Datasheet

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
10033-026
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.
RMS ERROR (%)
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
10033-027
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.)
10033-028
CONVERSION ERROR (%)
FREQUENCY (Hz)
–0.5
–1.5
0
–1.0
–2.0
1k10010
2
SEE
TEXT
CAVG = 0.22µF
0.47µF
2.2µF
4.7µF
10µF
22µF
50µF
1µF
Figure 28. Conversion Error vs. Frequency for Various Values of CAVG