UM10208_2
© NXP B.V. 2007. All rights reserved.
User manual
Rev. 02 — 1 June 2007
267 of 362
NXP Semiconductors
UM10208
Chapter 21: LPC2800 DADC
The analog signals can be AC-coupled to the AINL and AINR with series capacitors. If this
is done, AINL and AINR should be pulled down to analog ground with resistors of about
1 M
Ω
. For voltages that vary with frequencies between 30 Hz and 10 kHz, the series
capacitors should be about 22
μ
F.
Assuming that the V
REFN(DADC)
and V
REFP(DADC)
pins are connected to analog ground and
+
3.3 V per normal practice, such AC-coupled AINL and AINR signal sources can be up to
1 V RMS. The PGAs include a series 12 k
Ω
resistor that can be used with a similar
external series resistor connected between (the series capacitor and pull-down resistor)
and the AINL and AINR pins, to handle signals can be up to 2 V RMS.
shows how to use such a series resistor and set the gain of the PGA, to handle signals
with varying amounts of voltage range. The last two rows can be extrapolated to smaller
voltage ranges and higher gain settings, although signal-to-noise ratios will degrade.
4.
Dual ADC Block Diagrams
shows how the Dual ADC and its supporting modules are connected.
shows further detail of the Decimator block.
5.
Dual ADC registers
lists the LPC288x registers that are associated with the Dual ADC and its
supporting modules. Subsequent sections describe the registers in greater detail.
Table 301. Maximum source voltage swing vs. external series resistance and PGA gain
External 12 k
Ω
series R?
PGA gain
Maximum source voltage swing
Yes
0 dB
2 V RMS
Yes
+6 dB
1 V RMS
No
0 dB
1 V RMS
No
+6 dB
0.5 V RMS
Fig 28. Block diagram of the Dual ADC and associated modules
PGA
Single-ended
to Differential
Sigma-Delta
ADC
Decimator, DC Blocking
and digital Gain Control
0 to 24 db in
3 db steps
128 * fs
fs; 24-bit data
AINL,
AINR
SAI4
to CPU
or DMA
Fig 29. Decimator block diagram
Comb filter
16x decimator
DC block
filter 1
DC block
filter 2
Gain
Control
1
3 Half-band
filter stages
1 * fs
128 * fs
8 * fs
8 * fs
8 * fs
fs; 24-bit
data out
bit streams
from ADCs
1
-63.5 dB to +24 dB in
0.5 dB steps