AD536A Data Sheet
Rev. F | Page 12 of 15
C
AV
–V
S
IN
V
OUT
+V
S
14
13
12
11
10
9
8
1
2
3
4
5
6
7
–V
S
+V
S
AD536A
25kΩ
25kΩ
ABSOLUTE
VALUE
SQUARER/
DIVIDER
CURRENT
MIRROR
R4
50kΩ
OFFSET
ADJUST
R3
750kΩ
R2
365kΩ
00504-007
BUF
NC
–V
S
C
AV
+V
S
NC
NC
NC
dB
COM
BUF OUT
R
L
BUF IN
I
OUT
SCALE
FACTOR
ADJUST
R1
500kΩ
Figure 16. Optional External Gain and Output Offset Trims
SINGLE-SUPPLY OPERATION
Refer to Figure 17 for single supply-rail configurations between
5 V and 36 V. When powered from a single supply, the input
stage (VIN pin) is internally biased at a voltage between ground
and the supply, and the input signal ac coupled. Biasing the
device between the supply and ground is simply a matter of
connecting the COM pin to an external resistor divider and
bypassing to ground. The resistor values are large, minimizing
power consumption, as the COM pin current is only 5 μA.
Note that the 10 kΩ and 20 kΩ resistors connected to the COM pin
(Figure 17) are asymmetrical, that is, the voltage at the COM pin is
1/3 of the supply. This ratio of input bias to supply is optimum
for the precision rectifier (aka absolute value circuit) input
circuit employed for rectifying ac input waveforms and ensures
full input symmetry for low signal voltages.
Capacitor C2 is required for AC input coupling, however an
external dc return is unnecessary because biasing occurs
internally. SelectC2 for the desired low frequency breakpoint
using an input resistance of 16.7 kΩ for the 1/ωRC calculation;
C2 = 1 μF for a cutoff at 10 Hz. Figure 11 and Figure 12 show
the input and output signal ranges for dual and single supply
configurations, respectively. The load resistor, RL, provides a
path to sink output sink current when an input signal is
disconnected.
C
AV
V
IN
V
OUT
+V
S
14
13
12
11
10
9
8
1
2
3
4
5
6
7
AD536A
25kΩ
ABSOLUTE
VALUE
SQUARER/
DIVIDER
CURRENT
MIRROR
C2
1µF
NONPOLARIZED
R
L
0.1µF
20kΩ
10kΩ
0.1µF
10kΩ
TO
1kΩ
00504-008
V
IN
NC
–V
S
C
AV
+V
S
NC
NC
NC
dB
COM
BUF OUT
R
L
BUF IN
I
OUT
BUF
Figure 17. Single-Supply Connection
CHOOSING THE AVERAGING TIME CONSTANT
The AD536A computes the rms of both ac and dc signals. If the
input is a slowly varying dc signal, the output of the AD536A
tracks the input exactly.
At higher frequencies, the average output of the AD536A
approaches the rms value of the input signal. The actual output
of the AD536A differs from the ideal output by a dc (or average)
error and some amount of ripple, as shown in Figure 18.
DC ERROR = E
O
– E
O
(IDEAL)
IDEAL E
O
DOUBLE FREQUENCY
RIPPLE
AVERAGE E
O
– E
O
E
O
TIME
00504-009
Figure 18. Typical Output Waveform for Sinusoidal Input
The dc error is dependent on the input signal frequency and
the value of C
AV
. Use Figure 19 to determine the minimum value
of C
AV
, which yields a given percentage of dc error above a 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
AV
. Because the ripple is inversely proportional
to C
AV
, a tenfold increase in this capacitance affects a tenfold
reduction in ripple.
When measuring waveforms with high crest factors, such as low
duty cycle pulse trains, the averaging time constant should be at
least 10 times the signal period. For example, a 100 Hz pulse
rate requires a 100 ms time constant, which corresponds to a
4 μF capacitor (time constant = 25 ms per μF).
