Handbook for Sound Engineers

Tubes, Discrete Solid State Devices, and Integrated Circuits 347

(12-62)

To take the square root of V log,

(12-63)

12.3.4.5 Rms Level Detection ICs

Because rms level detectors are more complex than
either peak- or average-responding circuits, they benefit
greatly from integration. Fortunately, a few ICs are suit-
able for the professional audio applications. Two ICs
currently in production are the Analog Devices AD636
and the THAT Corporation THAT2252.

Analog Devices AD636. The AD636 has enjoyed wide
application in audio and instrumentation. Its prede-
cessor, the AD536, was used in the channel dynamics
processor of the SSL 4000 series console in conjunction
with a dbx VCA. Thousands of these channels are in
daily use worldwide.
The AD636 shown in Fig. 12-49 provides both a
linear-domain rms output and a dB-scaled logarithmic
output. The linear output at pin 8 is ideal for applica-
tions where the rms input voltage must be read with a dc
meter. Suitably scaled, 1 Vrms input can produce 1 Vdc
at the buffer output, pin 6.
In audio applications such as signal processors, it is
often most useful to express the signal level in dB. The
AD636 also provides a dB-scaled current output at pin

5. The linear dB output is particularly useful with expo-
   nentially controlled VCAs such as the SSM2018 or
   THAT 2180 series.

Averaging required to calculate the mean of the sum

of the squares is performed by a capacitor, CAV ,

connected to pin 4. Fig. 12-50 shows an AD636 used as

an audio dB meter for measurement applications.

THAT Corporation THAT2252. The 2252 IC uses the

technique taught by David Blackmer to provide wide

dynamic range, logarithmic linear dB output, and rela-

tively fast time constants. Blackmer’s detector delivers

a fast attack with a slow linear dB decay characteristic

in the log domain.^17 Because it was specifically devel-

oped for audio applications, it has become a standard

for use in companding noise reduction systems and

VCA-based compressor/limiters.

A simplified schematic of Blackmer’s rms detector,

used in the THAT2252, is shown in Fig. 12-51.

The audio input is first converted to a current Iin by

an external resistor (not shown in Fig. 12-51). Iin is

full-wave rectified by a current mirror rectifier formed

by OA1 and Q 1 - Q 3 , such that IC 4 is a full-wave rectified

version of Iin. Positive input currents are forced to flow

through Q 1 , and mirrored to Q 2 as IC 2 ; negative input

currents flow through Q 3 as IC 3 ; both IC 2 and IC 3 thus

flow through Q 4. (Note that pin 4 is normally connected

to ground through an external 20: resistor.)

Performing the absolute value before logarithmic

conversion avoids the problem that, mathematically, the

log of a negative number is undefined. This eliminates

the requirement for bipolar logarithmic conversion and

the PNP transistors required for log-domain VCAs.

OA2, together with Q 4 and Q 5 , forms a log amplifier.

Due to the two diode-connected transistors in the feed-

back loop of OA2, the voltage at its output is propor-

tional to twice the log of IC 4. This voltage, Vlog, is

therefore proportional to the log of Iin^2 (plus the bias

voltage V 2 ).

To average Vlog, pin 6 is usually connected to a

capacitor CT and a negative current source RT. see

Fig. 12-52. The current source establishes a quiescent

dc bias current, IT, through diode-connected Q 6. Over

time, CT charges to 1 Vbe below Vlog.

Q 6 ’s emitter current is proportional to the antilog of

its Vbe. The potential at the base (and collector) of Q 6

represents the log of Iin^2 while the emitter of Q 6 is held

at ac ground via the capacitor. Thus, the current in Q 6 is

proportional to the square of the instantaneous change

in input current. This dynamic antilogging causes the

capacitor voltage to represent the log of the mean of the

square of the input current. Another way to characterize

the operation of Q 6 , CT, and RT is that of a “log domain”

filter.^20

Figure 12-49. The AD636 block diagram. Courtesy Analog
Devices, Inc.

Vin^2 =antilog>@ logVinu 2

V log antilog

log V log

2

= --------------------------

Absolute value/voltage-current

converter

(^7) A4 6
5
3
84 9
10
14
A1
A2
A3
1
Com
Bufin Buffer
10k 7
Q 2 Q 4 Q^5
Q 1
Q 3
CAVIOUT
8k 7
8k 7

• |VIN|
  R 4
  I1
  I3
  I4
  IREF
  Current mirror
  VIN
  20kR^47
  10kR^37 One-quadrant
  square/divider –VS
  +VS
  RL
  dB out
  Buf out
  10kR^27
  20 μA
  25kR^17 FS
  10 μA
  FS CAV +VS