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LTC1287B

3V Single Chip 12-Bit Data Acquisition System

Linear Technology 

LTC1287

APPLICATI S I FOR ATIO

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

For high frequency bypassing a 0.1µF ceramic disk placed

in parallel with the 22µF 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 10µF to adequately bypass the

supply pin. Figure 4 shows the effect of poor VCC bypassing.

Figure 5 shows the settling of a LT1117 low dropout

regulator with a 22µF bypass capacitor. The noise and

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

of a lithium battery with a 10µF 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

5V/DIV

CS

0.5mV/DIV

VCC

HORIZONTAL: 20µs/DIV

Figure 6. Lithium Battery with 10µF Bypassing on VCC

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.

HORIZONTAL: 10µs/DIV

Figure 4. Poor VCC Bypassing. Noise and

Ripple Can Cause A/D Errors

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.

5V/DIV

CS

0.5mV/DIV

VCC

HORIZONTAL: 20µs/DIV

Figure 5. LT1117 Regulator with 22µF Bypassing on VCC

VIN +

RSOURCE +

VIN –

RSOURCE –

“+”

INPUT

C1

“–”

INPUT

C2

LTC1287

CS↑

RON = 1.5k

tWHCS +

1/2 CLK

CIN =

100pF

LTC1287 F07

Figure 7. Analog Input Equivalent Circuit

1287fa

9

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