AD7873
Data Sheet
ANALOG INPUT
Figure 26 shows an equivalent circuit of the analog input
structure of the AD7873 that contains a block diagram of the
input multiplexer, the differential input of the ADC, and the
differential reference.
V
CC
X+
X–
Table 6 shows the multiplexer address corresponding to each
Y+
Y–
REF
INT/
analog input, both for the SER/
bit in the control register
DFR
X+ Y+ EXT
set high and low. The control bits are provided serially to the
device via the DIN pin. For more information on the control
register, see the Control Register section.
3-TO-1
MUX
ON-CHIP SWITCHES
When the converter enters hold mode, the voltage difference
between the +IN and –IN inputs (see Figure 26) 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 37 pF). Once the capacitor is fully charged,
there is no further input current. The rate of charge transfer
from the analog source to the converter is a function of
conversion rate.
X+
Y+
Y–
REF+
ADC CORE
IN+
DATA OUT
IN+
6-TO-1
MUX
IN– REF–
V
BAT
AUX
3-TO-1
MUX
TEMP
X– Y– GND
Figure 26. Equivalent Analog Input Circuit
Table 6. Analog Input, Reference, and Touch Screen Control
A2
0
0
0
0
A1
0
0
1
1
A0
0
1
0
1
Analog Input
TEMP0
X+
VBAT
X+ (Z1)
X Switches
Off
Off
Y Switches
+REF1
VREF
VREF
VREF
–REF1
GND
GND
GND
GND
SER/ DFR
1
1
1
1
Off
On
Off
Y+ On
Y– Off
Y+ On
Y– Off
Off
Off
X+ Off
X– On
X+ Off
X– On
On
VREF
1
0
0
1
Y– (Z2)
VREF
GND
1
1
1
0
0
1
1
0
1
0
1
0
1
1
1
0
Y+
VREF
VREF
VREF
GND
GND
GND
AUX
TEMP1
Off
Off
Off
Off
Invalid Address. Test Mode: Switches out the TEMP0 diode to the PENIRQ pin.
0
0
0
0
1
1
1
0
1
0
0
0
X+
Off
On
Y+
Y+
Y+
X+
Y–
X–
X–
X–
Invalid Address
X+ (Z1)
X+ Off
X– On
X+ Off
X– On
ON
Y+ On
Y– Off
Y+ On
Y– Off
Off
1
0
0
0
Y– (Z2)
Y+
1
1
1
0
1
1
1
0
1
0
0
0
Outputs Identity Code, 1000 0000 0000.
Invalid address. Test mode: Switches out the TEMP1 diode to the PENIRQ pin.
1 Internal node, not directly accessible by the user.
Rev. F | Page 1ꢀ of 28
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