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LTC2484

2484fa

Automatic Differential Input Current Cancellation

In applications where the sensor output impedance is low

(up to 10k

Ω with no external bypass capacitor or up to

500

Ω with 0.001µF bypass), complete settling of the input

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

digitization of the sensor is possible.

For many applications, the sensor output impedance com-

bined with external bypass capacitors produces RC time

constants much greater than the 580ns required for 1ppm

accuracy. For example, a 10k

Ω bridge driving a 0.1µF

bypass capacitor has a time constant an order of magni-

tude greater than the required maximum. Historically,

settling issues were solved using buffers. These buffers

led to increased noise, reduced DC performance (Offset/

Drift), limited input/output swing (cannot digitize signals

near ground or VCC), added system cost and increased

power. The LTC2484 uses a proprietary switching algo-

rithm that forces the average differential input current to

zero independent of external settling errors. This allows

accurate direct digitization of high impedance sensors

without the need for buffers. Additional errors resulting

from mismatched leakage currents must also be taken into

account.

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 (VINCM) and the common mode reference

voltage (VREFCM).

In applications where the input common mode voltage is

equal to the reference common mode voltage, as in the

case of a balance bridge type application, both the differ-

ential and common mode input current are zero. The

accuracy of the converter is unaffected by settling errors.

Mismatches in source impedances between IN+ and IN–

also do not affect the accuracy.

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 VINCM and VREFCM. For a reference

common mode of 2.5V and an input common mode of

1.5V, the common mode input current is approximately

0.74

µA (in simultaneous 50Hz/60Hz rejection mode). This

common mode input current has no effect on the accuracy

if the external source impedances tied to IN+ and IN– are

matched. Mismatches in these source impedances lead to

a fixed offset error but do not affect the linearity or full-

scale reading. A 1% mismatch in 1k

Ω source resistances

leads to a 15ppm shift (74

µV) in offset voltage.

APPLICATIO S I FOR ATIO

Figure 11. LTC2484 Equivalent Analog Input Circuit

VREF

VIN

VCC

RSW (TYP)

10k

ILEAK

VCC

ILEAK

VCC

RSW (TYP)

10k

CEQ

12pF

(TYP)

RSW (TYP)

10k

ILEAK

IIN

VIN

–

IIN

–

IREF

–

2484 F11

ILEAK

VCC

ILEAK

SWITCHING FREQUENCY

fSW = 123kHz INTERNAL OSCILLATOR

fSW = 0.4 • fEOSC EXTERNAL OSCILLATOR

GND

RSW (TYP)

10k

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