High-Accuracy Adjustments
The accuracy of the MX536A/MX636 can be improved
by the addition of external trims as shown in Figure 4.
R4 trims the offset. The input should be grounded and
R4 adjusted to give zero volts output from pin 6. R1 is
trimmed to give the correct value for either a calibrated
DC input or a calibrated AC signal. For example: 200mV
DC input should give 200mV DC output; a ?00mV
peak-to-peak sine-wave should give 141mV DC output.
Single-Supply Operation
Both the MX536A and the MX636 can be used with a
single supply down to +5V (Figure 5). The major limita-
tion of this connection is that only AC signals can be
measured, since the differential input stage must be
biased off ground for proper operation. The load resis-
tor is necessary to provide output sink current. The
input signal is coupled through C2 and the value cho-
sen so that the desired low-frequency break point is
obtained with the input resistance of 16.7k& for the
MX536A and 6.7k&for the MX636.
Figure 5 shows how to bias pin 10 within the range of
the supply voltage (pin 2 on H packages). It is critical
that no extraneous signals are coupled into this pin. A
capacitor connected between pin 10 and ground is
recommended. The common pin requires less than 5礎(chǔ)
of input current, and if the current flowing through resis-
tors R1 and R2 is chosen to be approximately 10 times
the common pin current, or 50礎(chǔ), the resistor values
can easily be calculated.
Choosing the Averaging Time Constant
Both the MX536A and MX636 compute the RMS value
of AC and DC signals. At low frequencies and DC, the
output tracks the input exactly; at higher frequencies,
the average output approaches the RMS value of the
input signal. The actual output differs from the ideal by
an average (or DC) error plus some amount of ripple.
The DC error term is a function of the value of C
AV
and
the input signal frequency. The output ripple is inverse-
True RMS-to-DC Converters
8  _______________________________________________________________________________________
100
0.1
0.22
0.65
1
100
1k
1
10
10
0.01
0.1
1
FREQUENCY (Hz)
10
60
1%
0.1%
Figure 3. Lower Frequency for Stated % of Reading Error and
Settling Time for Circuit shown in Figure 2
14
13
12
11
10
9
8
1
2
3
4
5
6
7
ABSOLUTE
VALUE
SQUARER
DIVIDER
CURRENT
MIRROR
BUF
V
IN
-V
S
R1
+V
S
V
OUT
C
AV
R2
R3
R4
-V
S
+V
S
MX536A
500&
365&
750k&
50k&
MX636
200&
154&
470k&
500k&
R1
R2
R3
R4
MX536A
MX636
Figure 4. Optional External Gain and Output Offset Trims
14
13
12
11
10
9
8
1
2
3
4
5
6
7
ABSOLUTE
VALUE
SQUARER
DIVIDER
CURRENT
MIRROR
BUF
V
IN
+V
S
R
L
V
OUT
C
AV
R1
R2
MX536A
20k&
10k&
1礔
MX636
20k&
39k&
3.3礔
R1
R2
C2
10k TO 1k
0.1礔
0.1礔
C2
MX536A
MX636
Figure 5. Single-Supply Operation