LTC2489CDEPBF datasheet(16/24 Pages) LINER

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LTC2489

2489f

without changing the channel, a Stop may be issued after

acknowledgement of the global write command. Global

read commands are not allowed and the converters will

NAK a global read request.

Driving the Input and Reference

The input and reference pins of the LTC2489 are connected

directly to a switched capacitor network. Depending on

the relationship between the differential input voltage and

the differential reference voltage, these capacitors are

switched between these four pins. Each time a capacitor

is switched between two of these pins, a small amount

of charge is transferred. A simpliﬁed equivalent circuit is

shown in Figure 9.

When using the LTC2489’s internal oscillator, the input

capacitor array is switched at 123kHz. The effect of the

charge transfer depends on the circuitry driving the in-

put/reference pins. If the total external RC time constant

is less than 580ns the errors introduced by the sampling

process are negligible since complete settling occurs.

Typically, the reference inputs are driven from a low

impedance source. In this case, complete settling occurs

even with large external bypass capacitors. The inputs

(CH0-CH3, COM), on the other hand, are typically driven

from larger source resistances. Source resistances up

to 10k may interface directly to the LTC2489 and settle

completely; however, the addition of external capacitors

at the input terminals in order to ﬁlter unwanted noise

(antialiasing) results in incomplete settling.

Automatic Differential Input Current Cancellation

In applications where the sensor output impedance is

low (up to 10kΩ with no external bypass capacitor or up

to 500Ω with 0.001μF bypass), complete settling of the

input occurs. In this case, no errors are introduced and

direct digitization is possible.

For many applications, the sensor output impedance

combined with external input bypass capacitors produces

RC time constants much greater than the 580ns required

for 1ppm accuracy. For example, a 10kΩ bridge driving a

0.1μF capacitor has a time constant an order of magnitude

greater than the required maximum.

The LTC2489 uses a proprietary switching algorithm

that forces the average differential input current to zero

independent of external settling errors. This allows direct

digitization of high impedance sensors without the need

for buffers.

The switching algorithm forces the average input current

on the positive input (IIN+) to be equal to the average input

current on the negative input (IIN–). Over the complete

conversion cycle, the average differential input current

(IIN+ – IIN–) is zero. While the differential input current is

APPLICATIONS INFORMATION

IN+

IN–

10k

INTERNAL

SWITCH

NETWORK

10k

CEQ

μF

10k

IIN

–

REF+

IREF

IIN

IREF

–

2489 F09

SWITCHING FREQUENCY

fSW = 123kHz INTERNAL OSCILLATOR

fSW = 0.4 • fEOSC EXTERNAL OSCILLATOR

REF–

10k

100

INPUT

MULTIPLEXER

100

Figure 9. Equivalent Analog Input Circuit

IIN

I IN

AVG

IN CM

REF CM

() = () =

−

•

–

()

IIREF

AVG

REF

REF CM

IN CM

() ≈

()

.–

.•

()

REF

REF CM

where

REF

–

•

(

=−

+−

))

–

⎛

⎝

⎜⎜

⎞

⎠

⎟⎟

=−

+−

REF

WHERE IN AN

D IN ARE THE SELECTED INPUT CHANNELS

IN CM

−

()

–

–IN−

⎛

⎝

⎜⎜

⎞

⎠

⎟⎟

=Ω

2.98M INTERNAL OSCILLATOR

EOSC

0.833 10

EXTERNAL OSCILLATOR

=•

()

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