Low Cost, Low Power,

True RMS-to-DC Converter

AD8436

Rev. 0

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FEATURES

Computes true rms value instantly

Accuracy: ±10 μV ± 0.5% of reading

Wide dynamic input range

100 μV rms to 3 V rms (8.5 V p-p) full-scale input range

Larger inputs with external scaling

Wide bandwidth:

1 MHz for −3 dB (300 mV)

65 kHz for additional 1% error

Zero converter dc output offset

No residual switching products

Specified at 300 mV rms input

Accurate conversion with crest factors up to 10

Low power: 300

µA typical at ±2.4 V

Fast settling at all input levels

High-Z FET separately powered input buffer

≤ 2 pF

Precision dc output buffer

Wide supply range

Dual: ±2.4 V to ±18 V

Single: 4.8 V to 36 V

Small size: 4 mm × 4 mm package

ESD protected

FUNCTIONAL BLOCK DIAGRAM

VEE

CAVG

VCC

IGND

OUT

IBUFGN

RMS

IBUFOUT

IBUFIN+

SUM

8kΩ

100kΩ

16kΩ

100kΩ

OGND

10kΩ

10kΩ

FET OP AMP

10pF

IBUFIN–

OBUFOUT

OBUFIN+

OBUFIN–

16kΩ

DC BUFFER

AD8436

–

+

RMS CORE

–

+

CCF

Figure 1.

GENERAL DESCRIPTION

The AD8436 is a new generation, translinear precision, low power,

true rms-to-dc converter that is loaded with options. It computes

a precise dc equivalent of the rms value of ac waveforms, including

complex patterns such as those generated by switchmode power

supplies and triacs. Its accuracy spans a wide range of input levels

(see Figure 2) and temperatures. The ensured accuracy of ≤±0.5%

and ≤10 μV output offset result from the latest Analog Devices,

Inc., technology. The crest factor error is <0.5% for CF values

between 1 and 10.

The AD8436 delivers instant true rms results at less cost than

misleading peak, averaging, or digital solutions. There is no

programming expense or processor overhead to consider, and the

4 mm × 4 mm package easily fits into those tight applications.

On-board buffer amplifiers enable the widest range of options

for any rms-to-dc converter available, regardless of cost. For

minimal applications, only a single external averaging capacitor

is required. The built-in high impedance FET buffer provides an

interface for external attenuators, frequency compensation, or

driving low impedance loads. A matched pair of internal resistors

enables an easily configurable gain-of-two or more, extending

the usable input range even lower. The low power, precision input

buffer makes the AD8436 attractive for use in portable multi-

meters and other battery-powered applications. The precision

dc output buffer offers extremely low offset voltages, thanks to

bias current cancellation.

Unlike digital solutions, the AD8436 has no switching circuitry

limiting performance at high or low amplitudes (see Figure 2).

A usable response of <100 μV and >3 V extends the dynamic

range with no external scaling, accommodating the most

demanding low signal conditions.

1mV

10mV

1V

100mV

AD8436

GREATER INPUT DYNAMIC RANGE

100µV

3V

∆Σ SOLUTION

Figure 2. Usable Dynamic Range of the AD8436 vs.

ΔΣ

The AD8436 operates from single or dual supplies of ±2.4 V

(4.8 V) to ±18 V (36 V). A and J grades are available in a compact

4 mm × 4 mm, 20-lead chip-scale package. The operating

temperature ranges are −40°C to 125°C and 0°C to 70°C.