Minimizing The Effect Of High Input Source Resistance - Intel 8XC196MC User Manual

Typically, the (R / A
F V
determines the minimum sample time specified in the datasheet.
12.6.1.1

Minimizing the Effect of High Input Source Resistance

Under some conditions, the input source resistance (R
measurement. You can minimize this effect by increasing the sample time or by connecting an
external capacitor (C
EXT
the source voltage level. When the channel is sampled, C
charge the sample capacitor (C
sulting in a drop of the sampled voltage. The voltage drop is calculated using the following for-
mula.
Sampled Voltage Drop, %
If C is 0.005 µF or greater, the error will be less than –0.4 LSB in 10-bit conversion mode. The
EXT
use of C in conjunction with R
EXT
A/D converter.
High R resistance can also cause errors due to the input leakage (I
SOURCE
lower than its specified maximum (consult the datasheet for specifications). The combined effect
of I leakage and high R
LI 1
R
SOURCE
----------------------------------------------------------- -
error (LSBs) =
where:
R is the input source resistance, in ohms
SOURCE
I is the input leakage, in amperes
1
LI
V is the reference voltage, in volts
REF
External circuits with R
have a resultant error due to source impedance of 0.6 LSB or less.
+ 1) term is the major contributor to the total resistance and the factor that
) from the input pin to ANGND. The external signal will charge C
). A small portion of the charge in C
S
C
S
×
---------------------------
= 100%
C + C
EXT S
forms a low-pass filter that reduces noise input to the
SOURCE
resistance is calculated using the following formula.
SOURCE
× ×
I 1024
LI1
V
REF
resistance of 1 kilo-ohm or lower and V
SOURCE
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ANALOG-TO-DIGITAL (A/D) CONVERTER
) can be great enough to affect the
SOURCE
acts as a low-impedance source to
EXT
is transferred to C
EXT
).
LI 1
REF
to
EXT
, re-
S
I is typically much
LI 1
equal to 5.0 volts will
12-11