Data Sheet
FEATURES
Converts an ac voltage waveform to a dc voltage and then
converts to the true rms, average rectified, or absolute value
200 mV rms full-scale input range (larger inputs with input
attenuator)
High input impedance: 10
12
Ω
Low input bias current: 25 pA maximum
High accuracy: ±0.3 mV ± 0.3% of reading
RMS conversion with signal crest factors up to 5
Wide power supply range: +2.8 V, −3.2 V to ±16.5 V
Low power: 200 µA maximum supply current
Buffered voltage output
No external trims needed for specified accuracy
Related device: the
a power-down control
with standby current of only 25 μA; the dc output voltage
is negative and the output impedance is 8 kΩ
Low Cost, Low Power,
True RMS-to-DC Converter
AD736
FUNCTIONAL BLOCK DIAGRAM
CC 8kΩ
+VS
OUT
VIN
FULL WAVE
RECTIFIER
RMS
CORE
8kΩ
CF
CAV
CF
(OPT)
BIAS
SECTION
COM
–VS
CAV
Figure 1.
The AD736 allows the choice of two signal input terminals: a
high impedance FET input (10
12
Ω) that directly interfaces with
High-Z input attenuators and a low impedance input (8 kΩ) that
allows the measurement of 300 mV input levels while operating
from the minimum power supply voltage of +2.8 V, −3.2 V. The
two inputs can be used either single ended or differentially.
The AD736 has a 1% reading error bandwidth that exceeds
10 kHz for the input amplitudes from 20 mV rms to 200 mV rms
while consuming only 1 mW.
The AD736 is available in four performance grades. The
AD736J and AD736K grades are rated over the 0°C to +70°C
and −20°C to +85°C commercial temperature ranges. The
AD736A and AD736B grades are rated over the −40°C to +85°C
industrial temperature range. The AD736 is available in three
low cost, 8-lead packages: PDIP, SOIC, and CERDIP.
GENERAL DESCRIPTION
The AD736 is a low power, precision, monolithic true rms-to-
dc converter. It is laser trimmed to provide a maximum error of
±0.3 mV ± 0.3% of reading with sine wave inputs. Furthermore,
it maintains high accuracy while measuring a wide range of
input waveforms, including variable duty-cycle pulses and triac
(phase)-controlled sine waves. The low cost and small size of
this converter make it suitable for upgrading the performance
of non-rms precision rectifiers in many applications. Compared
to these circuits, the AD736 offers higher accuracy at an equal
or lower cost.
The AD736 can compute the rms value of both ac and dc input
voltages. It can also be operated as an ac-coupled device by
adding one external capacitor. In this mode, the AD736 can
resolve input signal levels of 100 μV rms or less, despite variations
in temperature or supply voltage. High accuracy is also maintained
for input waveforms with crest factors of 1 to 3. In addition,
crest factors as high as 5 can be measured (introducing only 2.5%
additional error) at the 200 mV full-scale input level.
The AD736 has its own output buffer amplifier, thereby pro-
viding a great deal of design flexibility. Requiring only 200 µA
of power supply current, the AD736 is optimized for use in
portable multimeters and other battery-powered applications.
PRODUCT HIGHLIGHTS
1. The AD736 is capable of computing the average rectified
value, absolute value, or true rms value of various input signals.
2. Only one external component, an averaging capacitor, is
required for the AD736 to perform true rms measurement.
3. The low power consumption of 1 mW makes the AD736
suitable for many battery-powered applications.
4. A high input impedance of 10
12
Ω eliminates the need for an
external buffer when interfacing with input attenuators.
5. A low impedance input is available for those applications that
require an input signal up to 300 mV rms operating from low
power supply voltages.
Rev. I
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