Data Sheet

AD8436

Rev. B | Page 13 of 24

For simplicity, Figure 29 shows ripple vs. frequency for four

combinations of CAVG and CLPF

INPUT FREQUENCY (Hz)

1

0.0001

100

1k

10

0.001

0.01

0.1

CAVG = 1µF, CLPF = 0.33µF

CAVG = 10µF, CLPF = 3.3µF

CAVG = 10µF, CLPF = 0.33µF

CAVG = 1µF, CLPF = 3.3µF

AC INPUT = 300mV rms

Figure 29. Residual Ripple Voltage for Various Filter Configurations

Figure 30 shows the effects of averaging and post-rms filter

capacitors on transition and settling times using a 10-cycle,

50 Hz, 1 second period burst signal input to demonstrate time-

domain behavior. In this instance, the averaging capacitor value

was 10 µF, yielding a ripple value of 6 mV rms. A postconversion

capacitor (CLPF) of 0.68 μF reduced the ripple to 1 mV rms. An

averaging capacitor value of 82 μF reduced the ripple to 1 mV

but extended the transition time (and cost) significantly.

INPUT

50Hz 10 CYCLE BURST

400mv/DIV

CAVG = 10µF FOR BOTH PLOTS,

BUT RED PLOT HAS NO LOW-PASS FILTER,

GREEN PLOT HAS CLPF = 0.68µF

10mV/DIV

TIME (100ms/DIV)

CAVG = 82µF

Figure 30. Effects of Various Filter Options on Transition Times

CAVG Capacitor Styles

When selecting a capacitor style for CAVG there are certain

tradeoffs.

For general usage, such as most DMM or power measurement

applications where input amplitudes are typically greater than

1 mV, surface mount tantalums are the best overall choice for

space, performance, and economy.

For input amplitudes less than around a millivolt, low dc leakage

capacitors, such as film or X8L MLCs, maintain rms conversion

accuracy. Metalized polyester or similar film styles are best, as

long as the temperature range is appropriate. X8L grade MLCs are

rated for high temperatures (125°C or 150°C), but are available only

up to 10 μF. Never use electrolytic capacitors, or X7R or lower grade

ceramics.

Basic Core Connections

Many applications require only a single external capacitor for

averaging. A 10 µF capacitor is more than adequate for acceptable

rms errors at line frequencies and below.

The signal source sees the input 8 kΩ voltage-to-current conversion

resistor at Pin RMS; thus, the ideal source impedance is a

voltage source (0 Ω source impedance). If a non-zero signal source

impedance cannot be avoided, be sure to account for any series

connected voltage drop.

An input coupling capacitor must be used to realize the near-zero

output offset voltage feature of the AD8436. Select a coupling

capacitor value that is appropriate for the lowest expected

operating frequency of interest. As a rule of thumb, the input

coupling capacitor can be the same as or half the value of the

averaging capacitor because the time constants are similar. For

a 10 μF averaging capacitor, a 4.7 μF or 10 μF tantalum capacitor

is a good choice (see Figure 31).

2

RMS

9

OUT

AD8436

11

IGND

19

CAVG

10

VEE

–5V

8

OGND

17

VCC

4.7µF

OR 10µF

+*

+5V

10µF

CAVG

+*

*FOR POLARIZED CAPACITOR STYLES.

Figure 31. Basic Applications Circuit

Using a Capacitor for High Crest Factor Applications

The AD8436 contains a unique feature to reduce large crest

factor errors. Crest factor is often overlooked when considering

the requirements of rms-to-dc converters, but it is very

important when working with signals with spikes or high peaks.

The crest factor is defined as the ratio of peak voltage to rms.

See Table 5 for crest factors for some common waveforms.

2

RMS

9

OUT

AD8436

11

IGND

19

CAVG

18

CCF

10

VEE

–5V

8

OGND

17

VCC

4.7µF

OR 10µF

+*

+5V

10µF

CAVG

+*

0.1µF

CCF

*FOR POLARIZED CAPACITOR STYLES.

Figure 32. Connection for Additional Crest Factor Performance

Crest factor performance is mostly applicable for unexpected

waveforms such as switching transients in switchmode power

supplies. In such applications, most of the energy is in these

peaks and can be destructive to the circuitry involved, although

the average ac value can be quite low.