
Semtech 2006
www.semtech.com
16-32
XE8805/05A
4)
For highest linearity and lowest noise performance, bypass all PGAs and use the ADC alone (applications
where no "zooming" is needed); i.e. set ENABLE[3:0] = '0001'.
5)
For low-noise applications where power consumption is not a primary concern, maintain the largest bias
currents in the PGAs and in the ADC; i.e. set IB_AMP_PGA[1:0] = IB_AMP_ADC[1:0] = '11'.
6)
For lowest output offset error at the output of the ADC, bypass PGA2 and PGA3. Indeed, PGA2 and
PGA3 typically introduce an offset of about 5 to 10 LSB (16 bit) at their output. Note, however, that the
ADC output offset is easily calibrated out by software.
16.9.4
Frequency Response
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 quantization noise introduced by the modulator.
As shown in Figure 16-26, this filter determines the frequency response of the transfer function between the output
of the ADC and the analog input VIN. Notice that the frequency axes are normalized to one elementary conversion
period OSR/fS. The plots of Figure 16-26 also show that the frequency response changes with the number of
elementary conversions NELCONV performed. In particular, notches appear for NELCONV ≥ 2. These notches occur at:
ELCONV
S
NOTCH
N
OSR
f
i
f
=
)
(
(Hz)
for
)
1
(
,...,
2
,
1
=
ELCONV
N
i
(Eq. 23)
and are repeated every fS/OSR.
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 NELCONV = 4 to 50Hz, i.e.
1.25
f
S/OSR = 50Hz. The sampling frequency is then calculated as fS = 20.48kHz for OSR = 512. Notice that this
choice yields also good attenuation of 50Hz harmonics.
0
0.2
0.4
0.6
0.8
1
1.2
01
2
34
Normalized Frequency - f *(OSR/fS) [-]
N
o
rm
a
li
z
e
d
M
a
g
n
itu
d
e
[-]
NELCONV = 1
0
0.2
0.4
0.6
0.8
1
1.2
012
34
Normalized Frequency - f *(OSR/fS) [-]
No
rm
a
li
z
e
d
M
a
g
n
it
u
d
e
[-]
NELCONV = 2
0
0.2
0.4
0.6
0.8
1
1.2
01
23
4
Normalized Frequency - f *(OSR/fS) [-]
N
o
rm
a
liz
e
d
M
a
g
n
it
u
d
e
[
-]
NELCONV = 4
0
0.2
0.4
0.6
0.8
1
1.2
01
23
4
Normalized Frequency - f *(OSR/fS) [-]
No
rm
a
li
z
e
d
M
a
g
n
itu
d
e
[-]
NELCONV = 8
Figure 16-26. Frequency response: normalized magnitude vs. frequency for different NELCONV
Not
Recommended
for
New
Designs