AD536A Analog Devices, AD536A Datasheet - Page 4

• Manufacturer Part Number: AD536A
• Description: Integrated Circuit True RMS-to-DC Converter
• Manufacturer: Analog Devices

AD536A
The input and output signal ranges are a function of the supply
voltages; these ranges are shown in Figure 14. The AD536A can
also be used in an unbuffered voltage output mode by discon-
necting the input to the buffer. The output then appears unbuf-
fered across the 25 kΩ resistor. The buffer amplifier can then be
used for other purposes. Further the AD536A can be used in a
current output mode by disconnecting the 25 kΩ resistor from
ground. The output current is available at Pin 8 (Pin 10 on the
“H” package) with a nominal scale of 40 µA per volt rms input
positive out.
OPTIONAL EXTERNAL TRIMS FOR HIGH ACCURACY
If it is desired to improve the accuracy of the AD536A, the
external trims shown in Figure 2 can be added. R4 is used to
trim the offset. Note that the offset trim circuit adds 365 Ω in
series with the internal 25 kΩ resistor. This will cause a 1.5%
increase in scale factor, which is trimmed out by using R1 as
shown. Range of scale factor adjustment is ± 1.5%.
The trimming procedure is as follows:
1. Ground the input signal, V
2. Connect the desired full scale input level to V
The major advantage of external trimming is to optimize device
performance for a reduced signal range; the AD536A is inter-
nally trimmed for a 7 V rms full-scale range.
SINGLE SUPPLY CONNECTION
The applications in Figures l and 2 require the use of approxi-
mately symmetrical dual supplies. The AD536A can also be
used with only a single positive supply down to +5 volts, as
shown in Figure 3. The major limitation of this connection is
that only ac signals can be measured since the differential input
stage must be biased off ground for proper operation. This
biasing is done at Pin 10; thus it is critical that no extraneous
signals be coupled into this point. Biasing can be accomplished
Figure 2. Optional External Gain and Output Offset Trims
volts output from Pin 6. Alternatively, R4 can be adjusted to
give the correct output with the lowest expected value of V
a calibrated ac signal (1 kHz is the optimum frequency);
then trim R1, to give the correct output from Pin 6, i.e.,
1000 V dc input should give 1.000 V dc output. Of course, a
± 1.000 V peak-to-peak sine wave should give a 0.707 V dc
output. The remaining errors, as given in the specifications
are due to the nonlinearity.
IN
, and adjust R4 to give zero
IN
, either dc or
IN
.
–4–
by using a resistive divider between +V
of the resistors can be increased in the interest of lowered power
consumption, since only 5 mA of current flows into Pin 10
(Pin 2 on the “H” package). AC input coupling requires only
capacitor C2 as shown; a dc return is not necessary as it is
provided internally. C2 is selected for the proper low frequency
break point with the input resistance of 16.7 kΩ; for a cutoff at
10 Hz, C2 should be 1 µF. The signal ranges in this connection
are slightly more restricted than in the dual supply connection.
The input and output signal ranges are shown in Figure 14. The
load resistor, R
CHOOSING THE AVERAGING TIME CONSTANT
The AD536A will compute the rms of both ac and dc signals.
If the input is a slowly-varying dc signal, the output of the
AD536A will track the input exactly. At higher frequencies, the
average output of the AD536A will approach the rms value of
the input signal. The actual output of the AD536A will differ
from the ideal output by a dc (or average) error and some
amount of ripple, as demonstrated in Figure 4.
The dc error is dependent on the input signal frequency and the
value of C
value of C
given frequency using the standard rms connection.
The ac component of the output signal is the ripple. There are
two ways to reduce the ripple. The first method involves using a
large value of C
C
reduction in ripple. When measuring waveforms with high crest
Figure 4. Typical Output Waveform for Sinusoidal Input
AV
, a tenfold increase in this capacitance will affect a tenfold
AV
AV
. Figure 5 can be used to determine the minimum
Figure 3. Single Supply Connection
which will yield a given percent dc error above a
C2
L
AV
, is necessary to provide output sink current.
. Since the ripple is inversely proportional to
S
and ground. The values
REV. B