ANALOG DEVICES AD8436 Service Manual

Low Cost, Low Power,

G

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

R

≥ 1012 Ω, CIN ≤ 2 pF

IN

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

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-

Rev. 0

Information furnished by Analog Devices is believed to be accurate and reliable. However, no
responsibility is assumed by Anal og Devices for its use, nor for any infringements of patents or ot her
rights of third parties that may result from its use. Specifications subject to change without notice. No
license is granted by implication or otherwise under any patent or patent rights of Analog Devices.
Trademarks and registered trademarks are the property of their respective owners.

True RMS-to-DC Converter

AD8436

FUNCTIONAL BLOCK DIAGRAM

CAV

CCF

VCC

SUM

RMS

IBUFGN

IBUFIN–

IBUFIN+

OBUFIN+

OBUFIN–

8k

10k 10k

16k

RMS CORE

10pF

–

+

+

–

Figure 1.

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.

GREATER INPUT DYNAMIC RANGE

AD8436

 SOLUTION

100µV 3V

1mV 10mV 1V100mV

Figure 2. Usable Dynamic Range of the AD8436 vs.

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.

One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
Tel: [phone] www.analog.com
Fax: [phone] ©2011 Analog Devices, Inc. All rights reserved.

100k

100k

VEE

FET OP AMP

DC BUFFER

AD8436

16k

IGND

OGND

OUT

IBUFOUT

OBUFOUT

ΔΣ

10033-001

10033-002

AD8436

TABLE OF CONTENTS

Features.............................................................................................. 1

Functional Block Diagram .............................................................. 1

General Description ......................................................................... 1

Revision History ............................................................................... 2

Specifications..................................................................................... 3

Absolute Maximum Ratings............................................................ 4

Thermal Resistance ...................................................................... 4

ESD Caution.................................................................................. 4

Pin Configuration and Function Descriptions............................. 5

Typical Performance Characteristics ............................................. 6

REVISION HISTORY

7/11—Revision 0: Initial Version

Test Circuits........................................................................................9

Theory of Operation ...................................................................... 10

Overview ..................................................................................... 10

Applications Information.............................................................. 12

Using the AD8436....................................................................... 12

AD8436 Evaluation Board......................................................... 16

Outline Dimensions....................................................................... 18

Ordering Guide .......................................................................... 18

Rev. 0 | Page 2 of 20

AD8436

SPECIFICATIONS

eIN = 300 mV ac (rms), frequency = 1 kHz sinusoidal, ac-coupled, ±VS = ±5 V, TA = 25°C, C

Table 1.

Parameter Test Conditions/Comments Min Typ Max Unit

RMS CORE

Conversion Error Default conditions ±10 − 0.5 ±0 ± 0 ±10 + 0.5 μV/% rdg

Vs. Temperature −40°C < T < 125 C 0.006 %/°C

Vs. Rail Voltage ±2.4 V to ±18 V ±0.013 ±%/V
Input Offset Voltage DC-coupled −500 0 +500 μV
Output Offset Voltage Default conditions, ac-coupled input 0 V

Vs. Temperature −40 C < T < 125°C 0.3 μV/°C
DC Reversal Error DC-coupled, VIN = ±300 mV ±0.5 ±2 %
Nonlinearity eIN = 10 mV to 300 mV ac (rms) 0.05 %
Crest Factor Error Additional error

1 < CF < 10 CCF = 0.1 μF −0.5 +0.5 %
Peak Input Voltage −VS − 0.7 +VS + 0.7 V
Input Resistance 7.92 8 8.08 kΩ
Frequency Response VIN = 300 mV rms

1% Additional Error 65 kHz

3 dB Bandwidth 1 MHz
Settling Time

0.1% Rising/falling 148/341 ms

0.01% Rising/falling 158/350 ms
Output Resistance 15.68 16 16.32 kΩ
Supply Current No input 325 400 μA

INPUT BUFFER

Signal Voltage Swing G = 1

Input AC- or dc-coupled −VS +VS V

Output AC-coupled to Pin RMS −VS + 0.2 +VS − 0.2 mV
Offset Voltage −1 0 +1 mV
Input Bias Current 50 pA
Input Resistance 1012 Ω
Frequency Response

0.1 dB 950 kHz

3 dB Bandwidth 2.1 MHz
Supply Current 100 160 200 μA
Optional Gain Resistor −9.9 +10 +10.1 kΩ
Gain Error G = ×1 0.05 %

OUTPUT BUFFER

Offset Voltage Connected to Pin OUT −200 0 +200 μV
Input Current 3 nA
Output Voltage Swing −VS + 0.0005 +VS − 1 V
Gain Error 0.003 0.01 %
Supply Current 40 70 μA

SUPPLY VOLTAGE

Dual ±2.4 ±18 V
Single 4.8 36 V

= 10 µF, unless otherwise specified.

AVG

Rev. 0 | Page 3 of 20

AD8436

ABSOLUTE MAXIMUM RATINGS

Table 2.

Parameter Rating

Supply Voltage ±18 V
Internal Power Dissipation 18 mW
Input Voltage ±VS
Output Short-Circuit Duration Indefinite
Differential Input Voltage +VS and −VS
Temperature

Operating Range −40°C to +125°C
Storage Range −65°C to +125°C
Lead Soldering (60 sec) 300°C

ESD Rating 2 kV

Stresses above those listed under Absolute Maximum Ratings
may cause permanent damage to the device. This is a stress
rating only; functional operation of the device at these or any
other conditions above those indicated in the operational
section of this specification is not implied. Exposure to absolute
maximum rating conditions for extended periods may affect
device reliability.

THERMAL RESISTANCE

θJA is specified for the worst-case conditions, that is, a device
soldered in a circuit board for surface-mount packages.

Table 3. Thermal Resistance

Package Type θJA Unit

CP-20-10 LFCSP Without Thermal Pad 86 °C/W
CP-20-10 LFCSP With Thermal Pad 48 °C/W

ESD CAUTION

Rev. 0 | Page 4 of 20

AD8436

VCC

2

M

PIN CONFIGURATION AND FUNCTION DESCRIPTIONS

SU

CAVG

CCF

1

DNC

PIN 1

RMS

IBUFOUT

IBUFIN–

IBUFIN+

5

IBUFGN

NOTES

1. DNC = DO NOT CONNECT. DO NOT CONNECT TO THIS PIN.
. THE EXPOSED PAD SHOULD NOT BE CONNECTED.

INDICATOR

AD8436

TOP VIEW

(Not to S cale)

OGND

Figure 3. Pin Configuration, Top View

Table 4. Pin Function Descriptions

Pin No. Mnemonic Description

1 DNC Do Not Connect. Used for factory test.
2 RMS AC Input to the RMS Core.
3 IBUFOUT Output Connection for the FET Input Buffer Amplifier.
4 IBUFIN– Inverting Input to the FET Input Buffer Amplifier.
5 IBUFIN+ Noninverting Input to the FET Input Buffer Amplifier.
6 IBUFGN Optional 10 kΩ Precision Gain Resistor.
7 DNC Do Not Connect. Used for factory test.
8 OGND

Internal 16 kΩ Current-to-Voltage Resistor. Connect to ground for voltage output at Pin 9; leave unconnected

for current output at Pin 9.
9 OUT Voltage or Current Output of the RMS Core.
10 VEE Negative Supply Rail.
11 IGND Half Supply Node. Leave open for single-supply operation.
12 OBUFIN+ Noninverting Input of the Optional Precision Output Buffer. OBUFIN+ is typically connected to OUT.
13 OBUFIN− Inverting Input of the Optional Precision Output Buffer. OBUFIN− is typically connected to OBUFOUT.
14 OBUFOUT Low Impedance Output for ADC or Other Loads.
15 OBUFV+ Power Pin for the Output Buffer.
16 IBUFV+ Power Pin for the Input Buffer.
17 VCC Positive Supply Rail for the RMS Core.
18 CCF Connection for Crest Factor Capacitor.
19 CAVG Connection for Averaging Capacitor.
20 SUM Summing Amplifier Input Node. An external resistor can be connected for custom scaling.
EP DNC Exposed Pad. The exposed pad should not be connected.

IBUFV+

1620

15

OBUFV+

OBUFOUT

OBUFIN–

OBUFIN+

IGND

11

106

VEE

OUTDNC

10033-003

Rev. 0 | Page 5 of 20

AD8436

V

TYPICAL PERFORMANCE CHARACTERISTICS

TA = 25°C, ±VS = ±5 V, C

5V

= 10 µF, 1 kHz sine wave, unless otherwise indicated.

AVG

5

1V

100mV

10mV

INPUT LEVEL (V rms)

1mV

100µV

50µV

100 1k

FREQUENCY (Hz)

Figure 4. RMS Core Frequency Response (See Figure 20)

5V

1V

100mV

10mV

INPUT LEVEL (V rms)

1mV

100µV

50µV

100 1k

FREQUENCY ( Hz)

Figure 5. RMS Core Frequency Response with V

3dB BW

100k10k

3dB BW

VS = ±2.4V

100k10k

= ±2.4 V (See Figure 20)

S

5M1M50

1V

100mV

10mV

INPUT LEVEL (V rms)

1mV

100µV

50µV

5M1M50

10033-004

100 1k

FREQUENCY (Hz)

Figure 7. RMS Core Frequency Response with V

15

e

= 3.5mV rms

IN

12

9

6

3

0

GAIN (dB)

–3

–6

–9

–12

–15

100 1k

10033-005

FREQUENCY (Hz)

100k10k

3dB BW

VS = 4.8V

100k10k

= +4.8 V (See Figure 21)

S

5M1M50

5M1M

10033-007

10033-008

Figure 8. Input Buffer, Small Signal Bandwidth at 0 dB and 6 dB Gain

5V

1V

100mV

10mV

INPUT LEVEL (V rms)

1mV

100µV

50µV

100 1k

FREQUENCY ( Hz)

Figure 6. RMS Core Frequency Response with V

3dB BW

VS = ±15V

100k10k

= ±15 V (See Figure 20)

S

5M1M50

10033-006

15

e

= 300mV rms

IN

12

9

6

3

0

GAIN (dB)

–3

–6

–9

–12

–15

100 1k

FREQUENCY (Hz)

100k10k

5M1M

Figure 9. Input Buffer, Large Signal Bandwidth at 0 dB and 6 dB Gain

10033-009

Rev. 0 | Page 6 of 20