AD8436 AD [Analog Devices], AD8436 Datasheet - Page 13

• Manufacturer Part Number: AD8436
• Description: Low Cost, Low Power
• Manufacturer: AD [Analog Devices]

For simplicity, Figure 28 shows ripple vs. frequency for four
combinations of CAVG and CLPF
Figure 29 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 .068 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.
Capacitor Construction
Although tolerant of most capacitor styles, rms conversion
accuracy can be affected by the type of capacitor that is selected.
Capacitors with low dc leakage yield best all around performance,
and many sources are available. Metalized polyester or similar
film styles are best, as long as the temperature range is appropriate.
For practical applications such as the rms-to-dc function in
DMMs or power monitoring circuits, surface mount tantalums
are the best over-all choice.
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.
Figure 28. Residual Ripple Voltage for Various Filter Configurations
Figure 29. Effects of Various Filter Options on Transition Times
0.0001
0.001
0.01
0.1
1
10
INPUT
50Hz 10 CYCLE BURST
400mV/DIV
CAVG = 10µF FOR BOTH
PLOTS, BUT RED PLOT HAS
NO LOW-PASS FILTER, GREEN
PLOT HAS CLPF = 68nF 100mV/DIV
TIME (100ms/DIV)
INPUT FREQUENCY (Hz)
AC INPUT = 300mV rms
100
CAVG = 1µF, CLPF = 0.33µF
CAVG = 1µF, CLPF = 3.3µF
CAVG = 10µF, CLPF = 0.33µF
CAVG = 10µF, CLPF = 3.3µF
CAVG = 82µF
1k
Rev. 0 | Page 13 of 20
The signal source sees the input 8 kΩ voltage-to-current conversion
resistor at Pin 2 (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 30).
Using a Capacitor for High Crest Factor Applications
The
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.
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.
Figure 13 shows the effects of an additional crest factor
capacitor of 0.1 μF and an averaging capacitor of 10 μF. The
larger capacitor serves to average the energy over long spaces
between pulses, while the CCF capacitor charges and holds the
energy within the relatively narrow pulse.
AD8436
Figure 31. Connection for Additional Crest Factor Performance
contains a unique crest factor feature. Crest factor
4.7µF
4.7µF
10µF
10µF
OR
OR
Figure 30. Basic Applications Circuit
2
2
CAVG CCF
RMS
CAVG
IGND
RMS
IGND
19
11
19
11
AD8436
AD8436
CAVG
CAVG
10µF
–5V
10µF
–5V
VEE
18
VEE
10
10
0.1µF
CCF
OGND
OGND
VCC
VCC
OUT
OUT
17
8
17
8
+5V
+5V
9
9
AD8436