Datasheet
R
X
TLV2780
C
RESET
INT
SCL
SDA
CH0
CH1
CH2
CH3
1
2
3
4
12
11
10
9
DVDD
16
A0
5
AVDD
15
DGND
6
ADCIN
14
PWRCON
7
MUXOUT
13
AGND
8
ADS7924
SensorSignals
MSP430
Microcontroller
SHDN
DVDD
AVDD
1 Fm
1 Fm
3kW 3kW
ADS7924
SBAS482A –JANUARY 2010–REVISED MAY 2010
www.ti.com
Using an Op Amp and RC Filter
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 5:
Settling Time = R × C × ln(2
12
) (5)
R
X
and C form a low-pass filter for removing sensor and noise from other sources at the op amp input pin. The
low-pass bandwidth is given by Equation 6:
f
–3dB
= 1/(2pRC) (6)
The f
–3dB
should be chosen such that the signals of interest are within half of the programmable sampling
frequency. The noise bandwidth is given by Equation 7:
f
NB
= 1/(4RC) (7)
This term should be set to reduce noise bandwidth but still allow for enough settling time. Note that the ADS7924
has internal registers ACQCONFIG (address = 14h), PWRCONF (address = 15h), and SLPCONFIG (address =
13h) that can be programmed to slow down the channel-to-channel power up, acquisition, and sleep periods if
needed to allow for a longer settling time requirement.
In Figure 36, R is the sum of the sensor output impedance R
SENSOR
, the internal MUX resistance R
MUX
(approximately 60Ω), and external resistor R
X
. The primary benefit of having the filter at the input of the op amp
is that the amplifier does not have to drive the filter, which can cause instability with large capacitor values that
may be needed in order to filter noise to low levels.
NOTE: f
–3dB
BW = 159kHz, R = 1kΩ, and C = 1nF where R = R
MUX
+ R
SENSOR
+ R
X
.
Figure 36. Sensor Data Acquisition with Filter and TLV2780 Buffer Amplifier
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Product Folder Link(s): ADS7924