CM +V
REF
+V
REF
CM Voltage
+IN
–IN = CM Voltage
–V
REF
CM –V
REF
+IN
+V
REF
CM Voltage
–V
REF
CM –1/2V
REF
–IN
Differential Inputs
(IN+) + (IN–)
t
t
Single-Ended Inputs
CM +1/2V
REF
Common-Mode Voltage (Single-Ended Mode) = IN–.
2
The maximum differential voltage between +IN and –IN of the ADS7861 is V
REF
. See Figures 3 and 4 for a further
explanation of the common voltage range for single-ended and differential inputs.
NOTES: Common-Mode Voltage (Differential Mode) =
FIGURE 2. Using the ADS7861 in the Single-Ended and Differential Input Modes.
5
V
CC
= 5V
4.1
4
Common Voltage Range (V)
Common Voltage Range (V)
5
4.7
V
CC
= 5V
4.0
4
3
Single-Ended Input
2.7
2.3
3
Differential Input
2
2
1
0.9
1
0.3
0
1.0
0
–1
1.0
1.2
1.5
2.0
V
REF
(V)
2.5
2.6
3.0
–1
1.0
1.2
1.5
2.0
V
REF
(V)
2.5
2.6
3.0
FIGURE 3. Single-Ended Input: Common-Mode Voltage
Range vs V
REF
.
In each case, care should be taken to ensure that the output
impedance of the sources driving the +IN and –IN inputs are
matched. Otherwise, this may result in offset error, gain
error and linearity error which will change with both tem-
perature and input voltage.
The input current on the analog inputs depend on a number
of factors: sample rate, input voltage, and source impedance.
Essentially, the current into the ADS7861 charges the inter-
nal capacitor array during the sampling period. After this
FIGURE 4. Differential Input: Common-Mode Voltage
Range vs V
REF
.
capacitance has been fully charged, there is no further input
current. The source of the analog input voltage must be able
to charge the input capacitance (15pF) to a 12-bit settling
level within 2 clock cycles. When the converter goes into the
hold mode, the input impedance is greater than 1GΩ.
Care must be taken regarding the absolute analog input
voltage. The +IN input should always remain within the
range of GND – 300mV to V
DD
+ 0.3V.
ADS7861
SBAS110D
9
TI [ TEXAS INSTRUMENTS ]
