Semtech 2006
www.semtech.com
16-4
XE8805/05A
pos.
RegAcCfg3
rw
reset
description
7
PGA1_GAIN
r w
0 resetsystem
PGA1 stage gain selection
6:0
PGA3_GAIN[6:0]
r w
0000000
resetsystem
PGA3 stage gain selection
Table 16-7: RegAcCfg3
pos.
RegAcCfg4
rw
reset
description
7
reserved
r
0
Unused
6:0
PGA3_OFFSET[6:0]
r w
0000000
resetsystem
PGA3 stage offset selection
Table 16-8: RegAcCfg4
pos.
RegAcCfg5
rw
reset
description
7
BUSY
r
0 resetsystem
Activity flag
6
DEF
w r0
0
Selects default configuration
5:1
AMUX[4:0]
r w
00000
resetsystem
Input channel configuration selector
0
VMUX
r w
0 resetsystem
Reference channel selector
Table 16-9: RegAcCfg5
16.4
ZoomingADC
Description
Figure 16-2 gives a more detailed description of the acquisition chain.
16.4.1
Acquisition Chain
Figure 16-1 shows the general block diagram of the acquisition chain (AC). A control block (not shown in Figure
16-1) manages all communications with the CoolRisc
microcontroller.
Analog inputs can be selected among eight input channels, while reference input is selected between two
differential channels.
The core of the zooming section is made of three differential programmable amplifiers (PGA). After selection of a
combination of input and reference signals VIN and VREF, the input voltage is modulated and amplified through
stages 1 to 3. Fine gain programming up to 1'000V/V is possible. In addition, the last two stages provide
programmable offset. Each amplifier can be bypassed if needed.
The output of the PGA stages is directly fed to the analog-to-digital converter (ADC), which converts the signal
VIN,ADC into digital.
Like most ADCs intended for instrumentation or sensing applications, the ZoomingADC
is an over-sampled
converter (See Note1). The ADC is a so-called incremental converter, with bipolar operation (the ADC accepts both
positive and negative input voltages). In first approximation, the ADC output result relative to full-scale (FS) delivers
the quantity:
1 Note: Over-sampled converters are operated with a sampling frequency f
S much higher than the input signal's Nyquist rate (typically fS is 20-
1'000 times the input signal bandwidth). The sampling frequency to throughput ratio is large (typically 10-500). These converters include digital
decimation filtering. They are mainly used for high resolution, and/or low-to-medium speed applications.
Not
Recommended
for
New
Designs