2252-DEMO THAT Corporation, 2252-DEMO Datasheet - Page 6

• Manufacturer Part Number: 2252-DEMO
• Description: Power Management Modules & Development Tools RMS Detector Demo Board
• Manufacturer: THAT Corporation
• Type: AC/DC Switching Convertersr

Specifications of 2252-DEMO

• Input Voltage: 12 V
• Maximum Operating Temperature: + 75 C
• Minimum Operating Temperature: - 20 C
• Product: Power Management Modules
• Supply Current: 1 mA
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

Page 6
pro duc tion of pos i tive-half in put sig nals. The base
of Q2 is brought out to pin 4 in or der to al low ad -
just ment for mis matches be tween these tran sis -
tors and for the in put off set volt age of OA1. Pin 4
should be con nected to a vari able low-impedance
volt age source ca pa ble of sup ply ing a few mil li volts
(V
through a 20
the base im ped ances of Q1 and Q2. A 20
should be used at pin 4 for op ti mum match ing.
In put Bias Cur rents and Volt ages
OA1 will rec tify any cur rents flow ing in its feed -
back net work, in clud ing the bias cur rent of OA1 it -
self. In put sig nals be low OA1’s bias cur rent will be
ob scured, with a steady dc out put “floor” the re -
sult. The in put bias com pen sa tion for OA1 largely
can cels OA1’s bias cur rent, im prov ing low-level
per for mance. Even if over com pen sated, any un cor -
rected bias cur rent (pos i tive or neg a tive) in OA1
re sults in a limit to the low-level res o lu tion of the
2252.
Any dc off set volt age at pin 1 will cause dc in put
cur rents to flow if the in put is dc cou pled. The dc
off set at pin 1 is typ i cally 8 mV, so the in put
should be ac cou pled (as shown in Fig ure 4) for
most ap pli ca tions. If low-level per for mance is not
re quired, dc cou pling may be used with out fur ther
mod i fi ca tions to the ap pli ca tion cir cuits. How ever,
dc cou pling and good low-level per for mance are
pos si ble with ad di tional ex ter nal cir cuitry, as de -
scribed in the sec tion on DC Mea sure ments on
Page 9.
Pos i tive equiv a lent in put bias cur rent in OA2 (af ter
cor rec tion) will also add to the out put level read -
ing. This is be cause the equiv a lent in put bias cur -
rent is sup plied through Q4 and Q5, which adds to
the dc level at the out put of OA2. OA2, like OA1,
has in put bias com pen sa tion. While gen er ally ex -
tend ing low-level per for mance, the com pen sa tion
can cause OA2’s net in put bias re quire ment to be -
come neg a tive, tend ing to re verse bias Q4 and Q5.
The cir cuit of Fig ure 4 uses a 22 M
to sup ply in creas ing dc in put cur rent as the 2252
out put drops. This forces cur rent to flow in Q2,
which over whelms any neg a tive in put bias at OA2
and pre vents the cur rent in Q4 and Q5 from re -
vers ing.
The in tended pur pose of the 2252 is to com pute
the log of the time-weighted root-mean-square of
an in put cur rent sig nal. Sev eral prac ti cal con sid er -
ations ap ply when at tempt ing to make full use of
the 100 dB dy namic range of the de vice.
In put Con sid er ations
The 2252 in put is in tended to ac cept a cur rent: as
is clear from Fig ure 3 (page 3), pin 1 is a sum ming
SYM
). The base of Q1 is con nected to its col lec tor
re sis tor to al low match ing be tween
THAT Corporation; 45 Sumner Street; Milford, Massachusetts 01757-1656; USA
Tel: [phone]; Fax: [phone]; Web: www.thatcorp.com
re sis tor, Rf,
source
Ap pli ca tions
Rf is re spon si ble for the flat ten ing of the low-level
re sponse shown in Fig ure 6 (Page
back cur rent pro vided en forces a lower limit to the
re solv able in put cur rent. With out this feed back,
those
overcorrected will ex hibit a sharp down ward swing
in out put level at low in put sig nals (see Fig ure 8).
For many ap pli ca tions, such low-level re sponse is
not nec es sary. In such cases, Rf may be de leted, as
shown in Fig ure 11. See fur ther dis cus sion of this
sub ject in Out put Con sid er ations, (Page 8).
junc tion sup plied with in ter nal neg a tive feed back.
Max i mum in put cur rent is on the or der of 1 mA,
lim ited by in ter nal de vice char ac ter is tics. The min -
i mum prac ti cal in put cur rent is de ter mined by the
ef fects of in ter nal bias cur rents (whether from
OA1, OA2, or the ef fect of V1). At cur rents as low
as 31 nA, ap prox i mately 1 dB er ror in read ing will
re sult from interal bias cur rents.
IN
Fig ure 9. Ap pli ca tion Cir cuit With out Sym me try
Fig ure 8. Low Level Out put from Sev eral 2252s
V-
2252s
1u
20u
Cin
10k
Rin
where
20
THAT 2252 RMS-Level De tec tor
Con trol
(no Rf)
1
the
750k
IN
SYM
4
V-
V-
Rb
2252
5
2M2
GND
2
OA2
IBIAS
R T
3
8
Rf
CAP
OUT
V+
6
in put
22M
7
5). The feed -
1k
10u
10u
C
bias
T
V+
OUT
is