SX8723CWLTDT SEMTECH [Semtech Corporation], SX8723CWLTDT Datasheet - Page 19

• Manufacturer Part Number: SX8723CWLTDT
• Description: ZoomingADC for sensing data acquisition
• Manufacturer: SEMTECH [Semtech Corporation]

Revision 1.01
© Semtech
The core of the zooming section is made of three differential programmable amplifiers (PGA). After selection of an
input and reference signals V
stages 1 to 3. Fine gain programming up to 1'000 V/V is possible. In addition, the last two stages provide
programmable offset. Each amplifier can be bypassed if needed.
The output of the cascade of PGA is directly fed to the analog-to-digital converter (ADC), which converts the signal
V
Like most ADCs intended for instrumentation or sensing applications, the ZoomingADC
converter
negative differential input voltages). In first approximation, the ADC output result relative to full-scale (FS) delivers the
quantity:
in two's complement (see
FS / 2 for V
the relationship:
where GD
Table 9, page
7.1.2 Programmable Gain Amplifiers
As seen in
These are:
All gain and offset settings are realized with ratios of capacitors. The user has control over each PGA activation and
gain, as well as the offset of stages 2 and 3. These functions are examined hereafter.
1.
ADVANCED COMMUNICATIONS & SENSING
IN,ADC
PGA1: coarse gain tuning
PGA2: medium gain and offset tuning
PGA3: fine gain and offset tuning. Should be set ON for high linearity data acquisition
Over-sampled converters are operated with a sampling frequency f
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.
into digital.
TOT
1
IN,ADC
Figure 10, page
. The ADC is a so-called incremental converter; with bipolar operation (the ADC accepts both positive and
is the total PGA gain, GD
21).
= -V
REF,ADC
January 2011
Equation 18
18, the zooming function is implemented with three programmable gain amplifiers (PGA).
/ 2 to + V
IN
and V
V
REF,ADC
REF,ADC
OFFTOT
IN
,
and
ADC
=
Equation 19, page 33
combination, the input voltage is modulated and amplified through
is the total magnitude of PGA offset and S is the sign of the offset (see
/ 2 respectively. As will be shown, V
GD
TOT
OUT
FS
⋅
V
Page 19
Equation 1
Equation 2
2 /
IN
ADC
−
GDoff
≅
V
V
S
REF
IN
much higher than the input signal's Nyquist rate (typically f
,
TOT
ADC
ZoomingADC for sensing data acquisition
2 /
for details). The output code OUT
⋅
S
⋅
V
REF
[
V
IN,ADC
]
is related to input voltage V
TM
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is an over-sampled
SX8723C
ADC
DATASHEET
is -FS / 2 to +
S
IN
is 20-
by