ADS7846 datasheet(9/29 Pages) SYC

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ADS7846

SBAS125H

www.ti.com

INTERNAL REFERENCE

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).

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

FIGURE 2. Simplified Diagram of Analog Input.

Converter

–REF

+REF

+IN

–IN

BAT

AUX

Battery

GND

A2-A0

(Shown 001

2.5V

Reference

Ref On/Off

SER/DFR

(Shown High)

X+

X–

TEMP1

PENIRQ

Y+

Y–

REF

TEMP0

7.5k

Ω

2.5k

Ω

Buffer

Band

Gap

Reference

Power Down

CDAC

Optional

REF

FIGURE 3. Simplified Diagram of the Internal Reference.

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