LTC2495 datasheet(23/32 Pages) LINER

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LTC2495

2495f

input/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-CH15, COM), on the other hand, are typically driven

from larger source resistances. Source resistances up

to 10k may interface directly to the LTC2495 and settle

completely; however, the addition of external capacitors

at the input terminals in order to ﬁlter unwanted noise

(antialiasing) results in incomplete settling.

The LTC2495 offers two methods of removing these

errors. The ﬁrst is automatic differential input current

cancellation (Easy Drive) and the second is the insertion

of an external buffer between the MUXOUT and ADCIN

pins, thus isolating the input switching from the source

resistance.

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

Figure 11. Equivalent Analog Input Circuit

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 10k

Ω bridge driving a

0.1µF capacitor has a time constant an order of magnitude

greater than the required maximum.

The LTC2495 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

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)).

APPLICATIONS INFORMATION

IN+

IN–

10k

Ω

INTERNAL

SWITCH

NETWORK

10k

Ω

CEQ

µF

10k

Ω

IIN

–

REF+

IREF

IIN

IREF

–

2495 F11

SWITCHING FREQUENCY

fSW = 123kHz INTERNAL OSCILLATOR

fSW = 0.4 • fEOSC EXTERNAL OSCILLATOR

REF–

10k

Ω

100

Ω

INPUT

MULTIPLEXER

EXTERNAL

CONNECTION

100

Ω

MUXOUTP

ADCINP

EXTERNAL

CONNECTION

MUXOUTN

ADCINN

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

−

()

–

M INTERNAL OSCILLATOR

−

⎛

⎝

⎜⎜

⎞

⎠

⎟⎟

271

Ω

0Hz MODE

2.98M INTERNAL OSCILLATOR 50Hz/60

EQ =Ω

Hz MODE

0.833 10

EXTERNAL OSCIL

EOSC

=•

()

LATOR

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