Datasheet LTC1052, LTC7652 (Analog Devices) - 8

LTC1052/LTC7652
U THEORY OF OPERATIO
C1 S3 VREF C2 l2 + IN S1 + S2 δl – – gm1 + gm2 g + V m4 gm5 – + OUT – IN l – 1 RL1 RL2 CEXT B RL4 RL5 CEXT A gm3 – V– l3 LTC1052/7652 • TO02 gm6 +
Figure 1b. LTC1052 Block Diagram Sampling Cycle U U W U APPLICATIO S I FOR ATIO EXTERNAL CAPACITORS
CEXTA and CEXTB are the holding elements of a sample-
On competitive devices, connecting CEXTA and CEXTB to
and-hold circuit. The important capacitor characteristics
V– causes an increase in amplifier noise. Design changes
are leakage current and dielectric absorption. A high
have eliminated this problem on the LTC1052. On the
quality film-type capacitor such as mylar or polypropylene
14-pin LTC1052 and 8-pin LTC7652, the capacitors can
provides excellent performance. However, low grade
be returned to V– or CRETURN with no change in noise
capacitors such as ceramic are suitable in many
performance.
applications. Capacitors with very high dielectric absorption (ceramic)
ACHIEVING PICOAMPERE/MICROVOLT PERFORMANCE
can take several seconds to settle after power is first turned on. This settling appears as clock ripple on the
Picoamperes
output and, as the capacitor settles, the ripple gradually In order to realize the picoampere level of accuracy of the disappears. If fast settling after power turn-on is LTC1052, proper care must be exercised. Leakage important, mylar or polypropylene is recommended. currents in circuitry external to the amplifier can Above 85°C, leakage, both from the holding capacitors significantly degrade performance. High quality insulation and the printed circuit board, becomes important. To should be used (e.g., Teflon, Kel-F); cleaning of all maintain the capabilities of the LTC1052 it may be insulating surfaces to remove fluxes and other residues necessary to use Teflon™ capacitors and Teflon standoffs will probably be necessary—particularly for high when operating at 125°C (see Achieving Picoampere/ temperature performance. Surface coating may be Microvolt Performance). necessary to provide a moisture barrier in high humidity environments. CEXTA and CEXTB are normally in the range of 0.1µF to 1.0µF. All specifications are guaranteed with 0.1µF and Board leakage can be minimized by encircling the input the broadband noise (refer to Typical Performance Char- connections with a guard ring operated at a potential acteristics) is only very slightly degraded with 0.1µF. close to that of the inputs: in inverting configurations, the Output clock ripple is not present for capacitors of 0.1µF guard ring should be tied to ground; in noninverting or greater at any temperature. Teflon is a trademark of Dupont. 1052fa 8