MX536A Maxim Integrated Products, MX536A Datasheet - Page 10

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

True RMS-to-DC Converters
Figure 7. MX536A/MX636 with a One-Pole Output Filter
Figure 9. Performance Features of Various Filter Types for
MX536A/MX636
10
V
V
+V
RMS
______________________________________________________________________________________
IN
S
OUT
C
MX536A
MX636
AV
0.1
10
N.C.
-V
1
dB
S
10
ONE-POLE FILTER
C2
C
TWO-POLE FILTER
C2
C3
C
1
2
3
4
5
6
7
AV
AF
MX536A
BUF
2.2 F
2.2 F
2.2 F
1 F
1 F
(ALL FILTERS)
ABSOLUTE
SQUARER
CURRENT
DC ERROR
DIVIDER
MIRROR
100
VALUE
FREQUENCY (Hz)
PK-PK RIPPLE
(TWO POLE)
C2 = C3 = 4.7 F
PK-PK RIPPLE
MX636
PK-PK RIPPLE
4.7 F
4.7 F
4.7 F
1 F
1 F
(ONE POLE)
C2 = 4.7 F
1k
14
13
12
11
10
9
8
+V
N.C.
N.C.
N.C.
COMMON
R
I
OUT
L
S
R
X
C2
= 0
10k
The MX536A/MX636 utilizes a logarithmic circuit in per-
forming the RMS computation of the input signal. The
bandwidth of the RMS converters is proportional to sig-
nal level. Figures 11 and 12 represent the frequency
response of the converters from 10mV to 7V
MX536A and 1mV to 1V for the MX636, respectively.
The dashed lines indicate the upper frequency limits for
1%, 10%, and ±3dB of reading additional error.
Caution must be used when designing RMS measuring
systems so that overload does not occur. The input
clipping level for the MX636 is ±12V, and for the
MX536A it is ±20V. A 7V
of 3 has a peak input of 21V.
A low-cost digital voltmeter (DVM) using just two inte-
grated circuits plus supporting circuitry and LCD dis-
play is shown in Figure 13. The MAX130 is a 3 1/2 digit
integrating A/D converter with precision bandgap refer-
ence. The 10MΩ input attenuator is AC coupled to pin
6 of the MX636 buffer amplifier. The output from the
MX636 is connected to the MAX130 to give a direct
reading to the LCD display.
Figure 8. MX536A/MX636 with a Two-Pole Output Filter
-V
V
* MX536A = 25k
IN
S
MX636 = 10k
C
AV
Application in a Low-Cost DVM
C2
1
2
3
4
5
6
7
BUF
ABSOLUTE
SQUARER
CURRENT
DIVIDER
MIRROR
RMS
VALUE
R
X
*
Frequency Response
signal with a crest factor
14
13
12
11
10
9
8
+V
C3
S
MX536A
RMS
MX636
V
RMS
for the
OUT