LTC1286CS8 datasheet(17/24 Pages) LINER

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LTC1286/LTC1298

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.

Reduced Reference Operation

The minimum reference voltage of the LTC1298 is limited

to 4.5V because the VCC supply and reference are inter-

nally tied together. However, the LTC1286 can operate

with reference voltages below 1V.

The effective resolution of the LTC1286 can be increased

by reducing the input span of the converter. The LTC1286

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

Voltage). However, care must be taken when operating at

low values of VREF because of the reduced LSB step size

and the resulting higher accuracy requirement placed on

the converter. The following factors must be considered

when operating at low VREF values:

1. Offset

2. Noise

3. Conversion speed (CLK frequency)

Offset with Reduced VREF

The offset of the LTC1286 has a larger effect on the output

code. When the ADC is operated with reduced reference

voltage. The offset (which is typically a fixed voltage)

becomes a larger fraction of an LSB as the size of the LSB

is reduced. The typical curve of Change in Offset vs

Reference Voltage shows how offset in LSBs is related to

reference voltage for a typical value of VOS. For example,

a VOS of 122µV which is 0.1LSB with a 5V reference

becomes 0.5LSB with a 1V reference and 2.5LSBs with a

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.

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 64

µs, the input current equals 1.56µA at VIN = 5V.

In this case, a filter resistor of 75

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

LTC1286/98 • F09

LTC1286

“+”

“–”

IDC

Figure 9. RC Input Filtering

LTC1286

REF+

ROUT

VREF

GND

LTC1286/98 • F10

Figure 10. Reference Input Equivalent Circuit

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 0.2LSB. This error will be much reduced

at lower temperatures because leakage drops rapidly (see

typical curve of Input Channel Leakage Current vs Tem-

perature).

REFERENCE INPUTS

The reference input of the LTC1286 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

APPLICATION INFORMATION

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