Tru e RMS -t o -DC Co n ve rt e rs
ELECTRICAL CHARACTERISTICS—MX636 (continued)
(T = +25°C, +V = +3V, -V = -5V, unless otherwise noted.)
A
S
S
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
POWER SUPPLY
Rated Performance
Dual Supplies
+3/-5
V
V
+2/-2.5
+5
±16.5
+24
1
Single Supply
V
Quiescent Current (Note 10)
0.8
mA
Note 1: Accuracy is specified for 0 to 7V
, DC or 1kHz sine-wave input with the MX536A connected as in Figure 2.
RMS
Note 2: Error vs. crest factor is specified as an additional error for 1V
rectangular pulse stream, pulse width = 200µs.
RMS
Note 3: Input voltages are expressed in volts RMS, and error as % of reading.
Note 4: With 2kΩ 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 ), with pin 9 tied to COMMON.
OUT
5A/MX36
Note 7: Error vs. crest factor is specified as an additional error for 200mV
rectangular pulse input, pulse width = 200µs.
RMS
Note 8: Input voltages are expressed in volts RMS.
Note 9: With 10kΩ external pull-down resistor from pin 6 (BUF OUT) to -V .
S
Note 10: With BUF input tied to COMMON.
The dB output is obtained by the voltage at the emitter
of Q3, which is proportional to the -log V . The emitter
IN
follower Q5 buffers and level shifts this voltage so that
the dB output is zero when the externally set emitter
_______________De t a ile d De s c rip t io n
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:
current for Q5 approximates I .
3
S t a n d a rd Co n n e c t io n
(Fig u re 2 )
The standard RMS connection requires only one exter-
2
V
RMS
= Avg. [V /V
]
IN
RMS
The input voltage, V , applied to the MX536A/MX636 is
IN
na l c omp one nt, C . In this c onfig ura tion the
AV
processed by an absolute-value/voltage to current con-
verter that produces a unipolar current I (Figure 1).
1
MX536A/MX636 measures the RMS of the AC and DC
levels present at the input, but shows an error for low-
This current drives one input of a squarer/divider that
frequency inputs as a function of the C
filter capaci-
AV
produces a current I that has a transfer function:
4
tor. Figure 3 gives practical values of C
for various
AV
2
I
1
I
3
values of averaging error over frequency for the stan-
d a rd RMS c onne c tions (no p os t filte ring ). If a 3µF
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.
I =
4
The current I drives the internal current mirror through
4
a lowpass filter formed by R1 and an external capaci-
tor, C . As long as the time constant of this filter is
AV
greater than the longest period of the input signal, I is
4
averaged. The current mirror returns a current, I , to the
3
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-
square/divider to complete the circuit. The current I is
4
2
then a function of the average of (I /I ), which is equal
1
4
to I
.
RMS
1
The current mirror also produces a 2 · I output current,
4
I
, that can be used directly or converted to a volt-
OUT
age using resistor R2 and the internal buffer to provide
a low-impedance voltage output. The transfer function
for the MX536A/MX636 is:
nal load resistor, R , from ground. The current output is
L
available at pin 8 (pin 10 on the “H” package) with a
nominal scale of 40µA/V
input for the MX536A and
RMS
V
OUT
= 2 · R2 · I = V
RMS IN
100µA/V
input for the MX636. The output is positive.
RMS
6
_______________________________________________________________________________________
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