MX536AKH Просмотр технического описания (PDF) - Maxim Integrated

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MX536AKH

True RMS-to-DC Converters

Maxim Integrated 

True RMS-to-DC Converters

ELECTRICAL CHARACTERISTICS—MX636 (continued)

(TA = +25°C, +VS = +3V, -VS = -5V, unless otherwise noted.)

PARAMETER

CONDITIONS

POWER SUPPLY

Rated Performance

Dual Supplies

Single Supply

Quiescent Current (Note 10)

MIN TYP

+2/-2.5

+5

+3/-5

0.8

MAX

±16.5

+24

1

UNITS

V

V

V

mA

Note 1: Accuracy is specified for 0 to 7VRMS, 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 1VRMS rectangular pulse stream, pulse width = 200µs.

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 (IOUT), with pin 9 tied to COMMON.

Note 7: Error vs. crest factor is specified as an additional error for 200mVRMS rectangular pulse input, pulse width = 200µs.

Note 8: Input voltages are expressed in volts RMS.

Note 9: With 10kΩ external pull-down resistor from pin 6 (BUF OUT) to -VS.

Note 10: With BUF input tied to COMMON.

_______________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:

VRMS = Avg. [VIN2/VRMS]

The input voltage, VIN, applied to the MX536A/MX636 is

processed by an absolute-value/voltage to current con-

verter that produces a unipolar current I1 (Figure 1).

This current drives one input of a squarer/divider that

produces a current I4 that has a transfer function:

I4

=

I12

I3

The current I4 drives the internal current mirror through

a lowpass filter formed by R1 and an external capaci-

tor, CAV. As long as the time constant of this filter is

greater than the longest period of the input signal, I4 is

averaged. The current mirror returns a current, I3, to the

square/divider to complete the circuit. The current I4 is

then a function of the average of (I12/I4), which is equal

to I1RMS.

The current mirror also produces a 2 · I4 output current,

IOUT, 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:

VOUT = 2 · R2 · IRMS = VIN

The dB output is obtained by the voltage at the emitter

of Q3, which is proportional to the -log VIN. 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 I3.

Standard Connection

(Figure 2)

The standard RMS connection requires only one exter-

nal component, CAV. 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 CAV filter capaci-

tor. Figure 3 gives practical values of CAV for various

values of averaging error over frequency for the stan-

dard RMS connections (no post filtering). 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.

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, RL, from ground. The current output is

available at pin 8 (pin 10 on the “H” package) with a

nominal scale of 40µA/VRMS input for the MX536A and

100µA/VRMS input for the MX636. The output is positive.

6 _______________________________________________________________________________________

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