LTC1052/LTC7652
THEORY OF OPERATIO
U
C1
S3
V
REF
C2
+ IN
S1
l
2
S2
g
m2
R
L1
R
L2
C
EXT B
C
EXT A
g
m3
g
m4
+
–
+
R
L4
g
m5
V
OUT
R
L5
V
–
LTC1052/7652 • TO02
+
g
m1
–
+
–
l
1
δl
– IN
–
–
g
m6
l
3
+
Figure 1b. LTC1052 Block Diagram
Sampling Cycle
APPLICATIO S I FOR ATIO
EXTERNAL CAPACITORS
C
EXTA
and C
EXTB
are the holding elements of a sample-
and-hold circuit. The important capacitor characteristics
are leakage current and dielectric absorption. A high
quality film-type capacitor such as mylar or polypropylene
provides excellent performance. However, low grade
capacitors such as ceramic are suitable in many
applications.
Capacitors with very high dielectric absorption (ceramic)
can take several seconds to settle after power is first
turned on. This settling appears as clock ripple on the
output and, as the capacitor settles, the ripple gradually
disappears. If fast settling after power turn-on is
important, mylar or polypropylene is recommended.
Above 85°C, leakage, both from the holding capacitors
and the printed circuit board, becomes important. To
maintain the capabilities of the LTC1052 it may be
necessary to use Teflon™ capacitors and Teflon standoffs
when operating at 125°C (see Achieving Picoampere/
Microvolt Performance).
C
EXTA
and C
EXTB
are normally in the range of 0.1µF
to 1.0µF. All specifications are guaranteed with 0.1µF and
the broadband noise (refer to Typical Performance Char-
acteristics) is only very slightly degraded with 0.1µF.
Output clock ripple is not present for capacitors of 0.1µF
or greater at any temperature.
8
U
On competitive devices, connecting C
EXTA
and C
EXTB
to
V
–
causes an increase in amplifier noise. Design changes
have eliminated this problem on the LTC1052. On the
14-pin LTC1052 and 8-pin LTC7652, the capacitors can
be returned to V
–
or C
RETURN
with no change in noise
performance.
ACHIEVING PICOAMPERE/MICROVOLT PERFORMANCE
Picoamperes
In order to realize the picoampere level of accuracy of the
LTC1052, proper care must be exercised. Leakage
currents in circuitry external to the amplifier can
significantly degrade performance. High quality insulation
should be used (e.g., Teflon, Kel-F); cleaning of all
insulating surfaces to remove fluxes and other residues
will probably be necessary—particularly for high
temperature performance. Surface coating may be
necessary to provide a moisture barrier in high humidity
environments.
Board leakage can be minimized by encircling the input
connections with a guard ring operated at a potential
close to that of the inputs: in inverting configurations, the
guard ring should be tied to ground; in noninverting
Teflon is a trademark of Dupont.
1052fa
W
U U
LINER [ LINEAR TECHNOLOGY ]
