_______________Detailed Description
The MX536A/MX636 uses an implicit method of RMS
computation that overcomes the dynamic range as well
as other limitations inherent in a straightforward compu-
tation of the RMS. The actual computation performed
by the MX536A/MX636 follows the equation:
V
RMS
= Avg. [V
IN
2
/V
RMS
]
The input voltage, V
IN
, applied to the MX536A/MX636 is
processed by an absolute-value/voltage to current con-
verter that produces a unipolar current I
1
(Figure 1).
This current drives one input of a squarer/divider that
produces a current I
4
that has a transfer function:
I
4
=
I
1
2
I
3
The current I
4
drives the internal current mirror through
a lowpass filter formed by R1 and an external capaci-
tor, C
AV
. As long as the time constant of this filter is
greater than the longest period of the input signal, I
4
is
averaged. The current mirror returns a current, I
3
, to the
square/divider to complete the circuit. The current I
4
is
then a function of the average of (I
1
2
/I
4
), which is equal
to I
1
RMS
.
The current mirror also produces a 2 ?I
4
output current,
I
OUT
, that can be used directly or converted to a volt-
age using resistor R2 and the internal buffer to provide
a low-impedance voltage output. The transfer function
for the MX536A/MX636 is:
V
OUT
= 2 ?R2 ?I
RMS
= V
IN
The dB output is obtained by the voltage at the emitter
of Q3, which is proportional to the -log V
IN
. The emitter
follower Q5 buffers and level shifts this voltage so that
the dB output is zero when the externally set emitter
current for Q5 approximates I
3
.
Standard Connection
(Figure 2)
The standard RMS connection requires only one exter-
nal component, C
AV
. In this configuration the
MX536A/MX636 measures the RMS of the AC and DC
levels present at the input, but shows an error for low-
frequency inputs as a function of the C
AV
filter capaci-
tor. Figure 3 gives practical values of C
AV
for various
values of averaging error over frequency for the stan-
dard RMS connections (no post filtering). If a 3礔
capacitor is chosen, the additional error at 100Hz will
be 1%. If the DC error can be rejected, a capacitor
should be connected in series with the input, as would
typically be the case in single-supply operation.
The input and output signal ranges are a function of the
supply voltages. Refer to the electrical characteristics for
guaranteed performance. The buffer amplifier can be
used either for lowering the output impedance of the cir-
cuit, or for other applications such as buffering high-
impedance input signals. The MX536A/MX636 can be
used in current output mode by disconnecting the inter-
nal load resistor, R
L
, from ground. The current output is
available at pin 8 (pin 10 on the H package) with a
nominal scale of 40礎(chǔ)/V
RMS
input for the MX536A and
100礎(chǔ)/V
RMS
input for the MX636. The output is positive.
True RMS-to-DC Converters
6  _______________________________________________________________________________________
ELECTRICAL CHARACTERISTICSMX636 (continued)
(T
A
= +25癈, +V
S
= +3V, -V
S
= -5V, unless otherwise noted.)
Rated Performance
+3/-5
V
Dual Supplies
+2/-2.5
?6.5
V
Single Supply
+5
+24
V
UNITS
MIN   TYP   MAX
CONDITIONS
PARAMETER
Quiescent Current (Note 10)
0.8    1
mA
Note 1: Accuracy is specified for 0 to 7V
RMS
, DC or 1kHz sine-wave input with the MX536A connected as in Figure 2.
Note 2: Error vs. crest factor is specified as an additional error for 1V
RMS
rectangular pulse stream, pulse width = 200祍.
Note 3: Input voltages are expressed in volts RMS, and error as % of reading.
Note 4: With 2k&external pull-down resistor.
Note 5: Accuracy is specified for 0 to 200mV, DC or 1kHz sine-wave input. Accuracy is degraded at higher RMS signal levels.
Note 6: Measured at pin 8 of DIP and SO (I
OUT
), with pin 9 tied to COMMON.
Note 7: Error vs. crest factor is specified as an additional error for 200mV
RMS
rectangular pulse input, pulse width = 200祍.
Note 8: Input voltages are expressed in volts RMS.
Note 9: With 10k&external pull-down resistor from pin 6 (BUF OUT) to -V
S
.
Note 10:With BUF input tied to COMMON.
POWERSUPPLY