Datasheet

V
IN
SCL
SDA
V
A
ADC081C021
0.1 PF
GND
4.7 PF
22:
INPUT
470 pF
ALERT
5 k:
R
P
R
P
Regulated Supply
V
DD
Controller
SCL
SDA
INTERRUPT
I
2
C BUS ...
ADC081C021, ADC081C027
SNAS447C –FEBRUARY 2008–REVISED MARCH 2013 www.ti.com
APPLICATIONS INFORMATION
TYPICAL APPLICATION CIRCUIT
A typical application circuit is shown in Figure 36. The analog supply is bypassed with a capacitor network
located close to the ADC081C021. The ADC uses the analog supply (V
A
) as its reference voltage, so it is very
important that V
A
be kept as clean as possible. Due to the low power requirements of the ADC081C021, it is
possible to use a precision reference as a power supply.
The bus pull-up resistors (R
P
) should be powered by the controller's supply. It is important that the pull-up
resistors are pulled to the same voltage potential as V
A
. This will ensure that the logic levels of all devices on the
bus are compatible. If the controller's supply is noisy, an appropriate bypass capacitor should be added between
the controller's supply pin and the pull-up resistors. For Hs-mode applications, this bypass capacitance will
improve the accuracy of the ADC.
The value of the pull-up resistors (R
P
) depends upon the characteristics of each particular I
2
C bus. The I
2
C
specification describes how to choose an appropriate value. As a general rule-of-thumb, we suggest using a 1kΩ
resistor for Hs-mode bus configurations and a 5kΩ resistor for Standard or Fast Mode bus configurations.
Depending upon the bus capacitance, these values may or may not be sufficient to meet the timing requirements
of the I
2
C bus specification. Please see the I
2
C specification for further information.
Figure 36. Typical Application Circuit
BUFFERED INPUT
A buffered input application circuit is shown in Figure 37. The analog input is buffered by a TI LMP7731. The
non-inverting amplifier configuration provides a buffered gain stage for a single ended source. This application
circuit is good for single-ended sensor interface. The input must have a DC bias level that keeps the ADC input
signal from swinging below GND or above the supply (+5V in this case).
The LM4132, with its 0.05% accuracy over temperature, is an excellent choice as a reference source for the
ADC081C021.
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