ad8436acpz-wp Analog Devices, Inc., ad8436acpz-wp Datasheet - Page 12

• Manufacturer Part Number: ad8436acpz-wp
• Description: Low Cost, Low Power, True Rms-to-dc Converter
• Manufacturer: Analog Devices, Inc.

AD8436
APPLICATIONS INFORMATION
USING THE
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
capacitor (CAVG) connected to the CAVG pin (see Figure 30).
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 absolute value circuit, where
the polarity of negative input current components is reversed
(rectified) prior to squaring. 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 27 is a plot of rms error vs. frequency for various
averaging capacitor values. For Figure 27, the additional error
was 0.001% at 40 Hz using a 10 μF metalized polyester capacitor.
Larger values yield diminished returns because the settling time
increases with negligible improvement in rms accuracy.
To use Figure 27, determine the minimum operating frequency
and accuracy of the application and then find the suggested
capacitor value on the chart. For example, for –0.5% rms at 100 Hz,
the capacitor value is 1 μF.
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 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.
AD8436
AD8436
applications require only a single external
–0.5
–1.0
–1.5
–2.0
0
10
22µF
10µF
4.7µF
47µF
2.2µF
Figure 27. Conversion Error vs. Frequency for Various Values of CAVG
1µF
Rev. 0 | Page 12 of 20
FREQUENCY (Hz)
100
0.47µF
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 25). The capacitor value correlates to the
simple frequency relation of ½ π R-C, where R is fixed at 16 kΩ.
As seen in Figure 26, CAVG alone determines the rms error,
and CLPF serves purely to reduce ripple. Figure 26 shows a
constant rms error for CLPF values of 0.33 μF and 3.3 μF; only
the ripple is affected.
–10
–1
–2
–3
–4
–5
–6
–7
–8
–9
Figure 26. RMS Error vs. Frequency for Two Values of CAVG and CLPF
1
0
CAVG = 0.22µF
10
Figure 25. Simple One-Pole Post Conversion Filter
note that CLPF does not affect rms error result.)
CAVG = 10µF
CLPF = 0.33µF OR 3.3µF
(Compare the effects of CAVG and CLPF, and
CORE
CAVG = 1µF
CLPF = 0.33µF OR 3.3µF
OGND
8
FREQUENCY (Hz)
16kΩ
OUT
9
100
CLPF
1k
DC OUTPUT
1k