LTC1091AC datasheet(22/32 Pages) LINER

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LTC1091/LTC1092

LTC1093/LTC1094

APPLICATI

I FOR ATIO

Figure 8. “+” and “–” Input Settling Windows

“–” Input Settling

At the end of the sample phase the input capacitor switches

to the “–” input and the conversion starts (see Figure 8).

During the conversion, the “+” input voltage is effectively

“held” by the sample-and-hold and will not affect the

conversion result. However, it is critical that the “–” input

voltage settle completely during the first CLK cycle of the

conversion time and be free of noise. Minimizing RSOURCE–

and C2 will improve settling time. If large “–” input source

resistance must be used, the time allowed for settling can

be extended by using a slower CLK frequency. At the

maximum CLK rate of 500kHz, RSOURCE– < 1kΩ and

C2 < 20pF will provide adequate settling.

Input Op Amps

When driving the analog inputs with an op amp it is

important that the op amp settle within the allowed time

(see Figure 8). Again, the “+” and “–” input sampling times

can be extended as previously described to accommodate

slower op amps. Most op amps, including the LT1006 and

LT1013 single supply op amps, can be made to settle well

even with the minimum settling windows of 3

µs (“+”

input) and 2

µs (“–” input) which occur at the maximum

clock rate of 500kHz. Figures 9 and 10 show examples of

adequate and poor op amp settling.

CLK

DIN

DOUT

“+” INPUT

“–” INPUT

SAMPLE

HOLD

“+” INPUT MUST

SETTLE DURING

THIS TIME

tSMPL

tCONV

SGL/DIFF

START

MSBF

1ST BIT TEST “–” INPUT MUST

SETTLE DURING THIS TIME

1091-4 F08

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