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AD8436ACPZ-WP

Low Cost, Low Power, True RMS-to-DC Converter

Analog Devices 

Data Sheet

The 16 kΩ resistor in the output converts the output current to

a dc voltage that can connect to the output buffer or to the

circuit that follows. The output appears as a voltage source in

series with 16 kΩ. If a current output is desired, the resistor

connection to ground is left open and the output current is

applied to a subsequent circuit, such as the summing node of

a current summing amplifier. Thus, the core has both current

and voltage outputs, depending on the configuration. For a

voltage output with 0 Ω source impedance, use the output

buffer. The offset voltage of the buffer is 25 μV or 50 μV,

depending on the grade.

FET Input Buffer

Because the V-to-I input resistor value of the AD8436 rms core

is 8 kΩ, a high input impedance buffer is often used between

rms-to-dc converters and finite impedance sources. The optional

JFET input op amp minimizes attenuation and uncouples common

input amenities, such as resistive voltage dividers or resistors used

to terminate current transformers. The wide bandwidth of the

FET buffer is well matched to the rms core bandwidth so that

no information is lost due to serial bandwidth effects. Although

the input buffer consumes little current, the buffer supply is

independently accessible and can disconnect to reduce power.

Optional matched 10 kΩ input and feedback resistors are provided

on chip. Consult the Applications Information section to learn

how to use these resistors. The 3 dB bandwidth of the input

buffer is 2.7 MHz at 10 mV rms input and approximately 1.5 MHz

at 1 V rms. The amplifier gain and bandwidth are sufficient for

applications requiring modest gain or response enhancement to

a few hundred kilohertz (kHz), if desired. Configurations of the

input buffer are discussed in the Applications Information section.

Precision Output Buffer

The precision output buffer is a bipolar input amplifier, laser

trimmed to cancel input offset voltage errors. As with the input

buffer, the supply current is very low (<50 μA, typically), and

the power can be disconnected for power savings if the buffer is

not needed. Be sure that the noninverting input is also

disconnected from the core output (OUT) if the buffer supply

pin is disconnected. Although the input current of the buffer is

very low, a laser-trimmed 16 kΩ resistor, connected in series

with the inverting input, offsets any self-bias offset voltage.

AD8436

The output buffer can be configured as a single or two-pole low-

pass filter using circuits shown in the Applications Information

section. Residual output ripple is reduced, without affecting the

converted dc output. As the response approaches the low frequency

end of the bandwidth, the ripple rises, dependent on the value

of the averaging capacitor. Figure 27 shows the effects of four

combinations of averaging and filter capacitors. Although the

filter capacitor reduces the ripple for any given frequency, the dc

error is unaffected. Of course, a larger value averaging capacitor can

be selected, at a larger cost. The advantage of using a low-pass filter

is that a small value of filter capacitor, in conjunction with the

16 kΩ output resistor, reduces ripple and permits a smaller

averaging capacitor, effecting a cost savings. The recommended

capacitor values for operation to 40 Hz are 10 μF for averaging

and 3.3 μF for filter.

Dynamic Range

The AD8436 is a translinear rms-to-dc converter with exceptional

dynamic range. Although accuracy varies slightly more at the

extreme input values, the device still converts with no spurious

noise or dropout. Figure 25 is a plot of the rms/dc transfer function

near zero voltage. Unlike processor or other solutions, residual

errors at very low input levels can be disregarded for most

applications.

30

∆Σ OR OTHER DIGITAL

20

SOLUTIONS CANNOT

WORK AT ZERO

VOLTS

10

AD8436

SOLUTION

0

–30

–20

–10

0

10

20

30

INPUT VOLTAGE (mV DC)

Figure 25. DC Transfer Function near Zero

Rev. E | Page 11 of 21

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