Datasheet

Data Sheet AD8436
Rev. B | Page 13 of 24
For simplicity, Figure 29 shows ripple vs. frequency for four
combinations of CAVG and CLPF
RIPPLE ERROR (V p-p)
INPUT FREQUENCY (Hz)
1
0.0001
100 1k10
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
10033-029
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.
10033-130
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).
10033-131
2
RMS
9
OUTAD8436
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.
10033-132
2
RMS
9
OUTAD8436
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.