ADS7924_15 Datasheet, PDF (33/50 Pages) Texas Instruments – 4-Channel, MicroPOWER Analog-to-Digital Converter

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8 Application and Implementation
ADS7924
SBAS482B – JANUARY 2010 – REVISED SEPTEMBER 2015
NOTE
Information in the following applications sections is not part of the TI component
specification, and TI does not warrant its accuracy or completeness. TI’s customers are
responsible for determining suitability of components for their purposes. Customers should
validate and test their design implementation to confirm system functionality.
8.1 Application Information
The ADS7924 device provides a break-out point in the signal path between the multiplexer output and the ADC
input for external signal conditioning, if desired. Typical uses include adding an operational amplifier, such as the
TLV2780, along with an RC filter circuit. Different application circuits are described in following sections.
8.1.1 Using an Operational Amplifier Between Multiplexer Output and ADC Input
Adding an operational amplifier provides a high input impedance to the sensor source and buffers the capacitive
ADC input from high-impedance sensor circuits, as shown in Figure 57. High-impedance input signals can be
momentarily disrupted when coupled directly to a capacitive input like that of a sampling ADC. This disruption
can create errors when sampling. The use of an operational amplifier is recommended in these cases.
SHDN
TLV2780
AVDD
DVDD
1mF
1mF
Sensor Signals
12
CH0
11
CH1
10
CH2
9
CH3
3kW
1
RESET
ADS7924
2
INT
3
SCL
4
SDA
3kW
MSP430
Microcontroller
Figure 57. Sensor Data Acquisition With TLV2780 Buffer Amplifier
8.1.2 Using an Operational Amplifier and RC Filter Between Multiplexer Output and ADC Input
Placing an RC low-pass filter in the signal path allows for filtering out noise. The RC component values should
allow for sufficient settling time when changing from channel to channel. The time required for a full-scale input
signal to settle to within 1LSB of a 12-bit ADC is given by Equation 3:
Settling Time = R × C × ln(2¹²)
(3)
R_X and C form a low-pass filter for removing sensor and noise from other sources at the operational amplifier
input pin. The low-pass bandwidth is given by Equation 4:
f_–3dB = 1/(2πRC)
(4)
The f_–3dB should be chosen so that the signals of interest are within half of the programmable sampling
frequency. The noise bandwidth is given by Equation 5:
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