SX8724C Datasheet, PDF (19/67 Pages) Semtech Corporation – ZoomingADC for sensing data acquisition

SX8724C
ZoomingADC for sensing data acquisition
ADVANCED COMMUNICATIONS & SENSING
DATASHEET
The core of the zooming section is made of three differential programmable amplifiers (PGA). After selection of an
input and reference signals VIN and VREF,ADC combination, the input voltage is modulated and amplified through
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
VIN,ADC into digital.
Like most ADCs intended for instrumentation or sensing applications, the ZoomingADC^TM is an over-sampled
converter ¹. The ADC is a so-called incremental converter; with bipolar operation (the ADC accepts both positive and
negative differential input voltages). In first approximation, the ADC output result relative to full-scale (FS) delivers the
quantity:
OUT_{ADC â‰} ^V_{IN ,ADC}
FS / 2 V_REF / 2
Equation 1
in two's complement (see Equation 18 and Equation 19, page 33 for details). The output code OUTADC is -FS / 2 to +
FS / 2 for VIN,ADC = -VREF,ADC / 2 to + VREF,ADC / 2 respectively. As will be shown, VIN,ADC is related to input voltage VIN by
the relationship:
V_IN,ADC = GD_TOT ⋅V_IN − GDoff_TOT ⋅ S ⋅V_REF [V ]
Equation 2
where GDTOT is the total PGA gain, GDOFFTOT is the total magnitude of PGA offset and S is the sign of the offset (see
Table 9, page 21).
7.1.2 Programmable Gain Amplifiers
As seen in Figure 10, page 18, the zooming function is implemented with three programmable gain amplifiers (PGA).
These are:
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
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. Over-sampled converters are operated with a sampling frequency fS 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.
Revision 1.01
© Semtech
January 2011
Page 19
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