+VCC
VREF
PENIRQ
TEMP1
TEMP0
A2-A0
SER/DFR
(Shown 001B)
(Shown High)
X+
X–
Ref On/Off
Y+
+REF
Converter
+IN
Y–
–IN
2.5V
Reference
–REF
7.5kΩ
VBAT
2.5kΩ
Battery
On
AUX
GND
FIGURE 2. Simplified Diagram of Analog Input.
INTERNAL REFERENCE
Reference
Power Down
The ADS7846 has an internal 2.5V voltage reference that can
be turned on or off with the control bit, PD1 = 1 (see Table V and
Figure 3). Typically, the internal reference voltage is only used
in the single-ended mode for battery monitoring, temperature
measurement, and for using the auxiliary input. Optimal touch
screen performance is achieved when using the differential
mode. The internal reference voltage of the ADS7846 must be
commanded to be off to maintain compatibility with the ADS7843.
Therefore, after power-up, a write of PD1 = 0 is required to
insure the reference is off (see the Typical Characteristics for
power-up time of the reference from power-down).
VREF
Band
Gap
Buffer
Optional
To
CDAC
FIGURE 3. Simplified Diagram of the Internal Reference.
REFERENCE INPUT
The voltage difference between +REF and –REF (shown in
Figure 2) sets the analog input range. The ADS7846 oper-
ates with a reference in the range of 1V to +VCC. 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 4096
in 12-bit mode. Any offset or gain error inherent in the ADC
appears to increase, in terms of LSB size, as the reference
voltage is reduced. For example, if the offset of a given
converter is 2LSBs with a 2.5V reference, it is typically
5LSBs with a 1V reference. In each case, the actual offset of
the device is the same, 1.22mV. With a lower reference
ADS7846
SBAS125H
9
www.ti.com
BB [ BURR-BROWN CORPORATION ]
