25
LTC1968
1968f
APPLICATIO S I FOR ATIO
WUUU
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 C
AVE
. 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, Z
IN
= 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 C
AVE
.
LTC1968
KEEP BOARD CLEAN
1968 TS09
200mV
RMS
IN
–0.125%
DMM
DCV
LTC1968
10M
5
12.5k
6
V
OUT
OUT RTN
1968 TS10
LOADING DRAGS DOWN GAIN
mV
