4301 View Datasheet(PDF) - THAT Corporation

Part Name

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4301

THAT Analog Engine® IC Dynamics Processor

THAT Corporation 

Page 8

Rev. 04/10/02

value of the input signal current. Some ac component

(at twice the input frequency) remains superimposed

on the dc output. The ac signal is attenuated by a

log-domain filter, which constitutes a single-pole

rolloff with cutoff determined by an external capaci-

tor and a programmable dc current.

As in the VCA, input signals are currents to the

RMS IN pin. This input is a virtual ground, so a re-

sistor (R6 in Figure 2) is normally used to convert in-

put voltages to the desired current. The level detector

is capable of accurately resolving signals well below

10 mV (with a 10 kW input resistor). However, if the

detector is to accurately track such low-level signals,

ac coupling is normally required.

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

5 Hz, or a 32 ms 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

(ICT) connected to CT. The current source is pro-

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

ICT with a gain of approximately 1.1. The resulting

time constant t is approximately equal to 0.026 CT/IT.

Note that, as a result of the mathematics of RMS de-

tection, 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: 6.5 mV/dB. The detector’s 0 dB reference

(Iin0, the input current which causes 0 V output), is

determined by IT as follows: Iin0= 9.6 mA It The de-

tector output stage is capable of sinking or sourcing

100 mA.

Differences between the 4301’s RMS-Level Detec-

tor circuitry and that of the THAT 2252 RMS Detec-

tor are as follows:

1) The rectifier in the 4301 RMS Detector is inter-

nally balanced by design, and cannot be balanced

via an external control. The 4301 will typically bal-

ance positive and negative halves of the input signal

within ±1.5%, but in extreme cases the mismatch

may reach ±15%. However, a 15% mismatch will

not significantly increase ripple-induced distortion

in dynamics processors over that caused by signal

ripple alone.

2) The time constant of the 4301’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 1.1 IT. Normally, a resistor is not con-

nected directly to the CT pin on the 4301.

3) The 0 dB reference point, or level match, is not

adjustable via an external current source. However,

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 4301 RMS detector uses

integrated P-channel FETs rather than a

bias-current corrected bipolar differential amplifier.

Input bias currents are therefore negligible, improv-

ing performance at low signal levels.

The Opamps — in Brief

The three opamps in the 4301 are intended for

general purpose applications. All are 5 MHz 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

most out of the 4301, it is useful to know the major

differences among these opamps.

OA3, being internally connected to the output of

the VCA, is intended for current-to-voltage conver-

sion. Its input noise performance, at 7.5nV Hz, com-

plements that of the VCA, adding negligible noise at

unity gain. Its output section is capable of driving a

2 kW load to within 2V of the power supply rails,

making it possible to use this opamp directly as the

output stage in single-ended designs.

OA1 is the quietest opamp of the three. Its input

noise voltage, at 6.5nV Hz, makes it the opamp of

choice for input stages. Note that its output drive ca-

pability is limited (in order to reduce the chip’s

power dissipation) to approximately ±3 mA. It is

comfortable driving loads of 5 kW or more to within

1V of the power supply rails.

OA2 is intended primarily as a control-voltage

processor. Its input noise parallels that of OA3, and

its output drive capability parallels that of OA1.

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

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

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