AD536 データシートの表示（PDF） - Analog Devices

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AD536

Integrated Circuit True RMS-to-DC Converter

Analog Devices 

AD536A

Figure 9 is a simplified schematic of the AD536A; it is subdi-

vided into four major sections: absolute value circuit (active

rectifier), squarer/divider, current mirror, and buffer amplifier.

The input voltage, VIN, which can be ac or dc, is converted to a

unipolar current I1, by the active rectifier A1, A2. I1 drives one

input of the squarer/divider, which has the transfer function:

I4 = I12/I3

The output current, I4, of the squarer/divider drives the current

mirror through a low-pass filter formed by R1 and the externally

connected capacitor, CAV. If the R1, CAV time constant is much

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

effectively averaged. The current mirror returns a current I3,

which equals Avg. [I4], back to the squarer/divider to complete

the implicit rms computation. Thus:

[ ] I4 = Avg. I12 / I4 = I1 rms

Figure 9. Simplified Schematic

The current mirror also produces the output current, IOUT,

which equals 2I4. IOUT can be used directly or converted to a

voltage with R2 and buffered by A4 to provide a low impedance

voltage output. The transfer function of the AD536A thus

results:

VOUT = 2R2 I rms = V IN rms

The dB output is derived from the emitter of Q3, since the

voltage at this point is proportional to –log VIN. Emitter fol-

lower, Q5, buffers and level shifts this voltage, so that the dB

output voltage is zero when the externally supplied emitter

current (IREF) to Q5 approximates I3.

CONNECTIONS FOR dB OPERATION

A powerful feature added to the AD536A is the logarithmic or

decibel output. The internal circuit computing dB works accu-

rately over a 60 dB range. The connections for dB measure-

ments are shown in Figure 10. The user selects the 0 dB level by

adjusting R1, for the proper 0 dB reference current (which is set

to exactly cancel the log output current from the squarer-divider

at the desired 0 dB point). The external op amp is used to pro-

vide a more convenient scale and to allow compensation of the

+0.33%/°C scale factor drift of the dB output pin. The special

T.C. resistor, R2, is available from Tel Labs in Londonderry,

N.H. (model Q-81) or from Precision Resistor Inc., Hillside,

N.J. (model PT146). The averaged temperature coefficients of

resistors R2 and R3 develop the +3300 ppm needed to reverse

compensate the dB output. The linear rms output is available at

Pin 8 on DIP or Pin 10 on header device with an output imped-

ance of 25 kΩ; thus some applications may require an additional

buffer amplifier if this output is desired.

dB Calibration:

1. Set VIN = 1.00 V dc or 1.00 V rms

2. Adjust R1 for dB out = 0.00 V

3. Set VIN = +0.1 V dc or 0.10 V rms

4. Adjust R5 for dB out = –2.00 V

Any other desired 0 dB reference level can be used by setting

VIN and adjusting R1, accordingly. Note that adjusting R5 for

the proper gain automatically gives the correct temperature

compensation.

Figure 10. dB Connection

–6–

REV. B

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