SSM2110_15 Datasheet, PDF (4/11 Pages) Analog Devices – TRUE RMS TO DC CONVERTER

- ANALOG DEVICES FAX-ON-DEMANDHOTLINE
Page 19
SSM~2110
INPUT
TheINPUT(pin 17) is an ACvirtualground withapproximately
+1.3V DC offset voltage. The useful dynamicrangeof input
current the device can process is 1COde (3mAp.p to 30nAp.p). The
input RC network is usually chosen to allow close to 20dB of
headroom inorder to process highcrest factor signals and pro-
'GOO
'110 ^,_{y...- '}
;
!;
vide a DC block below a given frequency band. Since in every
case the ratio of ROUT(RAVand RRMS)to RINdetermines the
overall gain of the circuit, ROUTmust also be considered when
selecting the low frequency breakpoint. The maximum recom.
.,
j
10
R,..e,OMO"
!
!
i=R.. 31010
mended values for ROUTvary from 4kfl to 1akfl for the circuits in
Figures 1,3,4, and 5.
Referring to the circuit in Figure 3, and assuming a gain of 1 is
desired, RINshould be set to 4kfl (RRMSand RAv'"4kfl). Based
!
.¹_',_O-_.-_.A ._{'_}.._A-
-_'"-^I _A
i ..... """'-
-40<1- -2Ode
,000A 'OOI>A
on this value of R'N'GINshould be 2~F to place the subsonic filter
INPUTCURRENT
pole at 20Hz. If you wish to limit the lower frequency to some-
thing otherthan 20 Hzthen GINshould be changed accordingly.
The low frequency pote 's governedby the simple equation
fMIN'" 1/27tRINC'N.
GIN should be a very low leakage capacitor to avoid impairing
O_{the dynamic range at low signal levels (many electrolytic types}
will not work).
B _{The choice of R'N;; 1akfl in Figure 1 is good for processing OdBV}
". ^{S reference signals. Given a nominal signal level ofOdBV (2.828Vp.}
p 1 VRMS)'this voltage across the 10kfl input resistor gives a
O _{nominal input current of 28311Ap-p,which allows 20dB of head-}
room. The three other common choices for R'Nare 4kfl, 5kfl and
L _{8kn. whichprovide 12dB, 14dB,and 18dBrespectively.}
". E _{The values of GIN'RINand ROUTcan be changed accordingly to}
TE ^{vary output levels to system requirements.}
FIGURE 2: Normalized Time Constant Relative to 1mAAM$
The PREBIAS pin (pin 18), can be used to increase the speed of
the RMS computing loop at low signal levels at some expense to
the dynamic range. Below a 10l!ARMS input level, the time con-
stant ot the RMS loop will increase from its nominal value by a
factor of 10 for every 20dB drop in level.
By use of the PRE BIAS pin, one can ensure thaI the loop time
constant will not increase above a chosen maximum as the sig-
nallevel continues to decrease. The equation relating the maxi.
mum time constant increase to the value of RpREBIASconnected
between pin 18 and V- is given by:
'MAX
'NOM
10~ x RPAEaAS
6.8V
(See Figure 2)
VAY
(orV"va)
v.....
41<U
~
AB5<X.UTEVALUE PREBIAS ,e'
2 LOG'..VAL
INPUT \7
~^{3 VA'"}
4 OND
V+ 16
\5
v- 14
!..tIUE
. I EMmeR
II 'LOGOUT
c J!!
LOG SCALE 112
" ^{-LOG IN J!.!.}
+LOG IN D
'OPTIONAL PREBIAS COHNECTIOH
"CAva CAN BE ADDeD TO AVERAGE THE AIISOI.UTE VALUE
VAl.l-IVS RR-'N
V.v.-IV...,RA.
RIN
~ RIN
V..
2"F - .,5V
-15V
FIGURE 3: Simple RMS/Absolute Value/Average Absolute Value Configuration