**FEATURES**

**APPLICATIONS**

**Description**

dc out put volt age. The out put is pro por tional to the

log of the true rms value of the in put sig nal. The

parts are housed in a space-efficient plas tic 8-pin

sin gle-in-line (SIP) pack age, and re quire min i mal

sup port cir cuitry. Based on dbx tech nol ogy and fab -

bines wide dy namic range with fre quency re sponse

to be yond 20 kHz. The log a rith mic out put is es pe -

cially con ve nient for au dio ap pli ca tions re quir ing

deci bel-linear scal ing. The in te gra tion time is ad -

just able via an ex ter nal R/C pair. With some ex ter -

nal cir cuitry, re sponse to dc is also pos si ble.

**IC RMS-Level Detector**

**SPECIFICATIONS**

**1**

**Ab so lute-Maximum Rat ings (TA = 25C)**

**Rec om mended Op er ating Con di tions**

*I*

*in*

**Elec tri cal Characteristics**

**2**

*I*

*in*

*I*

*in0*

*I*

*IN*

*I*

*in*0

*I*

*in*

*< 100*

*I*

*in*

*< 316*

*I*

*in*

*< 1mA*

*I*

*in*

*I*

*in*

*I*

*in*

*I*

*in*

*T*

*T*

**Theory of Operation**

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.

gram of the 2252. The input signal current,

*I*

*in*

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.

*I*

*in*

Q1. Positive input current is thereby forced to flow

through the collector-emitter of Q1. Pin 4 is nor-

mally connected through a 20

*Typical Application Circuit*, Page 4,

and

*Symmetry Adjustment*, Page 6), so the

Q1. Therefore, the current in the collector of Q2

(

*I*

*C2*

*I*

*C1*

rent in the emitter of Q4 (

*I*

*C4*

*I*

*C2*

*I*

*C3*

rent in Q4, therefore, is equal to the absolute value

of the input current.

*I*

*and I*

*I*

*C*

*in*

*in*

*in*

*C*

*C*

*in*

*in*

*in*

*I*

*I*

*I*

*I*

*I*

*I*

*I*

*I*

*I*

*I*

*I*

*C*

*C*

*C*

*C*

*C*

*I*

*I*

*I*

*I*

*in*

*in*

*in*

*in*

*I*

*in*

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

rent, plus a bias voltage (V2).

*V*

*V n*

*V*

*T*

*I*

*I*

*C*

*S*

*V*

*n I*

*V*

*n I*

*V*

*T*

*C*

*T*

*S*

*V*

*n I*

*V*

*n I*

*V*

*T*

*C*

*T*

*S*

*V*

*n I*

*V*

*n I*

*V*

*T*

*in*

*T*

*S*

*kT*

*q*

to be the same in each).

value, the time integral of the square of the signal

must be evaluated over infinite time. Obviously, for

a practical measurement, only a finite time is

available, which leads to the question of how to

weight events occuring at various times. Tradi-

tionally, the simplest and most meaningful weight-

ing is exponential in time, giving highest weight to

the most recent history, and exponentially less

weight to increasingly older events. This weighting

corresponds to convolution in time with the famil-

*e*

*t*

connected to a capacitor and a negative current

source. (Refer to the

*Typical Application Circuit*in

Figure 4. In this circuit, C

current source

tional to the antilog of its V

pin 6. The potential at the base of Q6 represents

the log of the

*square*of the input current, while

the emitter of Q6 is held at ac ground via the ca-

pacitor. Since Q6's emitter current is proportional

to the antilog of its V

current.

*dy-*

namicsignals. For a dc input, the output of OA2

namic

represents the square of the input current. After

charging, the external timing capacitor voltage

again approaches one diode drop below V

the bias current I

*I*

*in*

voltage to represent the log of the mean of the

square of the input current.

sider the action of Q6 and C

ance of Q6 at the bias current I

*C*

*T*

*V*

*I*

*T*

*T*

*C*

*T*

*I*

*T*

the average (or mean) of the square of the input

signal, averaged over the time constant

weighting of a simple RC circuit. Mathematically,

this is as follows:

*V*

*n*

*I e dt*

*T*

*T*

*in*

*t*

*e*

*t*

and how fast it returns to rest following a signal

(the "release"), are locked in relationship to each

other

Separate attack and release adjustments are not

possible within the constraint of rms response.

which shows the 2252's response to a 100 ms,

1 kHz tone burst at ~ +10 dBV followed by

~500 ms of 1 kHz at ~ 30 dBV. The top trace is

the input tone burst (at 10 V/div), the bottom trace

is the output at 50 mV/div. The time scale is

50 ms/div.

determined by the interaction of the exponential

response

decay follows from the fact that the natural release

of the exponential time weighting is a decaying ex-

ponential in the linear world. This maps to a

straight line in the log representation. The attack

in the photo appears exponential, but actually fol-

lows the (

*e*

*t*

steepens the apparent attack shape.

lease times. For

*f*

*in*

*V*

*R*

*V*

*f*

*T*

*in*

is im plied by the con stant of pro por tion al ity for

the out put volt age: it is not com puted ex plic itly.

the square root is equiv a lent to di vi sion by two.

The volt age at pin 6 is pro por tional to the mean of

the square at ap prox i mately 3 mV/dB, and pro por -

*square root*of the mean of the square

at ap prox i mately 6 mV/dB.

shifted down by the bias volt age V3. Level shift ing

is re quired so that the out put volt age will be zero

when the rms in put cur rent reaches a pre de ter -

mined value,

*I*

*in0*

*level*

*match*, and rep re sents the 0 dB ref er ence of the

cir cuit.

fol lows: V2 rep re sents one di ode drop, so the volt -

+3V

+2V

tors re spon si ble for each V

rent in pin 2 (I

*I*

*in*

*I*

*I*

*BIAS T*

*I*

*in0*

a 2252 in its rec om mended cir cuit con fig u ra tion

(Fig ure 4). In this plot, 0 dBr

stant-amplitude fre quency sweeps for the same cir -

cuit. The ver ti cal di vi sions are 60 mV apart,

au dio band width is main tained over a 60 dB dy -

slaved to the cur rent in pin 2, I

*I*

*in0*

the match ing be tween Q1 and Q2 for ac cu rate re -