AD7890 Datasheet, PDF (12/20 Pages) Analog Devices – LC2MOS 8-Channel, 12-Bit Serial, Data Acquisition System

AD7890
CONVST
SCLK
RFS
TFS
t_CONVERT
500ns MIN
CONVERSION IS
INITIATED AND
TRACK/HOLD GOES
INTO HOLD
CONVERSION µP INT
ENDS 5.9µs SERVICE OR
LATER
POLLING
ROUTINE
SERIAL READ
& WRITE
OPERATIONS
READ & WRITE
OPERATIONS SHOULD
END 500ns PRIOR TO
NEXT RISING EDGE OF
CONVST
NEXT CONVST
RISING EDGE
Figure 6. CONVST Used as Status Signal in External Clocking Mode
In the Self-Clocking Mode, the AD7890 indicates when conver-
sion is complete by bringing the RFS line low and initiating a
serial data transfer. In the external clocking mode, there is no
indication of when conversion is complete. In many applica-
tions, this will not be a problem as the data can be read from the
part during conversion or after conversion. However, applica-
tions which want to achieve optimum performance from the
AD7890 will have to ensure that the data read does not occur
during conversion or during 500 ns prior to the rising edge of
CONVST. This can be achieved in either of two ways. The first
is to ensure in software that the read operation is not initiated
until 5.9 µs after the rising edge of CONVST. This will only be
possible if the software knows when the CONVST command is
issued. The second scheme would be to use the CONVST sig-
nal as both the conversion start signal and an interrupt signal.
The simplest way to do this would be to generate a square wave
signal for CONVST with high and low times of 5.9 µs (see Fig-
ure 6). Conversion is initiated on the rising edge of CONVST.
The falling edge of CONVST occurs 5.9 µs later and can be
used as either an active low or falling edge-triggered interrupt
signal to tell the processor to read the data from the AD7890.
Provided the read operation is completed 500 ns before the ris-
ing edge of CONVST, the AD7890 will operate to specification.
This scheme limits the throughput rate to 11.8 µs minimum.
However, depending upon the response time of the micropro-
cessor to the interrupt signal and the time taken by the proces-
sor to read the data, this may the fastest which the system could
have operated. In any case, the CONVST signal does not have
to have a 50:50 duty cycle. This can be tailored to optimize the
throughput rate of the part for a given system.
Alternatively, the CONVST signal can be used as a normal nar-
row pulse width. The rising edge of CONVST can be used as an
active high or rising edge-triggered interrupt. A software delay
of 5.9 µs can then be implemented before data is read from the
part.
C_EXT FUNCTIONING
The C_EXT input on the AD7890 provides a means of determin-
ing how long after a new channel address is written to the part
that a conversion can take place. The reason behind this is
two-fold. Firstly, when the input channel to the AD7890 is
changed, the input voltage on this new channel is likely to be
very different from the previous channel voltage. Therefore, the
part’s track/hold has to acquire the new voltage before an accu-
rate conversion can take place. An internal pulse delays any con-
version start command (as well as the signal to send the track/
hold into hold) until after this pulse has timed out. The second
reason is to allow the user to connect external antialiasing or sig-
nal conditioning circuitry between MUX OUT and SHA IN.
This external circuitry will introduce extra settling time into the
system. The C_EXT pin provides a means for the user to extend
the internal pulse to take this extra settling time into account.
Basically, varying the value of the capacitor on the C_EXT pin var-
ies the duration of the internal pulse. Figure 7 shows the rela-
tionship between the value of the C_EXT capacitor and the
internal delay.
64
56
T_A = +85 °C
48
T_A = +25 °C
40
32
24
T_A = –40 °C
16
8
0
0 250 500 750 1000 1250 1500 1750 2000
C_EXT CAPACITANCE – pF
Figure 7. Internal Pulse Width vs. C_EXT
–12–
REV. A