LTC7652_15 datasheet(8/24 Pages) LINER

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LTC1052/LTC7652

1052fa

Figure 1b. LTC1052 Block Diagram

Sampling Cycle

THEORY OF OPERATIO

LTC1052/7652 • TO02

–

gm6

+IN

–IN

VREF

RL1

V–

–

RL5

VOUT

RL4

CEXT A

CEXT B

RL2

δl

–

gm1

gm2

gm3

gm4

gm5

APPLICATIO S I FOR ATIO

EXTERNAL CAPACITORS

CEXTA and CEXTB 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).

CEXTA and CEXTB 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.

On competitive devices, connecting CEXTA and CEXTB 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 CRETURN 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.

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