The log-domain filter cutoff frequency is usually

placed well below the frequency range of interest.

For an audio-band detector, a typical value would be

5Hz, or a 32ms time constant (t). The filter’s time

constant is determined by an external capacitor at-

tached to the CT pin, and an internal current source

grammed via the IT pin: current in IT is mirrored to

time constant t is approximately equal to

matics of rms detection, the attack and release time

constants are fixed in their relationship to each other.

The DC output of the detector is scaled with the

same constant of proportionality as the VCA gain

control: 6.1mV/dB. The detector’s zero dB reference

(Iin0, the input current which causes zero volts out-

put), is determined by IT as follows: Iin0=IT. The

detector output stage is capable of sinking or sour-

cing l00mA.

Differences between the 4311’s RMS-Level Detec-

tor circuitry and that of the THAT 2252 RMS Detec-

tor are as follows:

1. The rectifier in the 4311 RMS Detector is inter-

nally balanced by design, and cannot be balanced via

an external control. The 4311 will typically balance

positive and negative halves of the input signal within

±1.5%, but in extreme cases the mismatch may

reach +20%. However, a 20% mismatch will not sig-

nificantly increase ripple-induced distortion in dy-

namics processors over that caused by signal ripple

alone.

2. The time constant of the 4311’s RMS detector

is determined by the combination of an external ca-

pacitor (connected to the CT pin) and an internal,

programmable current source. The current source is

equal to IT. Normally, a resistor is not connected di-

rectly to the CT pin on the 4311.

3. The zero dB reference point, or level match, is

not adjustable via an external current source. How-

ever, as in the 2252, the level match is affected by the

timing current, which, in this case, is drawn from the

IT pin and mirrored internally to CT.

4. The input stage of the 4311 RMS detector uses

integrated P-channel FETs rather than a bias-current

corrected bipolar differential amplifier. Input bias

currents are therefore negligible, improving perfor-

mance at low signal levels.

The Opamps - in Brief

The three opamps in the 4311 are intended for

general purpose applications. All are 5MHz opamps

with slew rates of approximately 2V/ms. All use bipo-

lar PNP input stages. However, the design of each is

optimized for its expected use. Therefore, to get the

THAT Corporation; 45 Sumner Street; Milford, Massachusetts 01757-1656; USA

Tel: +1 508 478 9200; Fax: +1 508 478 0990; Web: www.thatcorp.com

Page 8

Preliminary Information

IN

17

12

OUT

VCA

EC+

SYM

EC-

OA3

Vref

U1A

THAT 4311

Iset

2

TC

5

IN

1

OUT

4

RMS

U1B

THAT 4311

3

7

OA2

Vref

6

U1C

THAT 4311

18

19

20

OA1

U1D

THAT 4311

Cap

8

Vee

10

Vref 9

Vcc

11

Vref

U1E

THAT 4311

Vref

R1

20k

R2

20k

R3

51R

R4

51k

C1

47u

C2

47p

C5

10u

C8

22u

C7

100n

R5

50k

+5

+5

Signal In

Signal Out

Control Port Drive

N/C

N/C

N/C

N/C

N/C

N/C

N/C

N/C

N/C

15

14

13

16

+

++