THAT Corporation THAT2252 Datasheet

K

THAT Corporation

IC RMS-Level Detector

THAT 2252

FEATURES

True RMS Response

·

Wide Dynamic Range: >80 dB

·

High Crest Factor: 8 (1 dB error)

·

Wide Bandwidth: to > 20 kHz

·

Logarithmic Output Scaling

·

Low Cost: $2.20 in ’000s

·

Single In-Line Package

·

Matches 2180 and 2181 Series

·

VCAs

Description

The THAT 2252 integrated-circuit rms-level detec­tor is designed to convert an ac input current into a
dc output voltage. The output is proportional to the
log of the true rms value of the input signal. The
parts are housed in a space-efficient plastic 8-pin
single-in-line (SIP) package, and require minimal
support circuitry. Based on dbx technology and fab-

APPLICATIONS

Meters

·

Spectrum Analyzers

·

Compressors

·

Expanders

·

Oscillators

·

Psychoacoustic Modeling

·

Noise Measurement

·

ricated in a super low-noise process, the 2252 com­bines wide dynamic range with frequency response
to beyond 20 kHz. The logarithmic output is espe­cially convenient for audio applications requiring
decibel-linear scaling. The integration time is ad­justable via an external R/C pair. With some exter­nal circuitry, response to dc is also possible.

BIAS

CURRENT

COMP

1

1

3

4

2

PIN 1

8

THAT

BIAS

CURRENT

COMP

2

+

V

-

Figure 1. 2252 Equivalent Circuit Diagram

7

5

6

N

B
D
C

ITEM

A 20.32 MAX.
B
C
D
E
F
G
H

I
J
K
L

MN3.2 ± 0.5

Figure 2. 2252 Physical Outline

dbx is a registered trademark of Carillon Electronics Corporation

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

Tel: [phone]; Fax: [phone]; Web: www.thatcorp.com

MODEL NO.

H

TYP.

A

MILLIMETERS INCHES

1.1 MIN.

0.5 ± 0.1

0.25

2.54

1.27 MAX.

0.51 MIN.

5.08 MAX.

2.8 ± 0.2

5.75 MAX.

1.5 MAX.

0.25 +0.10 -0.04

1.1 MIN.

GE

F

0.8 MAX.

0.043 MIN.

0.02 ± 0.004

0.01

0.1

0.05 MAX.

0.02 MIN.

0.2 MAX.

0.11 ± 0.008

0.227 MAX.

0.058 MAX.

0.01 +0.004 -0.002

0.126 ± 0.02

0.043 MIN.

J

M

L

I

Page 2 THAT 2252 RMS-Level Detector

SPECIFICATIONS

1

Absolute-Maximum Ratings (TA = 25°C)

Positive Supply Voltage (VCC) +18 V

Negative Supply Voltage (VEE) -18 V

Supply Current (ICC) 10 mA

Power Dissipation (PD) 330 mW

Operating Temperature Range (TOP) -20 to +75°C

Storage Temperature Range (TST) -40 to +125°C

Recommended Operating Conditions

Parameter Symbol Conditions Min Typ Max Units

Positive Supply Voltage V

Negative Supply Voltage V

Bias Set Current I

Signal Current I

Timing Current I

CC

EE

BIAS

I

in

T

=24mA— — 1 mA

BIAS

Electrical Characteristics

+4 +12 +15 V

-4 -12 -15 V

15 24 50 mA

1 7.5 50 mA

2

Parameter Symbol Conditions Min Typ Max Units

Supply Current I

Equiv. Input Bias Current I

Input Offset Voltage V

Symmetry Voltage V

CC

B

OFF(IN)

SYM

Output Scale Factor EO/20log(Iin/ I

Input Current for 0V Output I

in0

in0

No Signal — 1 3 mA

No Signal — 5 8 nA

No Signal 0 +8 +16 mV

-2 8 +18 mV

) 31.6nA<IIN<1mA

T

=25°C (T

A

»35°C) 6.0 6.1 6.2 mV/dB

CHIP

II

× II

BIAS T

3.5

×

BIAS T

2.9

II

×

BIAS T

2.4

Output Linearity fIN= 1kHz

1mA<I

100nA < I

31.6nA < I

< 100mA — 0.1 — dB

in

< 316mA — 0.5 — dB

in

< 1mA — 1.0 — dB

in

Crest Factor 1ms pulse repetition rate

0.2 dB error — 3.5 —

0.5 dB error — 5 —

1.0 dB error — 8 —

Maximum Frequency for 1 dB additional error Iin³ 100mA — 80 — kHz

I

³ 10mA 74 — kHz

in

I

³ 1mA 30 — kHz

in

I

³ 100nA 4 — kHz

in

C

Filtering Time Constant

().026

T

I

T

s

Output TempCo D E

0

/ D T

CHIP

Re: T

1. All specifications subject to change without notice.

2. Unless otherwise noted, T
minimum ripple at V

out

=25°C, VCC= +15V, VEE= -15V. Test circuit is as shown in Figure 4. SYM is adjusted for

A

with Vin=1 V

, 100 Hz.

RMS

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

Tel: [phone]; Fax: [phone]; Web: www.thatcorp.com

=27°C — 0.33 — %/°C

CHIP

600032 Rev 01 Page 3

Theory of Operation

The THAT 2252 RMS-Level Detector is designed
for high performance in audio-frequency applica
tions requiring logarithmic output, rms response,
and wide dynamic range. The parts compute rms
level by rectifying input current signals, converting
the resulting current waveform to a logarithmic
voltage, and applying this voltage to a log-domain
filter.

Current Rectification

Figure 3 presents a simplified internal circuit dia
gram of the 2252. The input signal current, I
flows in pin 1, the input pin. OA1 drives the base
of Q3 and the emitter of Q1 (through V1) to main
tain pin 1 at virtual ground potential. A negative
input current (flowing out of pin 1) will tend to
drive the inverting input of OA1 negative, driving
OA1’s output positive, turning on Q3. V1 is de
signed to cut off Q1 while Q3 is on. Therefore, neg
ative input currents are forced to flow through the
collector-emitter of Q3.

Positive I

will drive OA1’s output negative, cutting

in

off Q3 and turning on diode-connected transistor
Q1. Positive input current is thereby forced to flow
through the collector-emitter of Q1. Pin 4 is nor­mally connected through a 20 W resistor to ground
(see Figure 4, Typical Application Circuit, Page 4,
and Symmetry Adjustment, Page 6), so the
base-emitter potential of Q2 is the same as that of
Q1. Therefore, the current in the collector of Q2

) will mirror that in the collector of Q1 (IC1),

(I

C2

which equals the positive input current.

Since the input impedance of OA2 is high, the cur
rent in the emitter of Q4 (I
currents I

and IC3. The mirror action of Q1/Q2

C2

), is the sum of the

C4

in

reverses the positive input currents so that they

-

add to the negative input currents in Q4. The cur

­rent in Q4, therefore, is equal to the absolute value
of the input current.

Mathematically,

I

00

II

-£

IandII

===

C

312

{{

00

But,

IIIII

CCCCC43231

­,

-

See Figure 3 for definitions of these currents.

0

,

in in

,

,

I

>

in

CC

=+=+

-<

II

0

in in

=

{

= I

II

,,0

in in

.

in

>

,

in

II

,

in in

£

.

0

>

Logging Action

-

-

OA2, together with Q4 and Q5, forms a log ampli
fier. By using two diode-connected transistors in
the feedback loop of OA2, the 2252 produces a
voltage proportional to twice the log of IC4 at the
output of OA2. This voltage, V

, is therefore pro

log

portional to the log of the square of the input cur­rent, plus a bias voltage (V2).

Mathematically,

I

æ

C

4

VVn V

=

log

T

=-+21 21

=-+VnI VnI V

TC TS

=-+VnI VnI V

Tin T S

-

Where V

is the thermal voltage,

T

ö

ç

+21

÷

ç

I

è

VnI VnI V

TC TS

121

() ()

121

2

S

ø

() ()

42

2

4

2

()

2

,

2

kT

, and ISis the

q

reverse-saturation current of Q4 and Q5 (assumed
to be the same in each).

-

-

-

+

OA1

IC3

1

Iin

IC1

20

Q1

Q3

+

-

V1

IC2

IC4

Q2

+

Q4

-

-

+

OA2

V2

Q5

Vlog

Q6

V3

-

+

-

+

OA3

I

T

6

V6

7

Vout

4

Figure 3. Simplified Internal Schematic

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

Tel: [phone]; Fax: [phone]; Web: www.thatcorp.com