Datasheet

LPC111X All information provided in this document is subject to legal disclaimers. © NXP Semiconductors N.V. 2014. All rights reserved.
Product data sheet Rev. 9.2 — 26 March 2014 102 of 127
NXP Semiconductors
LPC1110/11/12/13/14/15
32-bit ARM Cortex-M0 microcontroller
12.8 ADC effective input impedance
A simplified diagram of the ADC input channels can be used to determine the effective
input impedance seen from an external voltage source. See Figure 53
.
The effective input impedance, R
in
, seen by the external voltage source, V
EXT
, is the
parallel impedance of ((1/f
s
x C
ia
) + R
mux
+ R
sw
) and (1/f
s
x C
io
), and can be calculated
using Equation 2
with
f
s
= sampling frequency
C
ia
= ADC analog input capacitance
R
mux
= analog mux resistance
R
sw
= switch resistance
C
io
= pin capacitance
(2)
Under nominal operating condition V
DD
= 3.3 V and with the maximum sampling
frequency fs = 400 kHz, the parameters assume the following values:
C
ia
= 1 pF (max)
R
mux
= 2 kΩ (max)
R
sw
= 1.3 kΩ (max)
C
io
= 7.1 pF (max)
The effective input impedance with these parameters is R
in
= 308 kΩ.
Fig 53. ADC input channel
C
ia
R
s
V
SS
V
EXT
002aah615
ADC
COMPARATOR
ADC Block
R
in
C
io
R
mux
R
sw
Source
<2 kΩ <1.3 kΩ
R
in
1
f
s
C
ia

------------------
R
mux
R
sw
++


1
f
s
C
io

------------------



=