xe8802 Semtech Corporation, xe8802 Datasheet - Page 141

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

17.9.4
The incremental ADC is an over-sampled converter with two main blocks: an analog modulator and a low-pass
digital filter. The main function of the digital filter is to remove the quantitative noise introduced by the modulator. As
shown in Figure 17-26, this filter determines the frequency response of the transfer function between the output of
the ADC and the analog input V
period OSR/f
elementary conversions N
and are repeated every f
Information on the location of these notches is particularly useful when specific frequencies must be filtered out by
the acquisition system. For example, consider a 5Hz-bandwidth, 16-bit sensing system where 50Hz line rejection is
needed. Using the above equation and the plots below, we set the 4th notch for N
1.25⋅f
choice yields also good attenuation of 50Hz harmonics.
17.9.5
The ZoominADC™ is particularly well suited for low-power applications. When very low power consumption is of
primary concern, such as in battery operated systems, several parameters can be used to reduce power
consumption as follows:
© Semtech 2006
1.2
0.8
0.6
0.4
0.2
1.2
0.8
0.6
0.4
0.2
1)
2)
3)
1
0
S
1
0
/OSR = 50Hz. The sampling frequency is then calculated as f
0
0
f
Operate the acquisition chain with a reduced supply voltage V
Disable the PGAs which are not used during analog-to-digital conversion with ENABLE[3:0].
Disable all PGAs and the ADC when the system is idle and no conversion is performed.
NOTCH
Frequency Response
Power Reduction
Figure 17-26. Frequency response: normalized magnitude vs. frequency for different N
S
. The plots of Figure 17-26 also show that the frequency response changes with the number of
Normalized Frequency - f *(OSR/f
Normalized Frequency - f *(OSR/f
) (
i
1
1
=
OSR
i
⋅
S
⋅
N
/OSR.
ELCONV
f
ELCONV
2
S
2
performed. In particular, notches appear for N
IN
. Notice that the frequency axes are normalized to one elementary conversion
N
N
ELCONV
ELCONV
3
S
3
S
) [-]
) [-]
(Hz)
= 1
= 4
XE8802 Sensing Machine Data Acquisition MCU
4
4
for
i
=
1
17-29
2 ,
1.2
0.8
0.6
0.4
0.2
1.2
0.8
0.6
0.4
0.2
,...,
1
0
1
0
0
0
(
with ZoomingADC™ and LCD driver
N
ELCONV
Normalized Frequency - f *(OSR/f
Normalized Frequency - f *(OSR/f
1
−
1
S
) 1
= 20.48kHz for OSR = 512. Notice that this
DD
.
ELCONV
2
2
(Eq. 23)
≥ 2. These notches occur at:
N
N
ELCONV
ELCONV
3
3
S
S
) [-]
) [-]
ELCONV
= 2
= 8
= 4 to 50Hz, i.e.
4
4
www.semtech.com
ELCONV