AD7873 Data Sheet
Rev. F | Page 22 of 28
16 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 38. This timing diagram also allows
the possibility of communication with other serial peripherals
between each byte (eight DCLKs) transfer between the
processor and the converter. However, the conversion must
complete within a short enough time frame to avoid capacitive
droop effects that could distort the conversion result. It should
also be noted that the AD7873 is fully powered while other
serial communications are taking place between byte transfers.
15 Clocks per Cycle
Figure 39 shows the fastest way to clock the AD7873. This
scheme does not work with most microcontrollers or DSPs
because 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 can 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 3 DCLKs to
acquire the analog input. This effectively increases the
throughput rate of the AD7873 beyond that used for the
specifications that are tested using 16 DCLKs per cycle, and
DCLK = 2 MHz.
8-Bit Conversion
The AD7873 can be set up to operate in an 8-bit mode rather
than a 12-bit mode by setting the MODE bit in the control
register to 1. This mode allows a faster throughput rate to be
achieved, assuming 8-bit resolution is sufficient. When using 8-bit
mode, a conversion is complete four clock cycles earlier than in
12-bit mode. This can be used with serial interfaces that provide
12 clock transfers, or two conversions can 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 AD7873 is not as critical, because settling to eight 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.
Figure 38. Conversion Timing, 16 DCLKs per Cycle, 8-Bit Bus Interface. No DCLK delay required with dedicated serial port.
Figure 39. Conversion Timing, 15 DCLKs per Cycle, Maximum Throughput Rate
DCLK
DIN
BUSY
DOUT
CS
1
S S
11 10 9 8 7 6 5 4 3 2 1 0 11 10 9
1 1
1
8 8
8
CONTROL BITS CONTROL BITS
02164-038
CS
DCLK
DIN
BUSY
DOUT
S A2 PD1 PD0A1 A0
MODE
SER/
DFR
MODE
SER/
DFR
1
11 10 9 8 7 6 5 4 3 2 1 0 11 10 9 8 7 6 5 4
15 1 15
1
S A2 S A2A1 PD1 PD0A0
02164-039
