2252-DEMO THAT Corporation, 2252-DEMO Datasheet - Page 9

• 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

600032 Rev 02
re sponse is quite im por tant, the 2252s may be se -
lected, and Rf may be omit ted, as shown in Fig -
ure 11. The 2252s should be se lected on the ba sis
of ac cu rate low-level, low-frequency per for mance
(see Fig ure 8, page 6).
The cir cuit of Fig ure 11 may also be used with un -
se lected 2252s for ap pli ca tions where ac cu rate
low-level per for mance is un nec es sary. How ever,
the rel a tively low cost of a 5% 22 M
ally ar gues in fa vor of its use.
DC Mea sure ments
As noted ear lier, the 2252 is pri mar ily in tended for
mea sur ing the rms value of ac sig nals. This fol lows
from the 2252’s rel a tively high in put off set volt age
(typ i cally +8 mV). If the in put is dc cou pled, the
in put off set volt age will cause a dc cur rent to flow
in the in put, which will in ter fere with pre cise
low-level rec ti fi ca tion, ul ti mately
“floor” be low which in put sig nals will be ob scured.
For typ i cal val ues of R
rent would be 800 nA, up set ting rec ti fi ca tion for ac
sig nals be low ap prox i mately 8
com pletely ob scur ing sig nals be low 800 nA peak.
C
tend ing low-level per for mance
100 nA.
How ever, as shown in Fig ure 12, with a few added
parts, it is pos si ble to ex tend re sponse to dc with
lit tle loss in low-level ac cu racy.
The es sen tial pur pose of the added cir cuitry (OA1,
OA2 and as so ci ated parts) is to buffer the in put
off set volt age (at pin 1) and add it to the in put sig -
nal. The cir cuit of Fig ure 12 op er ates as fol lows.
As has been noted, pin 1 is a vir tual ground. The o -
retically, the volt age at pin 1 is only the dc off set of
the 2252’s in ter nal in put am pli fier, V
ti cally, the volt age at pin 1 will con sist of this off set
plus a small ac sig nal de ter mined by the value of
R
is in tended to buffer V
ad di tional cur rent from the pin 1 node. For the
out put of OA2 to be an ac cu rate rep re sen ta tion of
V
cur rent opamp, and should have ei ther low in put
off set volt age (< 0.1 mV) or be trimmed to have
low in put off set. Typ i cal choices would in clude an
OP07 or an LM108A with a trim for off set volt age.
The volt age at OA2’s out put is di vided by 2
through the two 10 k
1 f ca pac i tor by passes any ac sig nal at the out put
of OA2 so that only dc is pres ent at the
noninverting in put of OA1. As suming that the in -
put to the en tire cir cuit is con nected to a 0
age source, the volt age gain from the noninverting
in put of OA1 to its out put will be 2.0, so the com -
in
OFF(IN)
in
in Fig ures 4 and 11 blocks this dc cur rent, ex -
and the loop gain of the in ter nal am pli fier. OA2
, OA2 must be a low drift, low in put bias
THAT Corporation; 45 Sumner Street; Milford, Massachusetts 01757-1656; USA
in
Tel: [phone]; Fax: [phone]; Web: www.thatcorp.com
OFF(IN)
re sis tors R
10 k , this dc in put cur -
with out draw ing any
to well be low
A, and al most
1
pro duc ing
re sis tor usu -
OFF(IN)
and R
. Prac -
2
. The
volt -
a
bined gain from the out put of OA2 to the out put of
OA1 is unity.
In this fash ion, the dc volt age at the out put of OA1
is equal to V
no-signal con di tions, no cur rent flows in R
sig nal, this cur rent is
also be a low-offset opamp, though its in put cur -
rent is not as crit i cal as that for OA2. Typ i cal
choices here would in clude an OP27 or an
LM308A type with off set-voltage trim ming.
An ad di tional con sid er ation is that R
R
age ap plied to R
V
mea sured will also af fect the gain to V
well-defined, but non-zero source im ped ances, re -
duce the value of R
un con trolled, non-zero source im ped ances, the in -
put sig nal should be buf fered.
OFF(IN)
4
IN
Fig ure 11. Ex tended Low-Level Range with Se lected
SYM
should be pre ci sion tol er ance types, or the volt -
50k
IN
R
10k
V+
2
R
10k
Fig ure 12. Mea suring DC and AC Signals
. The source im ped ance of the volt age to be
1u
3
+
10k
-
R
OA1
Cin
20u
4
SYM
50k
OFF(IN)
V+
10k
R
OA1
1
V-
24k
47k
20
in
Rin
10k
3
plus the in put volt age. Un der
will not ac cu rately rep re sent
by the value of the source. For
1u
OA2
2252s
V-
10k
24k
47k
Rin
20
R
V
OA2
1
in
in
+
-
. Of course, OA1 must
1u
560k
IN
4
SYM
V-
V-
Rb
5
2252
2M2
2
GND
R
IBIAS
1
T
560k
3
SYM
IN
4
8
V-
V-
Rb
CAP
OUT
2252
5
V+
GND
2M2
2
6
IBIAS
R
T
3
8
Rf 22M
1
CAP
7
OUT
V+
, R
1k
6
10u
OFF(IN)
7
1k
10u
C
10u
2
, R
T
in
10u
V+
C
OUT
. With
T
Page 9
V+
3
OUT
. For
and