AD7843 Analog Devices, AD7843 Datasheet - Page 18

• Manufacturer Part Number: AD7843
• Description: Touch Screen Digitizer
• Manufacturer: Analog Devices

Specifications of AD7843

• Resolution (bits): 12bit
• # Chan: 4
• Sample Rate: 125kSPS
• Interface: Ser
• Analog Input Type: SE-Uni
• Ain Range: Uni (Vref)
• Adc Architecture: SAR
• Pkg Type: SOP

AD7843
Sixteen Clocks per Cycle
The control bits for the next conversion can be overlapped with
the current conversion to allow for a conversion every 16 DCLK
cycles, as shown in Figure 26. This timing diagram also allows
for the possibility of communication with other serial peripherals
between each (eight DCLK) byte transfer between the processor
and the converter. However, the conversion must be completed
within a short enough time frame to avoid capacitive droop
effects that could distort the conversion result. It should also be
noted that the AD7843 is fully powered while other serial
communications are taking place between byte transfers.
Fifteen Clocks per Cycle
Figure 27 shows the fastest way to clock the AD7843. This
scheme does not work with most microcontrollers or DSPs
because, in general, they are not capable of generating a
15-clock-cycle-per-serial transfer. However, some DSPs allow
the number of clocks per cycle to be programmed; this method
could also be used with FPGAs (field programmable gate
arrays) or ASICs (application specific integrated circuits). As in
the 16-clocks-per-cycle case, the control bits for the next
conversion are overlapped with the current conversion to allow
a conversion every 15 DCLK cycles, using 12 DCLKs to
perform the conversion and three DCLKs to acquire the analog
input. This effectively increases the throughput rate of the
AD7843 beyond that used for the specifications that are tested
using 16 DCLKs per cycle, and DCLK = 2 MHz.
DCLK
BUSY
DOUT
DOUT
DCLK
BUSY
DIN
CS
DIN
CS
S
1
A2
S
1
A1
Figure 26. Conversion Timing, 16 DCLKS per Cycle, 8-Bit Bus Interface. No DCLK delay required with dedicated serial port.
A0
CONTROL BITS
MODE
SER/
DFR
PD1 PD0
Figure 27. Conversion Timing, 15 DCLKS per Cycle, Maximum Throughput Rate
8
11
10
1
11
9
10
8
Rev. B | Page 18 of 20
9
7
6
8
15
S
5
7
1
A2
4
6
8-Bit Conversion
By setting the MODE bit to 1 in the control register, the
AD7843 can operate in 8-bit rather than 12-bit mode. This
mode allows a faster throughput rate to be achieved, assuming
8-bit resolution is sufficient. When using the 8-bit mode, a
conversion is complete four clock cycles earlier than in the
12-bit mode. This could be used with serial interfaces that
provide 12 clock transfers, or two conversions could be
completed with three 8-clock transfers. The throughput rate
increases by 25% as a result of the shorter conversion cycle, but
the conversion itself can occur at a faster clock rate because the
internal settling time of the AD7843 is not as critical because
settling to 8 bits is all that is required. The clock rate can be as
much as 50% faster. The faster clock rate and fewer clock cycles
combine to provide double the conversion rate.
A1
3
5
8
A0
2
4
S
MODE
1
1
3
SER/
DFR
0
2
PD1 PD0
CONTROL BITS
1
0
11
10
9
8
8
1
7
11
6
15
10
S
5
1
9
A2
4