Datasheet

Microcontroller
+2.7V to +3.6V
1 F tom
10 Fm
+
2kW2kW
5W
1 F tom
10 Fm
+
0.1 Fm
ADS7830
REF /
IN
REF
OUT
CH0
V
DD
SDA
SCL
A0
A1
GND
CH1
CH2
CH3
CH4
CH5
CH6
CH7
COM
ADS7830
www.ti.com SBAS302C –DECEMBER 2003–REVISED OCTOBER 2012
THEORY OF OPERATION
REFERENCE
The ADS7830 is a classic Successive Approximation
Register (SAR) A/D converter. The architecture is The ADS7830 can operate with an internal 2.5V
based on capacitive redistribution which inherently reference or an external reference. If a +5V supply is
includes a sampleand- hold function. The converter is used, an external +5V reference is required in order
fabricated on a 0.6µ CMOS process. to provide full dynamic range for a 0V to +V
DD
analog
input. This external reference can be as low as
The ADS7830 core is controlled by an internally
50mV. When using a +2.7V supply, the internal +2.5V
generated free-running clock. When the ADS7830 is
reference will provide full dynamic range for a 0V to
not performing conversions or being addressed, it
+V
DD
analog input.
keeps the A/D converter core powered off, and the
internal clock does not operate. As the reference voltage is reduced, the analog
voltage weight of each digital output code is reduced.
The simplified diagram of input and output for the
This is often referred to as the LSB (least significant
ADS7830 is shown in Figure 13.
bit) size and is equal to the reference voltage divided
by 256. This means that any offset or gain error
ANALOG INPUT
inherent in the A/D converter will appear to increase,
in terms of LSB size, as the reference voltage is
When the converter enters the hold mode, the
reduced.
voltage on the selected CHx pin is captured on the
internal capacitor array. The input current on the
The noise inherent in the converter will also appear to
analog inputs depends on the conversion rate of the
increase with lower LSB size. With a 2.5V reference,
device. During the sample period, the source must
the internal noise of the converter typically contributes
charge the internal sampling capacitor (typically
only 0.02LSB peak-to-peak of potential error to the
25pF). After the capacitor has been fully charged,
output code. When the external reference is 50mV,
there is no further input current. The amount of
the potential error contribution from the internal noise
charge transfer from the analog source to the
will be 50 times larger—1LSB. The errors due to the
converter is a function of conversion rate.
internal noise are Gaussian in nature and can be
reduced by averaging consecutive conversion results.
Figure 13. Simplified I/O of the ADS7830
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