REFERENCE INPUT
THEORY OF OPERATION
The external reference sets the analog input range. The
ADS7823 will operate with a reference in the range of 50mV
to VDD. There are several important implications of this.
The ADS7823 is a classic Successive Approximation Register
(SAR) A/D converter. The architecture is based on capacitive
redistribution which inherently includes a sample-and-hold
function. The converter is fabricated on a 0.6µ CMOS process.
As the reference voltage is reduced, the analog voltage
weight of each digital output code is reduced. This is often
referred to as the LSB (least significant bit) size and is equal
to the reference voltage divided by 4096. This means that
any offset or gain error inherent in the A/D converter will
appear to increase, in terms of LSB size, as the reference
voltage is reduced.
The ADS7823 core is controlled by an internally-generated
free-running clock. When the ADS7823 is not performing
conversions or being addressed, it keeps the A/D converter
core powered off, and the internal clock does not operate.
The ADS7823 has an internal 4-word first-in last-out buffer
(FILO) that stores the results of up to four conversions while
they are waiting to be read out over the I2C bus.
The noise inherent in the converter will also appear to increase
with lower LSB size. With a 2.5V reference, the internal noise
of the converter typically contributes only 0.32LSB peak-to-
peak of potential error to the output code. When the external
reference is 50mV, the potential error contribution from the
internal noise will be 50 times larger—16LSBs. The errors due
to the internal noise are Gaussian in nature and can be
reduced by averaging consecutive conversion results.
The simplified diagram of input and output for the ADS7823
is shown in Figure 1.
ANALOG INPUT
When the converter enters the hold mode, the voltage on the
AIN pin is captured on the internal capacitor array. The input
current on the analog inputs depends on the conversion rate
of the device. During the sample period, the source must
charge the internal sampling capacitor (typically 25pF). After
the capacitor has been fully charged, there is no further input
current. The amount of charge transfer from the analog
source to the converter is a function of conversion rate.
DIGITAL INTERFACE
The ADS7823 supports the I2C serial bus and data transmis-
sion protocol, in all three defined modes: standard, fast, and
high-speed. A device that sends data onto the bus is defined
as a transmitter, and a device receiving data as a receiver.
+2.7V to +3.6V
5Ω
+
1µF to
10µF
2kΩ
2kΩ
VREF
VDD
+
0.1µF
1µF to
10µF
Microcontroller
AIN
A0
SDA
SCL
ADS7823
A1
GND
FIGURE 1. Simplified I/O of the ADS7823.
ADS7823
SBAS180B
8
BB [ BURR-BROWN CORPORATION ]
