LTC1293BCSW datasheet(19/28 Pages) LINER

19 / 28 page

LTC1293/LTC1294/LTC1296

129346fs

APPLICATI

I FOR ATIO

Source Resistance

The analog inputs of the LTC1293/4/6 look like a 100pF

capacitor (CIN) in series with a 500Ω resistor (RON). CIN

gets switched between (+) and (–) inputs once during each

conversion cycle. Large external source resistors and

capacitances will slow the settling of the inputs. It is

important that the overall RC time constant is short

enough to allow the analog inputs to settle completely

within the allowed time.

“+” Input Settling

The input capacitor is switched onto the “+” input during

the sample phase (tSMPL, see Figure 8). The sample period

2 1/2 CLK cycles before a conversion starts. The voltage on

the “+” input must settle completely within the sample

period. Minimizing RSOURCE+ and C1 will improve the

settling time. If large “+” input source resistance must be

used, the sample time can be increased by using a slower

CLK frequency. With the minimum possible sample time

of 2.5

µs RSOURCE+ < 1.5kΩ

Ω

Ω and C1 < 20pF will provide

adequate settling time.

“–” 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. It is critical that the “–” input voltage be

free of noise and settle completely during the first CLK

cycle of the conversion. Minimizing RSOURCE– and C2 will

improve settling time. If large “–” input source resistance

must be used the time can be extended by using a slower

CLK frequency. At the maximum CLK frequency of 1MHz,

RSOURCE– < 250Ω

Ω

Ω 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 settles within the allowed time

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

can be extended as described above to accommodate

slower op amps. Most op amps including the LT1006 and

LT1013 single supply op amps can be made to settle

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

DIN

CLK

START

HI-Z

LTC1293 F08

1ST BIT TEST (–) INPUT MUST

SETTLE DURING THIS TIME

tSMPL

(+) INPUT MUST SETTLE DURING THIS TIME

(+) INPUT

(–) INPUT

SGL/

DIFF

MSBF

DOUT

B11

SAMPLE

HOLD

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