Datasheet

ADS8345
SBAS177C
10
www.ti.com
THEORY OF OPERATION
The ADS8345 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 0.6µm
CMOS process.
The basic operation of the ADS8345 is shown in Figure 1.
The device requires an external reference and an external
clock. It operates from a single supply of 2.7V to 5.25V. The
external reference can be any voltage between 500mV and
+V
CC
/2. The value of the reference voltage directly sets the
input range of the converter. The average reference input
current depends on the conversion rate of the ADS8345.
The analog input to the converter is differential and is
provided via an eight-channel multiplexer. The input can be
provided in reference to a voltage on the COM pin (which is
generally +V
CC
/2) or differentially by using four of the eight
input channels (CH-CH7). The particular configuration is
selectable via the digital interface.
ANALOG INPUT
The analog input is bipolar and fully differential. There are
two general methods of driving the analog input of the
ADS8345: single-ended or differential (see Figure 2). When
the input is single-ended, the COM input is held at a fixed
voltage. The CHX input swings around the same voltage and
the peak-to-peak amplitude is 2 • V
REF
. The value of V
REF
FIGURE 1. Basic Operation of the ADS8345.
determines the range over which the common voltage may
vary (see Figure 3).
When the input is differential, the amplitude of the input is the
difference between the CHX and COM input (see Figure 4).
A voltage or signal is common to both of these inputs. The
peak-to-peak amplitude of each input is V
REF
about this
common voltage. However, since the input are 180°C out-of-
phase, the peak-to-peak amplitude of the difference voltage is
2 • V
REF
. The value of V
REF
also determines the range of the
voltage that may be common to both inputs (see Figure 5).
In each case, care should be taken to ensure that the output
impedance of the sources driving the CHX and COM inputs
are matched. If this is not observed, the two inputs could
have different settling times. This may result in offset error,
gain error, and linearity error which changes with both
temperature and input voltage. If the impedance cannot be
matched, the errors can be lessened by giving the ADS8345
additional acquisition time.
The input current on the analog inputs depends on a number
of factors: sample rate, input voltage, and source impedance.
Essentially, the current into the ADS8345 charges the inter-
nal capacitor array during the sample period. After this
capacitance has been fully charged, there is no further input
current.
Care must be taken regarding the absolute analog input
voltage. Outside of these ranges, the converter’s linearity
may not meet specifications. Please refer to the Electrical
Characteristics table for min/max ratings.
CH0
CH1
CH2
CH3
CH4
CH5
CH6
CH7
COM
SHDN
1
2
3
4
5
6
7
8
9
10
20
19
18
17
16
15
14
13
12
11
+V
CC
DCLK
CS
D
IN
BUSY
D
OUT
GND
GND
+V
CC
V
REF
Serial/Conversion Clock
Chip Select
Serial Data In
Serial Data Out
+2.7V to +5V
1µF to 10µF
ADS8345
Single-ended
or differential
analog inputs.
V
REF
1µF to 10µF
+1.25V to +2.5V
0.1µF