5962-8963701CA(2015) View Datasheet(PDF) - Analog Devices
Part Name
Description
Manufacturer
5962-8963701CA Datasheet PDF : 25 Pages
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Data Sheet
AD637
+VS
OUTPUT
OFFSET 50kΩ
ADJUST
1MΩ
–VS
1µF
V+
1 BUFF IN
BUFFER AD637
3.3MΩ 3.3MΩ
BUFF
1µF
OUT 14
3
7
AD548JN
2
4
6
FILTERED
V RMS OUTPUT
2 NIC
3 COMMON
OUTPUT
4 OFFSET
BIAS
SECTION
+VS 5 CS
4.7kΩ DEN
6 INPUT 25kΩ
dB OUTPUT
7
ABSOLUTE
VALUE
VIN 13
NIC 12
SIGNAL
INPUT
SQUARER/
DIVIDER
+VS 11 +VS
–VS 10 –VS
RMS
OUT 9
+
25kΩ
100µF
CAV 8
VIN2
V RMS
V–
6.8MΩ
1000pF
CAV1
3.3µF
499kΩ 1%
R
NOTES
1. VALUES CHOSEN TO GIVE 0.1% AVERAGING ERROR AT 1Hz.
2. NIC = NO INTERNAL CONNECTION.
Figure 21. AD637 as a Low Frequency RMS Converter
LOW FREQUENCY MEASUREMENTS
If the frequencies of the signals to be measured are below 10 Hz,
the value of the averaging capacitor required to deliver even 1%
averaging error in the standard rms connection becomes
extremely large. Figure 21 shows an alternative method of
obtaining low frequency rms measurements. Determine the
averaging time constant by the product of R and CAV1, in this
circuit, 0.5 sec/µF of CAV. This circuit permits a 20:1 reduction
in the value of the averaging capacitor, permitting the use of
high quality tantalum capacitors. It is suggested that the 2-pole,
Sallen-Key filter shown in Figure 21 be used to obtain a low
ripple level and minimize the value of the averaging capacitor.
If the frequency of interest is below 1 Hz, or if the value of
the averaging capacitor is still too large, increase the 20:1 ratio.
This is accomplished by increasing the value of R. If this is
done, it is suggested that a low input current, low offset voltage
amplifier, such as the AD548, be used instead of the internal
buffer amplifier. This is necessary to minimize the offset error
introduced by the combination of amplifier input currents and
the larger resistance.
VECTOR SUMMATION
Use two AD637s for vector summation as shown in Figure 22.
Here, the averaging capacitors are omitted (nominal 100 pF
capacitors are used to ensure stability of the filter amplifier), and
the outputs are summed as shown. The output of the circuit is
VOUT = VX 2 + VY 2
This concept can be expanded to include additional terms by
feeding the signal from Pin 9 of each additional AD637 through
a 10 kΩ resistor to the summing junction of the AD711 and
tying all of the denominator inputs (Pin 6) together.
If CAV is added to IC1 in this configuration, then the output is
VX 2 + VY 2
If the averaging capacitor is included on both IC1 and IC2, the
output is
VX 2 + VY 2
This circuit has a dynamic range of 10 V to 10 mV and is
limited only by the 0.5 mV offset voltage of the AD637.
The useful bandwidth is 100 kHz.
Rev. L | Page 19 of 25
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