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LTC2488

2488f

CH3, COM), on the other hand, are typically driven from

larger source resistances. Source resistances up to 10k

may interface directly to the LTC2488 and settle completely;

however, the addition of external capacitors at the input

terminals in order to ﬁlter unwanted noise (anti-aliasing)

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 LTC2488 uses a proprietary switching algorithm that

forces the average differential input current to zero indepen-

dent of external settling errors. This allows direct digitization

of high impedance sensors without the need of 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

zero, the common mode input current (IIN

+ + IIN–)/2 is

proportional to the difference between the common mode

input voltage (VIN(CM)) and the common mode reference

voltage (VREF(CM)).

In applications where the input common mode voltage is

equal to the reference common mode voltage, as in the

case of a balanced bridge, both the differential and com-

mon mode input currents are zero. The accuracy of the

converter is not compromised by settling errors.

In applications where the input common mode voltage is

constant but different from the reference common mode

voltage, the differential input current remains zero while

the common mode input current is proportional to the

difference between VIN(CM) and VREF(CM). For a reference

common mode voltage of 2.5V and an input common mode

of 1.5V, the common mode input current is approximately

0.74µA. This common mode input current does not degrade

the accuracy if the source impedances tied to IN+ and

APPLICATIONS INFORMATION

Figure 10. LTC2488 Equivalent Analog Input Circuit

IN+

IN–

10k

INTERNAL

SWITCH

NETWORK

10k

CEQ

12pF

10k

IIN

–

REF+

IREF

IIN

IREF

–

2488 F10

SWITCHING FREQUENCY

fSW = 123kHz INTERNAL OSCILLATOR

fSW = 0.4 • fEOSC EXTERNAL OSCILLATOR

REF–

10k

100

Ω

INPUT

MULTIPLEXER

100

Ω

IIN

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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