AD536A

Data Sheet

Rev. G | Page 8 of 15

THEORY OF OPERATION

The AD536A embodies an implicit solution of the rms equation

that overcomes the dynamic range as well as other limitations

inherent in a straightforward computation of rms. The actual

computation performed by the AD536A follows the equation



















rms

V

V

Avg

rms

V

IN

2

Figure 6 is a simplified schematic of the AD536A. Note that it is

subdivided 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 rectifiers

(A1, A2). I1 drives one input of the squarer/divider, which has

the transfer function

I4 = II2/I3

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

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

nally 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[II2/I4] = II rms

14

+VS

–VS

I3

I2

I1

IOUT

RL

VIN

|VIN|R–1

ABSOLUTE VALUE;

VOLTAGE-CURRENT

CONVERTER

ONE-QUADRANT

SQUARER/DIVIDER

CURRENT MIRROR

Q1

Q2

Q3

Q4

Q5

COM

49

dB

OUT

5

BUF

OUT

6

3

8

1

BUF

IN BUFFER

7

10

0.4mA

FS

A3

NOTES

1. PINOUTS ARE FOR 14-LEAD DIP.

0.2mA

FS

R1

25kΩ

R2

25kΩ

12kΩ

25kΩ

12kΩ

R4

50kΩ

R3

25kΩ

80kΩ

A4

A2

A1

Figure 6. Simplified Schematic

The current mirror also produces the output current, IOUT, which

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

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

voltage output. The transfer function of the AD536A results in

the following:

VOUT = 2R2 × I rms = VIN rms

The dB output is derived from the emitter of Q3 because the

voltage at this point is proportional to –log VIN. The emitter

follower, 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

The logarithmic (or decibel) output of the AD536A is one of

its most powerful features. The internal circuit computing dB

works accurately over a 60 dB range. The connections for dB

measurements are shown in Figure 7.

Select the 0 dB level by adjusting R1 for the proper 0 dB reference

current (which is set to cancel the log output current from the

squarer/divider at the desired 0 dB point). The external op amp

provides a more convenient scale and allows compensation of

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

The temperature-compensating resistor, R2, is available online

in several styles from Precision Resistor Company, Inc., (Part

Number AT35 and Part Number ST35). The average temperature

coefficients of R2 and R3 result in the +3300 ppm required to

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

at Pin 8 on the DIP or Pin 10 on the header device with an output

impedance of 25 kΩ. Some applications require an additional

buffer amplifier if this output is desired.

For dB calibration,

1.

Set VIN = 1.00 V dc or 1.00 V rms.

2.

Adjust R1 for dB output = 0.00 V.

3.

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

4.

Adjust R5 for dB output = −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 provides the correct temperature

compensation.