LTC1968 datasheet(25/28 Pages) LINER

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LTC1968

1968f

APPLICATIO S I FOR ATIO

7. Output is noisy with >200kHz inputs.

– This is a fundamental characteristic of this topol-

ogy. The LTC1968 is designed to work very well

with inputs of 100kHz or less. It works okay as high

as 1MHz, but it is limited by aliased ∆Σ noise.

Solution: Bandwidth limit the input or digitally filter

the resulting output.

8. Large errors occur at crest factors approaching, but

less than 4.

– Insufficient averaging.

Solution: Increase CAVE. See “Crest Factor and AC +

DC Waveforms” section for discussion of output

droop.

9. Screwy results, errors > spec limits, typically 1% to 5%.

– High impedance (12.5kΩ) and high accuracy (0.1%)

require clean boards! Flux residue, finger grime, etc.

all wreak havoc at this level.

Solution: Wash the board.

10. Gain is low by ≅1% or more, no other problems.

– Probably due to circuit loading. With a DMM or a

10× scope probe, ZIN = 10MΩ. The LTC1968

output is 12.5kΩ, resulting in – 0.125% gain

error. Output impedance is higher with the DC

accurate post filter.

Solution: Remove the shunt loading or buffer the

output.

– Loading can also be caused by cheap averaging

capacitors.

Solution: Use a high quality metal film capacitor

for CAVE.

LTC1968

KEEP BOARD CLEAN

1968 TS09

200mVRMS IN

–0.125%

DMM

DCV

LTC1968

10M

12.5k

VOUT

OUT RTN

1968 TS10

LOADING DRAGS DOWN GAIN

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