MX536A Maxim Integrated Products, MX536A Datasheet - Page 8

• Manufacturer Part Number: MX536A
• Description: True RMS-to-DC Converters
• Manufacturer: Maxim Integrated Products

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 ±200mV
peak-to-peak sine-wave should give 141mV DC output.
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 16.7kΩ for the
MX536A and 6.7kΩ 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 5µA
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 50µA, the resistor values
can easily be calculated.
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,
True RMS-to-DC Converters
Figure 3. Lower Frequency for Stated % of Reading Error and
Settling Time for Circuit shown in Figure 2
8
_______________________________________________________________________________________
Choosing the Averaging Time Constant
0.65
0.22
100
0.1
10
1
1
High-Accuracy Adjustments
10
FREQUENCY (Hz)
Single-Supply Operation
1%
60
100
0.1%
1k
10
1
0.1
0.01
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
the input signal frequency. The output ripple is inverse-
Figure 4. Optional External Gain and Output Offset Trims
Figure 5. Single-Supply Operation
V
IN
10k TO 1k
V
OUT
MX536A
MX536A
MX636
V
MX636
IN
R
V
R1
L
OUT
-V
C2
S
1
2
3
4
5
6
7
R1
R2
C2
1
2
3
4
5
6
7
R1
R2
R3
R4
MX536A
MX536A
BUF
20k
10k
750k
BUF
1 F
500
365
50k
ABSOLUTE
SQUARER
CURRENT
DIVIDER
MIRROR
ABSOLUTE
SQUARER
CURRENT
VALUE
DIVIDER
MIRROR
C
VALUE
C
AV
AV
MX636
20k
39k
3.3 F
MX636
470k
500k
200
154
14
13
12
11
10
8
9
14
13
12
11
10
9
8
+V
0.1 F
S
R2
+V
R2
R3
S
R1
AV
R4
0.1 F
+V
-V
and
S
S