Datasheet

()
(V )(Gain)
IN
2
AVSS+0.1V +
£ V
CMI
£
()
(V )(Gain)
IN
2
AVDD 0.1V- -
ADC
A1
454W
454W
7.5pF
A2
7.5pF
7.5pF
7.5pF
R
R
C
AIN
P
AIN
N
ADS1146
ADS1147
ADS1148
SBAS453F –JULY 2009–REVISED APRIL 2012 www.ti.com
LOW-NOISE PGA
The ADS1146/7/8 feature a low-drift, low-noise, high
(4)
input impedance programmable gain amplifier (PGA).
The PGA can be set to gain of 1, 2, 4, 8, 16, 32, 64,
MODULATOR
or 128 by register SYS0. A simplified diagram of the
PGA is shown in Figure 21.
A third-order modulator is used in the ADS1146/7/8.
The modulator converts the analog input voltage into
a pulse code modulated (PCM) data stream. To save
power, the modulator clock runs from 32kHz up to
512kHz for different data rates, as shown in Table 4.
DIGITAL FILTER
The ADS1146/7/8 use linear-phase finite impulse
response (FIR) digital filters that can be adjusted for
different output data rates. The digital filter always
settles in a single cycle.
Table 5 shows the exact data rates when an external
oscillator equal to 4.096MHz is used. Also shown is
the signal –3dB bandwidth, and the 50Hz and 60Hz
attenuation. For good 50Hz or 60Hz rejection, use a
data rate of 20SPS or slower.
The frequency responses of the digital filter are
Figure 21. Simplified Diagram of the PGA
shown in Figure 22 to Figure 32. Figure 25 shows a
detailed view of the filter frequency response from
48Hz to 62Hz for a 20SPS data rate. All filter plots
The PGA consists of two chopper-stabilized
are generated with 4.096MHz external clock.
amplifiers (A1 and A2) and a resistor feedback
network that sets the gain of the PGA. The PGA input
Table 4. Modulator Clock Frequency for Different
is equipped with an electromagnetic interference
Data Rates
(EMI) filter, as shown in Figure 21. Note that as with
any PGA, it is necessary to ensure that the input
DATA RATE f
MOD
voltage stays within the specified common-mode
(SPS) (kHz)
input range specified in the Electrical Characteristics.
5, 10, 20 32
The common-mode input (V
CMI
) must be within the
40, 80, 160 128
range shown in Equation 4:
320, 640, 1000 256
2000 512
Table 5. Digital Filter Specifications
(1)
ATTENUATION
DATA RATE –3dB BANDWIDTH f
IN
= 50Hz ±0.3Hz f
IN
= 60Hz ±0.3Hz f
IN
= 50Hz ±1Hz f
IN
= 60Hz ±1Hz
5SPS 2.26Hz –106dB –74dB –81dB –69dB
10SPS 4.76Hz –106dB –74dB –80dB –69dB
20SPS 14.8Hz –71dB –74dB –66dB –68dB
40SPS 9.03Hz
80SPS 19.8Hz
160SPS 118Hz
320SPS 154Hz
640SPS 495Hz
1000SPS 732Hz
2000SPS 1465Hz
(1) Values shown for f
OSC
= 4.096MHz.
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