AD8436

Data Sheet

Rev. B | Page 12 of 24

APPLICATIONS INFORMATION

USING THE AD8436

This section describes the power supply and feature options,

as well as the function and selection of averaging and filter

capacitor values. Averaging and filtering options are shown

graphically and apply to all circuit configurations.

Averaging Capacitor Considerations—RMS Accuracy

Typical AD8436 applications require only a single external

capacitor (CAVG) connected to the CAVG pin (see Figure 31).

The function of the averaging capacitor is to compute the mean

(that is, average value) of the sum of the squares. Averaging

(that is, integration) follows the rms core, where the input

current is squared. The mean value is the average value of the

squared input voltage over several input waveform periods.

The rms error is directly affected by the number of periods

averaged, as is the resultant peak-to-peak ripple.

The result of the conversion process is a dc component and a

ripple component whose frequency is twice that of the input. The

rms conversion accuracy depends on the value of CAVG, so the

value selected need only be large enough to average enough periods

at the lowest frequency of interest to yield the required rms

accuracy.

Figure 28 is a plot of rms error vs. frequency for various averaging

capacitor values. To use Figure 28, simply locate the frequency

of interest and acceptable rms error on the horizontal and

vertical scales, respectively. Then choose or estimate the next

highest capacitor value adjacent to where the frequency and

error lines intersect (for an example, see the orange circle in

Figure 28).

Post Conversion Ripple Reduction Filter

Input rectification included in the AD8436 introduces a residual

ripple component that is dependent on the value of CAVG and

twice the input signal frequency for symmetrical input wave-

forms. For sampling applications such as a high resolution ADC,

the ripple component may cause one or more LSBs to cycle, and

low value display numerals to flash.

Ripple is reduced by increasing the value of the averaging capacitor,

or by postconversion filtering. Ripple reduction following

conversion is far more efficient because the ripple average value

has been converted to its rms value. Capacitor values for post-

conversion filtering are significantly less than the equivalent

averaging capacitor value for the same level of ripple reduction.

This approach requires only a single capacitor connected to the

OUT pin (see Figure 26). The capacitor value correlates to the

simple frequency relation of ½ π R-C, where R is fixed at 16 kΩ.

OUT

16kΩ

OGND

CORE

CLPF

DC OUTPUT

9

8

Figure 26. Simple One-Pole Post Conversion Filter

As seen in Figure 27, CAVG alone determines the rms error,

and CLPF serves purely to reduce ripple. Figure 27 shows a

constant rms error for CLPF values of 0.33 µF and 3.3 µF; only

the ripple is affected.

FREQUENCY (Hz)

1

0

100

1k

–1

–2

10

–3

–4

–5

–6

–7

–8

–9

–10

CAVG = 10µF

CLPF = 0.33µF OR 3.3µF

CAVG = 1µF

CLPF = 0.33µF OR 3.3µF

Figure 27. RMS Error vs. Frequency for Two Values of CAVG and CLPF

(Note that only CAVG value affects rms error; CLPF has no effect.)

FREQUENCY (Hz)

–0.5

–1.5

0

–1.0

–2.0

1k

100

10

2

SEE

TEXT

CAVG = 0.22µF

50µ

F

Figure 28. Conversion Error vs. Frequency for Various Values of CAVG