XE8000EV120 Semtech, XE8000EV120 Datasheet - Page 69

• Manufacturer Part Number: XE8000EV120
• Description: BOARD EVAL FOR SX8722I070TRLF
• Manufacturer: Semtech
• Series: ZoomingADC™r

Specifications of XE8000EV120

• Number Of Adc's: 1
• Number Of Bits: 16
• Data Interface: I²C
• Inputs Per Adc: 7 Single or 4 Differential
• Voltage Supply Source: Single Supply
• Operating Temperature: -40°C ~ 125°C
• Utilized Ic / Part: SX8722
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Other names: XM8000EV120; XM8000EV120

to remain within the ADC input range.
12.9. Power Saving Modes
During low-speed operation, the bias current in the PGAs and ADC can be programmed to save power using the control
words IB_AMP_PGA[1:0] and IB_AMP_ADC[1:0] (see Table 57). If the system is idle, the PGAs and ADC can even be
disabled, thus, reducing power consumption to its minimum. This can considerably improve battery lifetime.
12.10. Input impedance
The PGAs of the ZoomingADC are a switched capacitor based blocks (see Switched Capacitor Principle chapter). This
means that it does not use resistors to fix gains, but capacitors and switches. This has important implications on the nature
of the input impedance of the block.
Using switched capacitors is the reason why, while a conversion is done, the input impedance on the selected channel of the
PGAs is inversely proportional to the sampling frequency fs and to stage gain as given in Equation 21.
The input impedance observed is the input impedance of the first PGA stage that is enabled or the input impedance of the
ADC if all three stages are disabled.
PGA1 (with a gain of 10), PGA2 (with a gain of 10) and PGA3 (with a gain of 10) each have a minimum input impedance of
256 kOhm at fs = 300 kHz. Larger input impedance can be obtained by reducing the gain and/or by reducing the sampling
frequency. Therefore, with a gain of 1 and a sampling frequency of 75 kHz, Zin > 10.2 MOhm.
The input impedance on channels that are not selected is very high (>100MOhm).
ACS - Revision 4.2
©2008 Semtech Corp.
ADVANCED COMMUNICATIONS & SENSING
Z
Table 57. ADC & PGA power saving modes and maximum sampling frequency
in
≥
768
IB_AMP_ADC
(Eq. 21)
f
s
⋅
[1:0]
10
⋅
00
01
10
11
gain
9
Ω
October 2008
Hz
IB_AMP_PGA
[1:0]
00
01
10
11
[
Ohm
Page 69
ADC Bias
1/4 x I
1/2 x I
3/4 x I
Current
I
]
ADC
ADC
ADC
ADC
High gain acquisition for sensor interface
1/4 x I
1/2 x I
3/4 x I
PGA Bias
Current
I
ADC
ADC
ADC
ADC
Max. f
37.5
37.5
150
300
150
300
75
75
S
[KHz]
DATASHEET
www.semtech.com
SX8722