabled power-down (2.2 mA → 350 µA), on-chip buffer amplifier flexibility, and ±3 V to ±18 V dual-supply operation with laser-trimmed accuracy.

Technical Context

The AD637SD implements an implicit analog solution to the RMS equation V_OUT = √(V_IN²)_avg using a squarer/divider architecture with external averaging capacitor (CAV) control. Its absolute value circuit feeds current into a feedback-controlled divider loop where denominator current I₃ equals the average of squared current I₄, enabling true RMS computation without digital sampling.

It supports three operating modes: true RMS (with CAV), absolute value (no CAV, stability via 5 pF), and mean square (external denominator voltage applied to DEN INPUT). The dB output provides −3 mV/dB scaling referenced to user-programmable 0 dBV (0.1–2.0 V rms), with ±0.3 dB error over 7 mV–7 V rms input range.

Key Specifications

Pinout & Package

AD637SD is supplied in a hermetically sealed, 14-lead SBDIP package (0.300" wide, JEDEC MS-012), rated for −55°C to +125°C operation. Pin compatibility matches AD637S-grade variants; no internal connection (NIC) pins are unused and must remain unconnected.

Key Features

Applications

Equivalent & Alternatives

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

Compared with AD637SD, AD637JRZ offers identical electrical performance in a surface-mount package but sacrifices military-grade reliability and temperature range, while AD736ARZ trades bandwidth, accuracy, and feature set for integration simplicity and single-supply operation - making AD637SD the sole choice for high-fidelity, wideband, multi-mode RMS measurement in harsh environments.

Availability

AD637SD is available at Aetrix Electronics and suitable for power quality analyzers, RF test equipment, precision audio level meters, and high-voltage industrial sensor interfaces requiring stable component supply across extended temperature and long product lifecycles.

Supply support for AD637SD 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.

The AD637 product line was engineered specifically for metrology-grade RMS conversion in test and measurement, energy monitoring, and communications infrastructure - emphasizing laser-trimmed accuracy, wide bandwidth, and multi-function analog signal processing without digital overhead.

FAQ

What is the maximum input frequency supported by the AD637SD at low signal levels?

The AD637SD supports 600 kHz bandwidth at 100 mV rms input, verified per datasheet Table 1. This enables accurate RMS measurement of switching power supply ripple, PWM motor drive fundamentals, and intermediate frequency signals in communication receivers. Performance degrades gradually above this point due to 1% additional error threshold - actual usable bandwidth depends on acceptable error margin and input amplitude.

Does the AD637SD require external passive components for basic RMS operation?

Yes - the AD637SD requires only one external component for standard RMS operation: an averaging capacitor (CAV) connected to Pin 8. A 1–10 µF capacitor sets the time constant (τ = 25 ms/µF) and determines low-frequency accuracy and ripple. No resistors, trims, or op-amps are needed for baseline functionality, though optional offset/scale factor trims (R1/R3 in Figure 8) improve accuracy beyond laser-trimmed specs.

Can the AD637SD compute RMS of bipolar DC-coupled signals?

Yes - the AD637SD accepts ac or ac+dc inputs directly at VIN (Pin 13). Its absolute value circuit handles negative excursions, and the internal computation V_OUT = √(V_IN²)_avg inherently includes dc offset in the RMS result. For example, a 2 V rms sine wave superimposed on 5 V dc yields an output equal to √(2² + 5²) ≈ 5.385 V dc, matching theoretical true RMS.

How does the chip select (CS) pin affect output behavior in power-down mode?

When CS (Pin 5) is pulled below 0.2 V, the AD637SD enters standby mode: quiescent current drops from 2.2 mA to 350 µA, and RMS OUT (Pin 9) transitions to high-impedance state (100 kΩ typical). This allows multiple AD637SD outputs to be wire-OR'd without contention - essential for building wideband RMS multiplexers in automated test equipment.

Is the dB output of the AD637SD compatible with standard audio reference levels like dBu or dBV?

Yes - the AD637SD dB OUTPUT (Pin 7) is fully programmable: applying I_REF = 20 µA sets 0 dBV = 1 V rms, matching professional audio dBV standards. For dBu (0 dBu = 0.775 V rms), I_REF ≈ 25.8 µA is required. The scale factor is fixed at −3 mV/dB, and error remains within ±0.3 dB across the full 60 dB range (7 mV–7 V rms), supporting direct connection to analog meter drivers.