The Timer register, for instance, is loaded with a value that will
provide an interrupt at the required sample interval. When an
interrupt is received, a value is transmitted with TFS/DT (ADC
control word). The TFS is used to control the RFS and therefore
the reading of data. The frequency of the serial clock is set in the
SCLKDIV Register. When the instruction to transmit with TFS
is given (i.e., AX0 = TX0), the state of the SCLK is checked.
The DSP will wait until the SCLK has gone High, Low, and
High before transmission will start. If the timer and SCLK values
are chosen such that the instruction to transmit occurs on or
near the rising edge of SCLK, then the data may be transmitted
or it may wait until the next clock edge.
For example, if the ADSP-2189 had a 20 MHz crystal such that
it had a master clock frequency of 40 MHz, then the master cycle
time would be 25 ns. If the SCLKDIV Register is loaded with the
value 3, then an SCLK of 5 MHz is obtained and eight master
clock periods will elapse for every one SCLK period. Depending
on the throughput rate selected, if the Timer Registers are loaded
with the value, say 803, 100.5 SCLKs will occur between inter-
rupts and subsequently between transmit instructions. This
situation will result in non-equidistant sampling as the transmit
instruction is occurring on a SCLK edge. If the number of
SCLKs between interrupts is a whole integer figure of N, then
equidistant sampling will be implemented by the DSP.
AD7927 to DSP563xx
The connection diagram in Figure 22 shows how the AD7927
can be connected to the ESSI (Synchronous Serial Interface) of
the DSP563xx family of DSPs from Motorola. Each ESSI (two on
board) is operated in Synchronous mode (SYN bit in CRB = 1)
with internally generated word length frame sync for both Tx
and Rx (bits FSL1 = 0 and FSL0 = 0 in CRB). Normal operation
of the ESSI is selected by making MOD = 0 in the CRB. Set the
word length to 16 by setting bits WL1 = 1 and WL0 = 0 in CRA.
The FSP bit in the CRB should be set to 1 so the frame sync is
negative. It should be noted that for signal processing applica-
tions, it is imperative that the frame synchronization signal from
the DSP563xx provides equidistant sampling.
In the example shown in Figure 22, the serial clock is taken from
the ESSI so the SCK0 pin must be set as an output, SCKD = 1.
The VDRIVE pin of the AD7927 takes the same supply voltage as
that of the DSP563xx. This allows the ADC to operate at a higher
voltage than the serial interface, i.e., DSP563xx, if necessary.
*ADDITIONAL PINS REMOVED FOR CLARITY
Figure 22. Interfacing to the DSP563xx
Grounding and Layout
The AD7927 has very good immunity to noise on the power
supplies as can be seen by the PSRR vs. Supply Ripple Frequency
plot, TPC 3. However, care should still be taken with regard to
grounding and layout.
The printed circuit board that houses the AD7927 should be
designed such that the analog and digital sections are separated
and confined to certain areas of the board. This facilitates the use
of ground planes that can be separated easily. A minimum etch
technique is generally best for ground planes as it gives the best
shielding. All three AGND pins of the AD7927 should be sunk
in the AGND plane. Digital and analog ground planes should be
joined at only one place. If the AD7927 is in a system where
multiple devices require an AGND to DGND connection, the
connection should still be made at one point only, a star ground
point that should be established as close as possible to the AD7927.
Avoid running digital lines under the device as these will couple
noise onto the die. The analog ground plane should be allowed to
run under the AD7927 to avoid noise coupling. The power supply
lines to the AD7927 should use as large a trace as possible to
provide low impedance paths and reduce the effects of glitches
on the power supply line. Fast switching signals, like clocks,
should be shielded with digital ground to avoid radiating noise
to other sections of the board, and clock signals should never be
run near the analog inputs. Avoid crossover of digital and analog
signals. Traces on opposite sides of the board should run at right
angles to each other. This will reduce the effects of feedthrough
through the board. A microstrip technique is by far the best but
is not always possible with a double-sided board. In this technique,
the component side of the board is dedicated to ground planes
while signals are placed on the solder side.
Good decoupling is also important. All analog supplies should be
decoupled with 10 mF tantalum in parallel with 0.1 mF capacitors
to AGND. To achieve the best from these decoupling components,
they must be placed as close as possible to the device, ideally
right up against the device. The 0.1 mF capacitors should have
low Effective Series Resistance (ESR) and Effective Series Induc-
tance (ESI), such as the common ceramic types or surface mount
types, which provide a low impedance path to ground at high
frequencies to handle transient currents due to internal logic
Evaluating the AD7927 Performance
The recommended layout for the AD7927 is outlined in the
evaluation board for the AD7927. The evaluation board package
includes a fully assembled and tested evaluation board, docu-
mentation, and software for controlling the board from the
PC via the Eval-Board Controller. The Eval-Board Controller
can be used in conjunction with the AD7927 Evaluation board
as well as many other Analog Devices evaluation boards ending
in the CB designator to demonstrate/evaluate the ac and dc
performance of the AD7927.
The software allows the user to perform ac (fast Fourier
transform) and dc (histogram of codes) tests on the AD7927.
The software and documentation are on a CD shipped with the