Datasheet

ADS8343
11
SBAS183C
www.ti.com
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
determines the range over which the common
voltage may vary, as shown in Figure 3.
FIGURE 4. Differential Input—Common Voltage Range vs V
REF.
0.0 1.0 1.5 2.0 2.5
V
REF
(V)
4.2
V
CC
= 5V
0.8
5.2
Common Voltage Range (V)
0
1
2
3
4
5
Differential Input
0.2
FIGURE 3. Single-Ended Input—Common Voltage Range vs V
REF
.
0.5 1.0 1.5 2.0 2.5
V
REF
(V)
Common Voltage Range (V)
–1
0
1
2
3
4
5
2.8
2.1
V
CC
= 5V
Single-Ended Input
0.1
4.9
When the input is differential, the amplitude of the input is the
difference between the CHX and COM input. 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 inputs are 180° 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, as shown in Figure 4.
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 change with both tem-
perature and input voltage. If the impedance cannot be
matched, the errors can be lessened by giving the ADS8343
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 ADS8343 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.
REFERENCE INPUT
The external reference sets the analog input range. The
ADS8343 will operate with a reference in the range of 500mV
to +V
CC
/2. Keep in mind that the analog input is the differ-
ence between the CHX input and the COM input, as shown
in Figure 5. For example, in the single-ended mode, with
V
REF
and COM pin set to 1.25V, the selected input channel
(CH0-CH3) will properly digitize a signal in the range of 0V to
2.50V relative to GND. If the COM pin is connected to 2.0V,
the input range on the selected channel is 0.75V to 3.25V.
FIGURE 5. Simplified Diagram of the Analog Input.
Converter
+IN
–IN
CH0
CH1
CH2
CH3
COM
A2-A0
(Shown 001
B
)
SGL/DIF
(Shown HIGH)
There are several critical items concerning the reference
input and its wide voltage range. As the reference voltage is
reduced, the analog voltage weight of each digital output
code is also reduced. This is often referred to as the LSB
(Least Significant Bit) size and is equal to the reference
voltage divided by 65,536. Any offset or gain error inherent
in the A/D converter will appear to increase, in terms of LSB