AD7889 datasheet(20/40 Pages) AD

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AD7879/AD7889

Data Sheet

Rev. D | Page 20 of 40

CONVERSION TIMING

Conversion timing or update rate is the rate at which the

AD7879/AD7889 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.

TMEASURE

TMEASURE TMEASURE TMEASURE

X+

× M

Y+

VBAT/AUX

TEMP

Figure 29. Conversion Timing 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 the start of the sequence. At the end of the sequence, there is

always another FCD.

TMEASURE is the time required to perform one measurement in

the conversion sequence.

TMEASURE = (ACQ (2 μs, 4 μs, 8 μs, 16 μs) + TCONV (7.5 μs) +

TSORT (2 μs))

where:

ACQ is the acquisition time which is programmable in Control

TCONV (typical ADC conversion time) is specified at 7.5 μs.

TSORT is the time needed to sort the new sample within the

median filter array. The TSORT value is approximately 2 μs. If a

median filter is not used (MED =0), the TSORT value is 0.

TMEASURE_MIN = 9.5 μs (ACQ = 2 μs, no median filter)

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:

TCHANNEL = TMEASURE × MED

TCHANNEL_MIN =9.5 μs (ACQ = 2 μs, MED = 0)

TCHANNEL_MAX = 376 μs (ACQ = 16 μs, MED = 16)

Update Rate = (FCD + (TMEASURE × MED)) × N + FCD +

TMR

where:

MED = median filter setting (1, 4, 8, 16).

N = number of channels to be measured (1 to 6).

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 μs

MED = 8

N = 2

FCD = 1.024 ms

TMR = 620 μs

TMEASURE = 4 + 7.5 + 2 = 13.5 μs

TCHANNEL = (13.5 × 8) = 108 μs

Then, Update Rate = (1024 + 108) × 2 + 1024 + 620 = 3.9 ms

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