xe8802 Semtech Corporation, xe8802 Datasheet - Page 122

• Manufacturer Part Number: xe8802
• Description: Data Acquisition Soc With 16+10 Bit Zoomingadc? Embedded Risc Controller And Lcd Driver
• Manufacturer: Semtech Corporation

17.6.1
Depending on the application objectives, the user may enable or bypass each PGA stage. This is done using the
word ENABLE and the coding given in Table 17-13. To reduce power dissipation, the ADC can also be inactivated
while idle.
17.6.2
The first stage can have a buffer function (unity gain) or provide a gain of 10 (see Table 17-14). The voltage V
the output of PGA1 is:
where GD
17.6.3
The second PGA has a finer gain and offset tuning capability, as shown in Table 17-15 and Table 17-16. The
voltage V
where GD
PGA2_GAIN[1:0] and PGA2_OFFSET[3:0] .
17.6.4
The finest gain and offset tuning is performed with the third and last PGA stage, according to the coding of Table
17-17 and Table 17-18. The output of PGA3 is also the input of the ADC. Thus, similarly to PGA2, we find that the
voltage entering the ADC is given by:
© Semtech 2006
V
V
D2
D
D
1
2
1
2
PGA & ADC Enabling
PGA1
PGA2
PGA3
is the gain of PGA1 (in V/V) controlled with the bit PGA1_GAIN.
at the output of PGA2 is given by:
and GDoff
=
=
GD
GD
1
2
⋅
⋅
V
V
IN
D
1
2
−
are respectively the gain and offset of PGA2 (in V/V). These are controlled with the words
GDoff
2
(V)
⋅
V
REF
PGA3_OFFSET[6:0]
(V)
0000000
0000001
0000010
0010000
0100000
0111111
1000000
1000001
1000010
1010000
1100000
1111111
Table 17-18. PGA3 offset settings
XE8802 Sensing Machine Data Acquisition MCU
...
...
...
...
...
...
17-10
with ZoomingADC™ and LCD driver
+1/12(=+0.083)
+63/12(=+5.25)
-1/12(=-0.083)
-63/12(=-5.25)
PGA3 Offset
GDoff
+16/12
+32/12
-16/12
-32/12
+2/12
-2/12
...
...
...
...
...
...
0
0
3
(V/V)
(Eq. 5)
(Eq. 6)
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D1
at