REV. B

AD536A

–6–

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.

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

3

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

the implicit rms computation. Thus:

I

2

/

I

4

Figure 9. Simplified Schematic

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

voltage output. The transfer function of the AD536A thus

results:

V

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

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

output voltage is zero when the externally supplied emitter

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 k

Ω; thus some applications may require an additional

buffer amplifier if this output is desired.

dB Calibration:

2. Adjust R1 for dB out = 0.00 V

4. Adjust R5 for dB out = –2.00 V

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

the proper gain automatically gives the correct temperature

compensation.

Figure 10. dB Connection