AD7889 Analog Devices, AD7889 Datasheet - Page 20

• Manufacturer Part Number: AD7889
• Description: Low Voltage Controller for Touch Screens
• Manufacturer: Analog Devices

Specifications of AD7889

• Resolution (bits): 12bit
• # Chan: 6
• Sample Rate: 105kSPS
• Interface: I²C/Ser 2-Wire,Ser,SPI
• Analog Input Type: SE-Uni
• Ain Range: Uni Vdd
• Adc Architecture: SAR
• Pkg Type: CSP

AD7879/AD7889
CONVERSION TIMING
Conversion timing or update rate is the rate at which the
AD7879 provides converted values from the ADC so that the
XY positions in the touch screen can be updated. In other
words, the update rate is the timing required to give valid
measurements in the sequencer.
Figure 29 shows conversion timing for a conversion sequence.
FCD is required before each touch screen measurement (X+,
Y+, Z1, and Z2). This time is required to allow the screen inputs
to settle before converting. If the sequence does not contain any
screen channel (VBAT, AUX, or TEMP), only one FCD is added
at start of the sequence. At the end of the sequence, there is
always another FCD.
T
the conversion sequence.
T
(2 μs)]
where:
ACQ is the acquisition time which is programmable in Control
Register 1. For temperature measurements, ACQ is fixed at 16 μs.
T
T
median filter array. The T
median filter is not used (MED =0), the T
T
C
D
F
MEASURE
MEASURE
CONV
SORT
MEASURE
T
MEASURE
X+
is the time needed to sort the new sample within the
(typical ADC conversion time) is specified at 7.5 μs.
× M
_
= [ACQ (2 μs, 4 μs, 8 μs, 16 μs) + T
is the time required to perform one measurement in
MIN
C
D
F
= 9.5 μs (ACQ = 2 μs, no median filter)
T
MEASURE
Y+
Figure 29. Conversion Timing Sequence
× M
F
C
D
T
MEASURE
SORT
Z1
× M
value is approximately 2 μs. If a
F
C
D
T
MEASURE
Z2
× M
SORT
VBAT/AUX
CONV
T
MEASURE
value is 0.
× M
(7.5 μs) + T
T
MEASURE
TEMP
× M
SORT
F
C
D
Rev. C | Page 20 of 40
Conversion time per channel depends on the number of
samples to be converted. The number of samples is pro-
grammed using the following median filter settings:
T
T
T
Update Rate = [FCD + (T
where:
N = number of channels to be measured (1 to 6).
MED = median filter setting (1, 4, 8, 16).
TMR = timer setting (0 μs to 9.4 ms).
The total update rate depends on the median filter settings and
the number of channels in the conversion sequence. The timer
setting (TMR) allows the user more flexibility to program the
update rate.
For example, if
ACQ = 4 us
MED = 8
N = 2
FCD = 1.024 ms
TMR = 620 μs
T
T
Then
Update rate = [1024 + 108] × 2 + 1024 + 620 = 3.9 ms
CHANNEL
CHANNEL
CHANNEL
MEASURE
CHANNEL
_
_
= T
= 4 + 7.5 + 2 = 13.5 μs
= (13.5 × 8) = 108 μs
MIN
MAX
MEASURE
=9.5 μs (ACQ = 2 μs, MED = 0)
= 376 μs (ACQ = 16 μs, MED = 16)
× MED
MEASURE
× MED)] × N + FCD + TMR