LTC1287 datasheet(9/16 Pages) LINER

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LTC1287

1287fa

APPLICATI

I FOR ATIO

such as obtained from a voltage regulator (e.g., LT1117).

For high frequency bypassing a 0.1

µF ceramic disk placed

in parallel with the 22

µF is recommended. Again the leads

should be kept to a minimum. Using a battery to power the

LTC1287 will help reduce the amount of bypass capacitance

required on the VCC pin. A battery placed close to the

device will only require 10

µF to adequately bypass the

supply pin. Figure 4 shows the effect of poor VCCbypassing.

Figure 5 shows the settling of a LT1117 low dropout

regulator with a 22

µF bypass capacitor. The noise and

ripple is kept around 0.5mV. Figure 6 shows the response

of a lithium battery with a 10

µF bypass capacitor. The

noise and ripple is kept below 0.5mV.

Analog Inputs

Because of the capacitive redistribution A/D conversion

techniques used, the analog inputs of the LTC1287 have

Figure 5. LT1117 Regulator with 22

µF Bypassing on VCC

5V/DIV

VCC

0.5mV/DIV

HORIZONTAL: 20

µs/DIV

Figure 6. Lithium Battery with 10

µF Bypassing on VCC

5V/DIV

VCC

0.5mV/DIV

HORIZONTAL: 20

µs/DIV

capacitive switching input current spikes. These current

spikes settle quickly and do not cause a problem. If large

source resistances are used or if slow settling op amps

drive the inputs, take care to insure the transients caused

by the current spikes settle completely before the

conversion begins.

Source Resistance

The analog inputs of the LTC1287 look like a 100pF

capacitor (CIN) in series with a 1.5k resistor (RON). This

value for RON is for VCC = 2.7V. With larger supply voltages

RON will be reduced. For example, with VCC = 2.7V and V –

= – 2.7V, RON becomes 500Ω. 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.

Figure 7. Analog Input Equivalent Circuit

HORIZONTAL: 10

µs/DIV

Figure 4. Poor VCC Bypassing. Noise and

Ripple Can Cause A/D Errors

↑

RON = 1.5k

tWHCS +

1/2 CLK

CIN =

100pF

LTC1287

“+”

INPUT

RSOURCE +

VIN +

“–”

INPUT

RSOURCE –

VIN –

LTC1287 F07

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