LTC1288CS8 datasheet(17/24 Pages) LINER

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LTC1285/LTC1288

APPLICATION INFORMATION

Input Leakage Current

Input leakage currents can also create errors if the source

resistance gets too large. For instance, the maximum

input leakage specification of 1

µA (at 125°C) flowing

through a source resistance of 240

Ω will cause a voltage

drop of 240

µV or 0.4LSB. This error will be much

reduced at lower temperatures because leakage drops

rapidly (see typical curve of Input Channel Leakage

Current vs Temperature).

REFERENCE INPUTS

The reference input of the LTC1285 is effectively a 50k

Ω

resistor from the time CS goes low to the end of the

conversion. The reference input becomes a high impedence

node at any other time (see Figure 10). Since the voltage

on the reference input defines the voltage span of the A/D

converter, the reference input should be driven by a

reference with low ROUT(ex. LT1004, LT1019 and LT1021)

or a voltage source with low ROUT.

selected “+” 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 constants be short enough to allow the

analog inputs to completely settle within the allowed time.

LTC1285

REF+

ROUT

VREF

GND

LTC1285/88 • F10

Figure 10. Reference Input Equivalent Circuit

Reduced Reference Operation

The minimum reference voltage of the LTC1288 is limited

to 2.7V because the VCC supply and reference are inter-

nally tied together. However, the LTC1285 can operate

with reference voltages below 1.5V.

The effective resolution of the LTC1285 can be increased

by reducing the input span of the converter. The LTC1285

exhibits good linearity and gain over a wide range of

reference voltages (see typical curves of Change in Linear-

ity vs Reference Voltage and Change in Gain vs Reference

RON = 500Ω

CIN = 20pF

LTC1285

LTC1288

“+”

INPUT

RSOURCE

VIN +

“–”

INPUT

RSOURCE

–

VIN –

LTC1285/88 • F08

Figure 8. Analog Input Equivalent Circuit

RC Input Filtering

It is possible to filter the inputs with an RC network as

shown in Figure 9. For large values of CF (e.g., 1µF), the

capacitive input switching currents are averaged into a

net DC current. Therefore, a filter should be chosen with

a small resistor and large capacitor to prevent DC drops

across the resistor. The magnitude of the DC current is

approximately IDC = 20pF × VIN/tCYC and is roughly

proportional to VIN. When running at the minimum cycle

time of 133.3

µs, the input current equals 0.375µA at VIN

= 2.5V. In this case, a filter resistor of 160

Ω will cause

0.1LSB of full-scale error. If a larger filter resistor must

be used, errors can be eliminated by increasing the cycle

time.

RFILTER

VIN

CFILTER

LTC1285/88 • F09

LTC1285

“

+”

“

–”

IDC

Figure 9. RC Input Filtering

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